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Sample records for co2-based heusler compounds

  1. Electronic and magnetic properties of the Co2-based Heusler compounds under pressure: first-principles and Monte Carlo studies

    NASA Astrophysics Data System (ADS)

    Zagrebin, M. A.; Sokolovskiy, V. V.; Buchelnikov, V. D.

    2016-09-01

    Structural, magnetic and electronic properties of stoichiometric Co2 YZ Heusler alloys (Y  =  Cr, Fe, Mn and Z  =  Al, Si, Ge) have been studied by means of ab initio calculations and Monte Carlo simulations. The investigations were performed in dependence on different levels of approximations in DFT (FP and ASA modes, as well as GGA and GGA  +  U schemes) and external pressure. It is shown that in the case of the GGA scheme the half-metallic behavior is clearly observed for compounds containing Cr and Mn transition metals, while Co2FeZ alloys demonstrate the pseudo half-metallic behavior. It is demonstrated that an applied pressure and an account of Coulomb repulsion (U) lead to the stabilization of the half-metallic nature for Co2 YZ alloys. The strongest ferromagnetic inter-sublattice (Co-Y) interactions together with intra-sublattice (Co-Co and Y-Y) interactions explain the high values of the Curie temperature obtained by Monte Carlo simulations using the Heisenberg model. It is observed that a decrease in valence electrons of Y atoms (i.e. Fe substitution by Mn and Cr) leads to the weakening of the exchange interactions and to the reduction of the Curie temperature. Besides, in the case of the FP mode Curie temperatures were found in a good agreement with available experimental and theoretical data, where the latter were obtained by applying the empirical relation between the Curie temperature and the total magnetic moment.

  2. Basics and prospective of magnetic Heusler compounds

    SciTech Connect

    Felser, Claudia Wollmann, Lukas; Chadov, Stanislav; Fecher, Gerhard H.; Parkin, Stuart S. P.

    2015-04-01

    Heusler compounds are a remarkable class of materials with more than 1000 members and a wide range of extraordinary multi-functionalities including halfmetallic high-temperature ferri- and ferromagnets, multi-ferroics, shape memory alloys, and tunable topological insulators with a high potential for spintronics, energy technologies, and magneto-caloric applications. The tunability of this class of materials is exceptional and nearly every functionality can be designed. Co{sub 2}-Heusler compounds show high spin polarization in tunnel junction devices and spin-resolved photoemission. Manganese-rich Heusler compounds attract much interest in the context of spin transfer torque, spin Hall effect, and rare earth free hard magnets. Most Mn{sub 2}-Heusler compounds crystallize in the inverse structure and are characterized by antiparallel coupling of magnetic moments on Mn atoms; the ferrimagnetic order and the lack of inversion symmetry lead to the emergence of new properties that are absent in ferromagnetic centrosymmetric Heusler structures, such as non-collinear magnetism, topological Hall effect, and skyrmions. Tetragonal Heusler compounds with large magneto crystalline anisotropy can be easily designed by positioning the Fermi energy at the van Hove singularity in one of the spin channels. Here, we give a comprehensive overview and a prospective on the magnetic properties of Heusler materials.

  3. Thin film Heusler compounds manganese nickel gallium

    NASA Astrophysics Data System (ADS)

    Jenkins, Catherine Ann

    Multiferroic Heusler compounds Mn3--xNi xGa (x=0,1,2) have a tetragonal unit cell that can variously be used for magneto-mechanically coupled shape memory ( x=1,2) and spin-mechanical applications (x=0). The first fabrication of fully epitaxial thin films of these and electronically related compounds by sputtering is discussed. Traditional and custom lab characterization of the magnetic and temperature driven multiferroic behavior is augmented by more detailed synchrotron-based high energy photoemission spectroscopic techniques to describe the atomic and electronic structure. Integration of the MnNi2Ga magnetic shape memory compound in microwave patch antennas and active free-standing structures represents a fraction of the available and promising applications for these compounds. Prototype magnetic tunnel junctions are demonstrated by Mn3Ga electrodes with perpendicular anisotropy for spin torque transfer memory structures. The main body of the work concentrates on the definition and exploration of the material series Mn3--xNi xGa (x=0,1,2) and the relevant multiferroic phenomena exhibited as a function of preparation and external stimuli. Engineering results on each x=0,1,2 are presented with device prototypes where relevant. In the appendices the process of the materials design undertaken with the goal of developing new ternary intermetallics with enhanced properties is presented with a full exploration of the road from band structure calculations to device implementation. Cobalt based compounds in single crystal and nanoparticle form are fabricated with an eye to developing the production methods for new cobalt- and iron-based magnetic shape memory compounds for device applications in different forms. Mn2CoSn, a compound isolectronic and with similar atomic ordering to Mn2NiGa is experimentally determined to be a nearly half-metallic ferromagnet in contrast to the metallic ferrimagnetism in the parent compound. High energy photoemission spectroscopy is shown to

  4. EDITORIAL: Cluster issue on Heusler compounds and devices Cluster issue on Heusler compounds and devices

    NASA Astrophysics Data System (ADS)

    Felser, Claudia; Hillebrands, Burkard

    2009-04-01

    This is the third cluster issue of Journal Physics D: Applied Physics devoted to half-metallic Heusler compounds and devices utilizing this class of materials. Heusler compounds are named after Fritz Heusler, the owner of a German copper mine, the Isabellenhütte, who discovered this class of materials in 1903 [1]. He synthesized mixtures of Cu2Mn alloys with various main group metals Z = Al, Si, Sn, Sb, which became ferromagnetic despite all constituents being non-magnetic. The recent success story of Heusler compounds began in 1983 with the discovery of the half-metallic electronic structure in NiMnSb [2] and Co2MnZ [3], making these and similar materials, in particular PtMnSb, also useful for magneto-optical data storage media applications due to their high Kerr rotation. The real breakthrough, however, came in 2000 with the observation of a large magnetoresistance effect in Co2Cr0.6Fe0.4Al [4]. The Co2YZ (Y = Ti, Cr, Mn, Fe) compounds are a special class of materials, which follow the Slater-Pauling rule [5], and most of them are half-metallic bulk materials. The electronic structure of Heusler compounds is well understood [6] and Curie temperatures up to 1100 K have been observed [7]. In their contribution to this cluster issue, Thoene et al predict that still higher Curie temperatures can be achieved. A breakthrough from the viewpoint of materials design is the synthesis of nanoparticles of Heusler compounds as reported in the contribution by Basit et al. Nano-sized half- metallic ferromagnets will open new directions for spintronic applications. The challenge, however, is still to produce spintronic devices with well defined interfaces to take advantage of the half-metallicity of the electrodes. Several groups have succeeded in producing excellent tunnel junctions with high magnetoresistance effects at low temperatures and decent values at room temperature [8-11]. Spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions with a

  5. Magnetism in tetragonal manganese-rich Heusler compounds

    NASA Astrophysics Data System (ADS)

    Wollmann, Lukas; Chadov, Stanislav; Kübler, Jürgen; Felser, Claudia

    2015-08-01

    A comprehensive study of the total energy of manganese-rich Heusler compounds using density functional theory is presented. Starting from a large set of cubic parent systems, the response to tetragonal distortions is studied in detail. We single out the systems that remain cubic from those that most likely become tetragonal. The driving force of the tetragonal distortion and its effect on the magnetic properties, especially where they deviate from the Slater-Pauling rule, as well as the trends in the Curie temperatures, are highlighted. By means of partial densities of states, the electronic structural changes reveal the microscopic origin of the observed trends. We focus our attention on the magnetocrystalline anisotropy and find astonishingly high values for tetragonal Heusler compounds containing heavy transition metals accompanied by low magnetic moments, which indicates that these materials are promising candidates for spin-transfer-torque magnetization-switching applications.

  6. Computational investigation of half-Heusler compounds for spintronics applications

    NASA Astrophysics Data System (ADS)

    Ma, Jianhua; Hegde, Vinay I.; Munira, Kamaram; Xie, Yunkun; Keshavarz, Sahar; Mildebrath, David T.; Wolverton, C.; Ghosh, Avik W.; Butler, W. H.

    2017-01-01

    We present first-principles density functional calculations of the electronic structure, magnetism, and structural stability of 378 XYZ half-Heusler compounds (with X = Cr, Mn, Fe, Co, Ni, Ru, Rh; Y = Ti, V, Cr, Mn, Fe, Ni; Z = Al, Ga, In, Si, Ge, Sn, P, As, Sb). We find that a "Slater-Pauling gap" in the density of states (i.e., a gap or pseudogap after nine states in the three atom primitive cell) in at least one spin channel is a common feature in half-Heusler compounds. We find that the presence of such a gap at the Fermi energy in one or both spin channels contributes significantly to the stability of a half-Heusler compound. We calculate the formation energy of each compound and systematically investigate its stability against all other phases in the open quantum materials database (OQMD). We represent the thermodynamic phase stability of each compound as its distance from the convex hull of stable phases in the respective chemical space and show that the hull distance of a compound is a good measure of the likelihood of its experimental synthesis. We find low formation energies and mostly correspondingly low hull distances for compounds with X = Co, Rh, or Ni, Y = Ti or V, and Z = P, As, Sb, or Si. We identify 26 18-electron semiconductors, 45 half-metals, and 34 near half-metals with negative formation energy that follow the Slater-Pauling rule of three electrons per atom. Our calculations predict several new, as-yet unknown, thermodynamically stable phases, which merit further experimental exploration—RuVAs, CoVGe, FeVAs in the half-Heusler structure, and NiScAs, RuVP, RhTiP in the orthorhombic MgSrSi-type structure. Further, two interesting zero-moment half-metals, CrMnAs and MnCrAs, are calculated to have negative formation energy. In addition, our calculations predict a number of hitherto unreported semiconducting (e.g., CoVSn and RhVGe), half-metallic (e.g., RhVSb), and near half-metallic (e.g., CoFeSb and CoVP) half-Heusler compounds to lie close to

  7. High-throughput screening for antiferromagnetic Heusler compounds using density functional theory

    NASA Astrophysics Data System (ADS)

    Balluff, Jan; Diekmann, Kevin; Reiss, Günter; Meinert, Markus

    2017-08-01

    Commonly used antiferromagnets contain expensive precious metals, which limits their applicability. Novel materials that are made of abundant elements are thus required for a large scale application, e.g., in spintronic devices. We propose a combinatorial, high-throughput approach based on density functional theory calculations to search for such new antiferromagnets. The power of the method is demonstrated by screening the ternary Heusler compounds for antiferromagnetic phases. We utilize the AFLOWLib, a computational materials database that contains over one million ternary phases. Among these we identify 291 potentially stable magnetic Heusler compounds. By explicitly checking for antiferromagnetic configurations we identify 70 antiferromagnetic Heusler compounds. Comparison with available experimental data shows that the method has excellent selectivity: all known antiferromagnetic Heusler compounds are correctly identified and no material is erroneously assigned an antiferromagnetic ground state. By calculating the Néel temperatures we predict 21 antiferromagnetic Heusler compounds with a Néel temperature above room temperature.

  8. Origin of the Tetragonal Ground State of Heusler Compounds

    NASA Astrophysics Data System (ADS)

    Faleev, Sergey V.; Ferrante, Yari; Jeong, Jaewoo; Samant, Mahesh G.; Jones, Barbara; Parkin, Stuart S. P.

    2017-03-01

    We describe the general mechanism of tetragonal distortion in Heusler compounds X2Y Z . From 286 compounds studied using density-functional theory, 62% are found to be tetragonal at zero temperature. Such a large share of compounds with tetragonal distortions can be explained by the peak-and-valley character of the density of states (DOS) of these compounds in the cubic phase (arising from localized d bands and van Hove singularities) in conjunction with a smooth shift of peaky DOS structure relative to the Fermi energy, EF, when valence electrons are added to the system. A shift of the DOS in the Y or Z series leads to an alternation of stable and nonstable cubic phases that depends on the value of the DOS at EF in the cubic phase. Groups of compounds with a large share of tetragonal distortions are identified and explained.

  9. Ordered Structures and Thermoelectric Properties of MNiSn (M = Ti, Zr, Hf)-Based Half-Heusler Compounds Affected by Close Relationship with Heusler Compounds

    NASA Astrophysics Data System (ADS)

    Kimura, Yoshisato; Chai, Yaw-Wang

    2015-01-01

    Half-Heusler compounds are excellent thermoelectric materials. A characteristic of the half-Heusler-type ordered structure is the vacancy site that occupies one-fourth of all the lattice points. Therefore, a half-Heusler ABX phase (where A and B are typically transition metal elements, such as Ti, Zr, and Hf, and X represents a half-metal element such as Sn or Sb) has a crystallographically close relationship with a Heusler AB2X phase in the sense that the vacancy site in the half-Heusler phase is filled with B atoms in the Heusler phase. The thermoelectric properties are improved or affected by point lattice defects related to the vacancy site and the B site, such as the antisite atom B in the vacancy site, vacancies in the B site, and vacancy-site occupancy by quaternary C atoms. A modulated-like nanostructure due to point defects regarding vacancies and Ni atoms is formed for an instance in ZrNiSn alloys even close to the stoichiometric composition. Ni-rich nanoclusters are locally formed by excessive Ni antisite atoms in the vacancy site, which work as precursors of Heusler precipitates (TiNi2Sn, ZrNi2Sn, and so forth). The vacancy-site occupation in ZrNiSn with Co and Ir results in the drastic conversion of thermoelectric properties from n type to p type, and the effective reduction of the lattice thermal conductivity.

  10. Spin-filter and spin-gapless semiconductors: The case of Heusler compounds

    SciTech Connect

    Galanakis, I.; Özdoğan, K.; Şaşıoğlu, E.

    2016-05-15

    We review our recent first-principles results on the inverse Heusler compounds and the ordered quaternary (also known as LiMgPdSn-type) Heusler compounds. Among these two subfamilies of the full-Heusler compounds, several have been shown to be magnetic semiconductors. Such material can find versatile applications, e.g. as spin-filter materials in magnetic tunnel junctions. Finally, a special case are the spin-gapless semiconductors, where the energy gap at the Fermi level for the one spin-direction is almost vanishing, offering novel functionalities in spintronic/magnetoelectronic devices.

  11. Influence of disorder on anomalous Hall effect for Heusler compounds

    NASA Astrophysics Data System (ADS)

    Vilanova Vidal, E.; Schneider, H.; Jakob, G.

    2011-05-01

    The anomalous Hall effect (AHE) is a long known but still not fully understood transport effect. Most theory papers focus on the influence of one particular contribution to the AHE. Actual measured experimental data, however, often are not in accord with idealized assumptions. In this work we discuss the data analysis for materials with low residual resistivity ratios. As prototypical materials we study half metallic Heusler compounds. Here the influence of defects and disorder is apparent in a material with a complex topology of the Fermi surface. Using films of different degree of disorder, we show how different scattering mechanisms can be separated. For Co2FeSi0.6Al0.4 and Co2FeGa0.5Ge0.5 the AHE induced by B2-type disorder and temperature-dependent scattering is positive, while DO3-type disorder and possible intrinsic contributions possess a negative sign.

  12. Modeling of half-Heusler compound NiMnSb within tight-binding approximation

    NASA Astrophysics Data System (ADS)

    Sugiyanto, Majidi, M. A.; Nanto, D.

    2017-07-01

    Heusler compounds are families of magnetic materials with general stoichiometry of either X2YZ (full-Heusler compound) or XYZ (half-Heusler compound), with X and Y being transition metal elements, and Z a main-group element. Their various potentials for technology development make them be still relevant as a subject of both experimental and theoretical studies. Half-Heusler compounds are generally crystallized in the C1b-type structure. The magnetic moments of such materials may be predicted using Slater-Pauling rule, giving m = (Nvalence electrons - 18)µB per formula unit. However, this simple counting rule does not always work for all compounds in this group. This motivates us to perform a theoretical study to investigate the mechanism of magnetic moment formation microscopically. As a case study, we focus on NiMnSb, a particular half-Heusler compound, for which comparison between existing experimental results and theoretical predictions of its magnetic moment has not yet been quite convincing. We model the system by constructing a tight-binding-based Hamiltonian, incorporating Hubbard repulsive as well as spin-spin interactions for the electrons occupying the d-orbitals. We solve the model using Green's function approach, and treat the interaction terms within the mean-field approximation. At this stage, we aim to formulate the computational algorithm for the overall calculation process. Our final goal is to compute the total magnetic moment per unit cell of this system and compare it with available experimental data.

  13. Extrinsic doping of the half-Heusler compounds

    NASA Astrophysics Data System (ADS)

    Stern, Robin; Dongre, Bonny; Madsen, Georg K. H.

    2016-08-01

    Controlling the p- and n-type doping is a key tool to improve the power-factor of thermoelectric materials. In the present work we provide a detailed understanding of the defect thermochemistry in half-Heusler compounds. We calculate the formation energies of intrinsic and extrinsic defects in state of the art n-type TiNiSn and p-type TiCoSb thermoelectric materials. It is shown how the incorporation of online repositories can reduce the workload in these calculations. In TiNiSn we find that Ni- and Ti-interstitial defects play a crucial role in the carrier concentration of TiNiSn. Furthermore, we find that extrinsic doping with Sb can substantially enhance the carrier concentration, in agreement with experiment. In case of TiCoSb, we find ScTi, FeCo and SnSb being possible p-type dopants. While experimental work has mainly focussed on Sn-doping of the Sb site, the present result underlines the possibility to p-dope TiCoSb on all lattice sites.

  14. Extrinsic doping of the half-Heusler compounds.

    PubMed

    Stern, Robin; Dongre, Bonny; Madsen, Georg K H

    2016-08-19

    Controlling the p- and n-type doping is a key tool to improve the power-factor of thermoelectric materials. In the present work we provide a detailed understanding of the defect thermochemistry in half-Heusler compounds. We calculate the formation energies of intrinsic and extrinsic defects in state of the art n-type TiNiSn and p-type TiCoSb thermoelectric materials. It is shown how the incorporation of online repositories can reduce the workload in these calculations. In TiNiSn we find that Ni- and Ti-interstitial defects play a crucial role in the carrier concentration of TiNiSn. Furthermore, we find that extrinsic doping with Sb can substantially enhance the carrier concentration, in agreement with experiment. In case of TiCoSb, we find ScTi, FeCo and SnSb being possible p-type dopants. While experimental work has mainly focussed on Sn-doping of the Sb site, the present result underlines the possibility to p-dope TiCoSb on all lattice sites.

  15. Titanium nitride as a seed layer for Heusler compounds

    SciTech Connect

    Niesen, Alessia Glas, Manuel; Ludwig, Jana; Schmalhorst, Jan-Michael; Reiss, Günter; Sahoo, Roshnee; Ebke, Daniel; Arenholz, Elke

    2015-12-28

    Titanium nitride (TiN) shows low resistivity at room temperature (27 μΩ cm), high thermal stability and thus has the potential to serve as seed layer in magnetic tunnel junctions. High quality TiN thin films with regard to the crystallographic and electrical properties were grown and characterized by x-ray diffraction and 4-terminal transport measurements. Element specific x-ray absorption spectroscopy revealed pure TiN inside the thin films. To investigate the influence of a TiN seed layer on a ferro(i)magnetic bottom electrode in magnetic tunnel junctions, an out-of-plane magnetized Mn{sub 2.45}Ga as well as in- and out-of-plane magnetized Co{sub 2}FeAl thin films were deposited on a TiN buffer, respectively. The magnetic properties were investigated using a superconducting quantum interference device and anomalous Hall effect for Mn{sub 2.45}Ga. Magneto optical Kerr effect measurements were carried out to investigate the magnetic properties of Co{sub 2}FeAl. TiN buffered Mn{sub 2.45}Ga thin films showed higher coercivity and squareness ratio compared to unbuffered samples. The Heusler compound Co{sub 2}FeAl showed already good crystallinity when grown at room temperature on a TiN seed-layer.

  16. LaPtSb: a half-Heusler compound with high thermoelectric performance.

    PubMed

    Xue, Q Y; Liu, H J; Fan, D D; Cheng, L; Zhao, B Y; Shi, J

    2016-07-21

    The electronic and transport properties of the half-Heusler compound LaPtSb are investigated by performing first-principles calculations combined with semi-classical Boltzmann theory and deformation potential theory. Compared with many typical half-Heusler compounds, LaPtSb exhibits an obviously larger power factor at room temperature, especially for the n-type system. Together with the very low lattice thermal conductivity, the thermoelectric figure of merit (ZT) of LaPtSb can be optimized to a record high value of 2.2 by fine tuning the carrier concentration.

  17. LaPtSb: a half-Heusler compound with high thermoelectric performance

    NASA Astrophysics Data System (ADS)

    Xue, Q. Y.; Liu, H. J.; Fan, D. D.; Cheng, L.; Zhao, B. Y.; Shi, J.

    The electronic and transport properties of the half-Heusler compound LaPtSb are investigated by performing first-principles calculations combined with semi-classical Boltzmann theory and deformation potential theory. Compared with many typical half-Heusler compounds, the LaPtSb exhibits obviously larger power factor at room temperature, especially for the n-type system. Together with the very low lattice thermal conductivity, the thermoelectric figure of merit (ZT) of LaPtSb can be optimized to a record high value of 2.2 by fine tuning the carrier concentration.

  18. Evidence for localized moment picture in Mn-based Heusler compounds.

    PubMed

    Karel, J; Bernardi, F; Wang, C; Stinshoff, R; Born, N-O; Ouardi, S; Burkhardt, U; Fecher, G H; Felser, C

    2015-12-21

    X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) were used to probe the electronic structure and magnetic moment of Mn in Heusler compounds with different crystallographic structure. The results were compared with theoretical calculations of the magnetic and electronic properties, and it was found that in full and half Heusler alloys, Mn is metallic on both sublattices. The magnetic moment is large and localized when octahedrally coordinated by the main group element, consistent with previous theoretical work, and reduced when the main group coordination is tetrahedral. The magnetic and electronic properties of Mn in full and half Heusler compounds are strongly dependent on the structure and sublattice, a fact that can be exploited to design new materials.

  19. Ultrafast laser induced local magnetization dynamics in Heusler compounds

    NASA Astrophysics Data System (ADS)

    Elliott, P.; Müller, T.; Dewhurst, J. K.; Sharma, S.; Gross, E. K. U.

    2016-12-01

    The overarching goal of the field of femtomagnetism is to control, via laser light, the magnetic structure of matter on a femtosecond time scale. The temporal limits to the light-magnetism interaction are governed by the fact that the electron spin interacts indirectly with light, with current studies showing a laser induced global loss in the magnetic moment on a time scale of the order of a few 100 s of femtoseconds. In this work, by means of ab-initio calculations, we show that more complex magnetic materials - we use the example of the Heusler and half-Heusler alloys - allow for purely optical excitations to cause a significant change in the local moments on the order of 5 fs. This, being purely optical in nature, represents the ultimate mechanism for the short time scale manipulation of spins. Furthermore, we demonstrate that qualitative behaviour of this rich magnetic response to laser light can be deduced from the ground-state spectrum, thus providing a route to tailoring the response of some complex magnetic materials, like the Heuslers, to laser light by the well established methods for material design from ground-state calculations.

  20. Ultrafast laser induced local magnetization dynamics in Heusler compounds

    PubMed Central

    Elliott, P.; Müller, T.; Dewhurst, J. K.; Sharma, S.; Gross, E. K. U.

    2016-01-01

    The overarching goal of the field of femtomagnetism is to control, via laser light, the magnetic structure of matter on a femtosecond time scale. The temporal limits to the light-magnetism interaction are governed by the fact that the electron spin interacts indirectly with light, with current studies showing a laser induced global loss in the magnetic moment on a time scale of the order of a few 100 s of femtoseconds. In this work, by means of ab-initio calculations, we show that more complex magnetic materials - we use the example of the Heusler and half-Heusler alloys - allow for purely optical excitations to cause a significant change in the local moments on the order of 5 fs. This, being purely optical in nature, represents the ultimate mechanism for the short time scale manipulation of spins. Furthermore, we demonstrate that qualitative behaviour of this rich magnetic response to laser light can be deduced from the ground-state spectrum, thus providing a route to tailoring the response of some complex magnetic materials, like the Heuslers, to laser light by the well established methods for material design from ground-state calculations. PMID:27966585

  1. Ultrafast laser induced local magnetization dynamics in Heusler compounds.

    PubMed

    Elliott, P; Müller, T; Dewhurst, J K; Sharma, S; Gross, E K U

    2016-12-14

    The overarching goal of the field of femtomagnetism is to control, via laser light, the magnetic structure of matter on a femtosecond time scale. The temporal limits to the light-magnetism interaction are governed by the fact that the electron spin interacts indirectly with light, with current studies showing a laser induced global loss in the magnetic moment on a time scale of the order of a few 100 s of femtoseconds. In this work, by means of ab-initio calculations, we show that more complex magnetic materials - we use the example of the Heusler and half-Heusler alloys - allow for purely optical excitations to cause a significant change in the local moments on the order of 5 fs. This, being purely optical in nature, represents the ultimate mechanism for the short time scale manipulation of spins. Furthermore, we demonstrate that qualitative behaviour of this rich magnetic response to laser light can be deduced from the ground-state spectrum, thus providing a route to tailoring the response of some complex magnetic materials, like the Heuslers, to laser light by the well established methods for material design from ground-state calculations.

  2. Half-Heusler ternary compounds as new multifunctional experimental platforms for topological quantum phenomena.

    PubMed

    Lin, Hsin; Wray, L Andrew; Xia, Yuqi; Xu, Suyang; Jia, Shuang; Cava, Robert J; Bansil, Arun; Hasan, M Zahid

    2010-07-01

    Recent discovery of spin-polarized single-Dirac-cone insulators, whose variants can host magnetism and superconductivity, has generated widespread research activity in condensed-matter and materials-physics communities. Some of the most interesting topological phenomena, however, require topological insulators to be placed in multiply connected, highly constrained geometries with magnets and superconductors, all of which thus require a large number of functional variants with materials design flexibility as well as electronic, magnetic and superconducting tunability. Given the optimum materials, topological properties open up new vistas in spintronics, quantum computing and fundamental physics. We have extended the search for topological insulators from the binary Bi-based series to the ternary thermoelectric Heusler compounds. Here we show that, although a large majority of the well-known Heuslers such as TiNiSn and LuNiBi are rather topologically trivial, the distorted LnPtSb-type (such as LnPtBi or LnPdBi, Ln = f(n) lanthanides) compounds belonging to the half-Heusler subclass harbour Z(2) = -1 topological insulator parent states, where Z(2) is the band purity product index. Our results suggest that half-Heuslers provide a new platform for deriving a host of topologically exotic compounds and their nanoscale or thin-film device versions through the inherent flexibility of their lattice parameter, spin-orbit strength and magnetic moment tunability paving the way for the realization of multifunctional topological devices.

  3. Electronic structure and thermoelectric property of Co2YGe (Y=Mn, Fe) Heusler compounds: a first principle study

    NASA Astrophysics Data System (ADS)

    Joshi, Himanshu; Rai, D. P.; Sandeep; Thapa, R. K.

    2016-10-01

    The electronic and thermoelectric properties of Co2YGe (Y=Mn, Fe) Heusler compounds have been studied by first principle density functional theory and compared with the known experimental and theoretical results. Results of the density of states (DOS) and band structures shows the half-metallicity of the Heusler alloy Co2MnGe, whereas the Heusler alloy Co2FeGe fails to give half-metallicity when treated with GGA. The ZT value calculated for these materials is much below the benchmark value 1.

  4. Half-Heusler Compounds as a New Class of Three-Dimensional Topological Insulators

    NASA Astrophysics Data System (ADS)

    Xiao, Di; Yao, Yugui; Feng, Wanxiang; Wen, Jun; Zhu, Wenguang; Chen, Xing-Qiu; Stocks, G. Malcolm; Zhang, Zhenyu

    2010-08-01

    Using first-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of three-dimensional topological insulators (3DTI). We demonstrate that the electronic structure of unstrained LaPtBi as a prototype system exhibits a distinct band-inversion feature. The 3DTI phase is realized by applying a uniaxial strain along the [001] direction, which opens a band gap while preserving the inverted band order. A definitive proof of the strained LaPtBi as a 3DTI is provided by directly calculating the topological Z2 invariants in systems without inversion symmetry. We discuss the implications of the present study to other half-Heusler compounds as 3DTI, which, together with the magnetic and superconducting properties of these materials, may provide a rich platform for novel quantum phenomena.

  5. Half-Heusler Compounds as a New Class of Three-Dimensional Topological Insulators

    SciTech Connect

    Xiao, Di; Yao, yugui; Feng, wanxiang; Wen, Jun; Zhu, Wenguang; Chen, Xingqiu; Stocks, George Malcolm; Zhang, Zhenyu

    2010-01-01

    Using rst-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of three-dimensional topological insu- lators (3DTI). We demonstrate that the electronic structure of unstrained LaPtBi as a prototype system exhibits distinct band-inversion feature. The 3DTI phase is realized by applying a uni- axial strain along the [001] direction, which opens a bandgap while preserving the inverted band order. A denitive proof of the strained LaPtBi as a 3DTI is provided by directly calculating the topological Z2 invariants in systems without inversion symmetry. We discuss the implications of the present study to other half-Heusler compounds as 3DTI, which, together with the magnetic and superconducting properties of these materials, may provide a rich platform for novel quantum phenomena.

  6. Half-Heusler compounds as a new class of three-dimensional topological insulators.

    PubMed

    Xiao, Di; Yao, Yugui; Feng, Wanxiang; Wen, Jun; Zhu, Wenguang; Chen, Xing-Qiu; Stocks, G Malcolm; Zhang, Zhenyu

    2010-08-27

    Using first-principles calculations within density functional theory, we explore the feasibility of converting ternary half-Heusler compounds into a new class of three-dimensional topological insulators (3DTI). We demonstrate that the electronic structure of unstrained LaPtBi as a prototype system exhibits a distinct band-inversion feature. The 3DTI phase is realized by applying a uniaxial strain along the [001] direction, which opens a band gap while preserving the inverted band order. A definitive proof of the strained LaPtBi as a 3DTI is provided by directly calculating the topological Z2 invariants in systems without inversion symmetry. We discuss the implications of the present study to other half-Heusler compounds as 3DTI, which, together with the magnetic and superconducting properties of these materials, may provide a rich platform for novel quantum phenomena.

  7. Tunable damping in the Heusler compound Co2 -xIrxMnSi

    NASA Astrophysics Data System (ADS)

    Köhler, Albrecht; Wollmann, Lukas; Ebke, Daniel; Chadov, Stanislav; Kaiser, Christian; Diao, Zhitao; Zheng, Yuankai; Leng, Qunwen; Felser, Claudia

    2016-03-01

    Here we report on the realization of tuning the intrinsic damping in the half-metallic Heusler compound Co2MnSi by substituting Co by Ir. The work includes theoretical calculations and experimental measurements on bulk and thin films samples. Control of damping is to remove unwanted magnetization motion and suppress signal echoes through uncontrolled precession of the magnetization for future implementation of this material into, e.g., current perpendicular plane-giant-magnetoresistance sensors. Density functional calculations revealed stable magnetization and increasing damping parameter with Iridium concentration, whereas the half metallicity could be retained. The calculations are consistent with experimental results from bulk and thin film samples of this report and elucidate the linear dependence of the Gilbert damping parameter on the substituent concentration. This report again demonstrates the inherent tunability of Heusler compounds, which constitutes a pivotal feature of this material class.

  8. Multilayer heterostructures of magnetic Heusler and binary compounds from first principles

    NASA Astrophysics Data System (ADS)

    Garoufalis, Christos; Galanakis, Iosif

    2016-03-01

    Employing first-principles state-of-the-art electronic structure calculations, we study a series of multilayer heterostructures composed of ferro/ferrimagnetic half-metallic Heusler compounds and binary compounds presenting perpendicular magnetic anisotropy. We relax these heterostructures and study both their electronic and magnetic properties. In most studied cases the Heusler spacer keeps a large value of spin-polarization at the Fermi level even for ultrathin films which attends the maximum value of 100% in the case of the Mn2VSi/MnSi multilayer. Our results pave the way both experimentally and theoretically towards the growth of such multilayer heterostructures and their incorporation in spintronic/magnetoelectronic devices.

  9. Recent Advances in Nanostructured Thermoelectric Half-Heusler Compounds

    PubMed Central

    Xie, Wenjie; Weidenkaff, Anke; Tang, Xinfeng; Zhang, Qingjie; Poon, Joseph; Tritt, Terry M.

    2012-01-01

    Half-Heusler (HH) alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In this article, we briefly review the recent progress and advances in these HH nanocomposites. We begin by presenting the structure of HH alloys and the different strategies that have been utilized for improving the TE properties of HH alloys. Next, we review the details of HH nanocomposites as obtained by different techniques. Finally, the review closes by highlighting several promising strategies for further research directions in these very promising TE materials.

  10. Highly-dispersive spin gapless semiconductors in rare-earth-element contained quaternary Heusler compounds

    NASA Astrophysics Data System (ADS)

    Xu, Guizhou; You, Yurong; Gong, Yuanyuan; Liu, Er; Xu, Feng; Wang, Wenhong

    2017-03-01

    The acquisition of high mobility electrons in the zero-gap band of spin gapless semiconductors is crucial for their practical applications in spintronic devices. In this work, we propose to design a higher dispersive band by importing the rare-earth atom into the Heusler compounds. With first principles calculations, we identify several new spin gapless semiconductor candidates in the 21-electron LiMgPdSn-type quaternary Heusler alloys of (Y, La, Lu)CoCr/FeMn(Al, Ga). Densities of states for most of them reveal large band gaps in the minority spin direction, and relatively low states near the Fermi level in the majority spin. According to the electron projected band analysis, we find the import of the rare earth atom can enhance the sp component in the band across the Fermi level, which is conducive to form a linear-dispersive band that is promising to enhance the carrier mobility of spin gapless semiconductors.

  11. EDITORIAL: New materials with high spin polarization: half-metallic Heusler compounds

    NASA Astrophysics Data System (ADS)

    Felser, Claudia; Hillebrands, Burkard

    2007-03-01

    The development of magnetic Heusler compounds, specifically designed as materials for spintronic applications, has made tremendous progress in the very recent past [1-21]. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% [1]. These materials are exceptionally well suited for applications in magnetic tunnel junctions acting, for example, as sensors for magnetic fields. The tunnelling magneto-resistance (TMR) effect is the relative change in the electrical resistance upon application of a small magnetic field. Tunnel junctions with a TMR effect of 580% at 4 K were reported by the group of Miyazaki and Ando [1], consisting of two Co2MnSi Heusler electrodes. High Curie temperatures were found in Co2 Heusler compounds with values up to 1120 K in Co2FeSi [2]. The latest results are for a TMR device made from the Co2FeAl0.5Si0.5 Heusler compound and working at room temperature with a TMR effect of 174% [3]. The first significant magneto-resistance effect was discovered in Co2Cr0.6Fe0.4Al (CCFA) in Mainz [4]. With the classical Heusler compound CCFA as one electrode, the record TMR effect at 4 K is 240% [5]. Positive and negative TMR values at room temperature utilizing magnetic tunnel junctions with one Heusler compound electrode render magnetic logic possible [6]. Research efforts exist, in particular, in Japan and in Germany. The status of research as of winter 2005 was compiled in a recent special volume of Journal of Physics D: Applied Physics [7-20]. Since then specific progress has been made on the issues of (i) new advanced Heusler materials, (ii) advanced characterization, and (iii) advanced devices using the new materials. In Germany, the Mainz and Kaiserslautern based Research Unit 559 `New Materials with High Spin Polarization', funded since 2004 by the Deutsche Forschungsgemeinschaft, is a basic science approach to Heusler compounds, and it addresses the first two topics in particular

  12. Observation of unusual topological surface states in half-Heusler compounds LnPtBi (Ln=Lu, Y).

    PubMed

    Liu, Z K; Yang, L X; Wu, S-C; Shekhar, C; Jiang, J; Yang, H F; Zhang, Y; Mo, S-K; Hussain, Z; Yan, B; Felser, C; Chen, Y L

    2016-09-27

    Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.

  13. Observation of unusual topological surface states in half-Heusler compounds LnPtBi (Ln=Lu, Y)

    SciTech Connect

    Liu, Z. K.; Yang, L. X.; Wu, S. -C.; Shekhar, C.; Jiang, J.; Yang, H. F.; Zhang, Y.; Mo, S. -K.; Hussain, Z.; Yan, B.; Felser, C.; Chen, Y. L.

    2016-09-27

    Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.

  14. Observation of unusual topological surface states in half-Heusler compounds LnPtBi (Ln=Lu, Y)

    NASA Astrophysics Data System (ADS)

    Liu, Z. K.; Yang, L. X.; Wu, S.-C.; Shekhar, C.; Jiang, J.; Yang, H. F.; Zhang, Y.; Mo, S.-K.; Hussain, Z.; Yan, B.; Felser, C.; Chen, Y. L.

    2016-09-01

    Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.

  15. Observation of unusual topological surface states in half-Heusler compounds LnPtBi (Ln=Lu, Y)

    PubMed Central

    Liu, Z. K.; Yang, L. X.; Wu, S.-C.; Shekhar, C.; Jiang, J.; Yang, H. F.; Zhang, Y.; Mo, S.-K.; Hussain, Z.; Yan, B.; Felser, C.; Chen, Y. L.

    2016-01-01

    Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors. PMID:27671444

  16. Observation of unusual topological surface states in half-Heusler compounds LnPtBi (Ln=Lu, Y)

    DOE PAGES

    Liu, Z. K.; Yang, L. X.; Wu, S. -C.; ...

    2016-09-27

    Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observe the unusual topological surface states onmore » these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.« less

  17. Unified explanation of chemical ordering, the Slater-Pauling rule, and half-metallicity in full Heusler compounds

    NASA Astrophysics Data System (ADS)

    Faleev, Sergey V.; Ferrante, Yari; Jeong, Jaewoo; Samant, Mahesh G.; Jones, Barbara; Parkin, Stuart S. P.

    2017-01-01

    In the present work we developed an orbital coupling model for cubic full Heusler compounds that provides a unified set of rules that account for the chemical ordering, magnetic moment, and composition of the most promising candidates for half-metallicity. The origin and limitations of the rules are clearly described. To the best of our knowledge all of the several dozen half-metallic Heusler compounds known in the literature that follow the Mt=Nt-24 or Mt=Nt-28 generalized Slater-Pauling behavior satisfy the derived half-metallicity rule. Calculations performed by using density functional theory—performed for 259 compounds—confirm the validity of our model and derived rules for broad classes of Heusler compounds.

  18. Electronic and Piezoelectric properties of half-Heusler compounds: A first principles study

    NASA Astrophysics Data System (ADS)

    Rai, D. P.; Sandeep; Shankar, A.; Aly, Abeer E.; Patra, P. K.; Thapa, R. K.

    2016-10-01

    We have investigated the semiconducting and piezoelectric properties of bulk MNiSn (M=Ti, Zr, Hf) type a half-Heusler compound with cubic F-43m symmetry by means of density functional theory (DFT). For electron exchange correlation a generalized gradient approximation (GGA) was used. Special attention was paid to establish a most favourble ground state configuration on magnetic as well as non-magnetic ordering. With fully optimized structure the electronic and ferroelectric calculation was performed. The formation of band gap was discussed on the basis of d-d orbital hybridization. Further we have calculated the spontaneous polarization by means of structural deformation.

  19. Weyl points in the ferromagnetic Heusler compound Co2MnAl

    NASA Astrophysics Data System (ADS)

    Kübler, J.; Felser, C.

    2016-05-01

    The anomalous Hall conductivity (AHC) in some ferromagnetic and antiferromagnetic Heusler compounds was theoretically and experimentally found to be exceptionally large. For the case of ferromagnetic Co2MnAl we here argue that the large AHC is connected with the appearance of Weyl points near the Fermi energy. We find four Weyl points slightly above the Fermi edge. We describe our analysis for a magnetization being in the (110)-direction. For the possible (100)-direction we find at least four Weyl points, too. We predict that Co2MnGa also possesses Weyl points near or at the Fermi energy.

  20. NMR Evidence for the Topologically Nontrivial Nature in a Family of Half-Heusler Compounds

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaoming; Hou, Zhipeng; Wang, Yue; Xu, Guizhou; Shi, Chenglong; Liu, Enke; Xi, Xuekui; Wang, Wenhong; Wu, Guangheng; Zhang, Xi-Xiang

    2016-03-01

    Spin-orbit coupling (SOC) is expected to partly determine the topologically nontrivial electronic structure of heavy half-Heusler ternary compounds. However, to date, attempts to experimentally observe either the strength of SOC or how it modifies the bulk band structure have been unsuccessful. By using bulk-sensitive nuclear magnetic resonance (NMR) spectroscopy combined with first-principles calculations, we reveal that 209Bi NMR isotropic shifts scale with relativity in terms of the strength of SOC and average atomic numbers, indicating strong relativistic effects on NMR parameters. According to first-principles calculations, we further claim that nuclear magnetic shieldings from relativistic p1/2 states and paramagnetic contributions from low-lying unoccupied p3/2 states are both sensitive to the details of band structures tuned by relativity, which explains why the hidden relativistic effects on band structure can be revealed by 209Bi NMR isotropic shifts in topologically nontrivial half-Heusler compounds. Used in complement to surface-sensitive methods, such as angle resolved photon electron spectroscopy and scanning tunneling spectroscopy, NMR can provide valuable information on bulk electronic states.

  1. Electronic band structure, doping, and defects in the semiconducting Half Heusler compound CoTiSb

    NASA Astrophysics Data System (ADS)

    Kawasaki, Jason; Johansson, Linda; Hjort, Martin; Timm, Rainer; Schultz, Brian; Balasubramanian, Thiagarajan; Mikkelsen, Anders; Palmstrom, Chris

    2013-03-01

    We report transport and electronic band structure measurements on epitaxial films of the Half Heusler compound CoTiSb. CoTiSb belongs to the family of Half Heuslers with 18 valence electrons per formula unit that are predicted to be semiconducting despite being composed of all metallic components. Here the CoTiSb films were grown by molecular beam epitaxy on a lattice matched InAlAs buffer. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction and X-ray diffraction. Scanning tunnelling spectroscopy and temperature-dependent transport measurements reveal that the films are semiconducting, with unintentionally doped carrier concentrations comparable to that of highly doped conventional compound semiconductors. These carrier concentrations can be modulated by doping with Sn. The band structure of the films was measured by angle resolved photoemission spectroscopy at the MAX-Lab Synchrotron facility. The bulk bands are in general agreement with density functional theory calculations, with a valence band maximum at Γ and surface states within the bulk band gap. The effects of defects are explored in order to explain the ARPES results. This work was supported by the ARO, AFOSR, ONR, and NSF.

  2. NMR Evidence for the Topologically Nontrivial Nature in a Family of Half-Heusler Compounds

    PubMed Central

    Zhang, Xiaoming; Hou, Zhipeng; Wang, Yue; Xu, Guizhou; Shi, Chenglong; Liu, EnKe; Xi, Xuekui; Wang, Wenhong; Wu, Guangheng; Zhang, Xi-xiang

    2016-01-01

    Spin-orbit coupling (SOC) is expected to partly determine the topologically nontrivial electronic structure of heavy half-Heusler ternary compounds. However, to date, attempts to experimentally observe either the strength of SOC or how it modifies the bulk band structure have been unsuccessful. By using bulk-sensitive nuclear magnetic resonance (NMR) spectroscopy combined with first-principles calculations, we reveal that 209Bi NMR isotropic shifts scale with relativity in terms of the strength of SOC and average atomic numbers, indicating strong relativistic effects on NMR parameters. According to first-principles calculations, we further claim that nuclear magnetic shieldings from relativistic p1/2 states and paramagnetic contributions from low-lying unoccupied p3/2 states are both sensitive to the details of band structures tuned by relativity, which explains why the hidden relativistic effects on band structure can be revealed by 209Bi NMR isotropic shifts in topologically nontrivial half-Heusler compounds. Used in complement to surface-sensitive methods, such as angle resolved photon electron spectroscopy and scanning tunneling spectroscopy, NMR can provide valuable information on bulk electronic states. PMID:26980406

  3. Magnetic properties and interfacial characteristics of all-epitaxial Heusler-compound stacking structures

    NASA Astrophysics Data System (ADS)

    Yamada, S.; Honda, S.; Hirayama, J.; Kawano, M.; Santo, K.; Tanikawa, K.; Kanashima, T.; Itoh, H.; Hamaya, K.

    2016-09-01

    We study magnetic properties and interfacial characteristics of all-epitaxial D 03-Fe3Si /L 21 - Fe3 -xMnxSi /L 21-Co2FeSi Heusler-compound trilayers grown on Ge(111) by room-temperature molecular beam epitaxy. We find that the magnetization reversal processes can be intentionally designed by changing the chemical composition of the intermediate Fe3 -xMnxSi layers because of their tunable ferromagnetic-paramagnetic phase-transition temperature. From first-principles calculations, interfacial half metallicity in the Co2FeSi layer is nearly expected when the sequence of stacking layers along <111 > of the Fe2MnSi /Co2FeSi interface includes the atomic row of L 21 - or B 2 -ordered structures. We believe that Co2FeSi /Fe2MnSi /Co2FeSi trilayer systems stacked along <111 > will open a new avenue for high-performance current-perpendicular-to-plane giant magnetoresistive devices with Heusler compounds.

  4. NMR Evidence for the Topologically Nontrivial Nature in a Family of Half-Heusler Compounds.

    PubMed

    Zhang, Xiaoming; Hou, Zhipeng; Wang, Yue; Xu, Guizhou; Shi, Chenglong; Liu, EnKe; Xi, Xuekui; Wang, Wenhong; Wu, Guangheng; Zhang, Xi-Xiang

    2016-03-16

    Spin-orbit coupling (SOC) is expected to partly determine the topologically nontrivial electronic structure of heavy half-Heusler ternary compounds. However, to date, attempts to experimentally observe either the strength of SOC or how it modifies the bulk band structure have been unsuccessful. By using bulk-sensitive nuclear magnetic resonance (NMR) spectroscopy combined with first-principles calculations, we reveal that (209)Bi NMR isotropic shifts scale with relativity in terms of the strength of SOC and average atomic numbers, indicating strong relativistic effects on NMR parameters. According to first-principles calculations, we further claim that nuclear magnetic shieldings from relativistic p1/2 states and paramagnetic contributions from low-lying unoccupied p3/2 states are both sensitive to the details of band structures tuned by relativity, which explains why the hidden relativistic effects on band structure can be revealed by (209)Bi NMR isotropic shifts in topologically nontrivial half-Heusler compounds. Used in complement to surface-sensitive methods, such as angle resolved photon electron spectroscopy and scanning tunneling spectroscopy, NMR can provide valuable information on bulk electronic states.

  5. Long-term stability of phase-separated half-Heusler compounds.

    PubMed

    Krez, J; Balke, B; Ouardi, S; Selle, S; Höche, T; Felser, C; Hermes, W; Schwind, M

    2015-11-28

    Half-Heusler (HH) compounds have shown high figure of merit up to 1.5. Here, we address the long-term stability of n- and p-type HH materials. For this purpose, we investigated HH materials based on the Ti0.3Zr0.35Hf0.35NiSn-system after 500 cycles (1700 h) from 373 to 873 K. Both compounds exhibit a maximum Seebeck coefficient of |α|≈ 210 μV K(-1) and a phase separation into two HH phases. The dendritic microstructure is temperature resistant and upon cycling the changes in the microstructure are so marginal that the low thermal conductivity values (κ < 4 W m(-1) K(-1)) could be maintained. Our results emphasize that phase-separated HH compounds are suitable low cost materials and can lead to enhanced thermoelectric efficiencies beyond the set benchmark for industrial applications.

  6. Anomalous Hall effect and current spin polarization in Co2Fe X Heusler compounds (X =Al , Ga , In , Si , Ge , and Sn ): A systematic ab initio study

    NASA Astrophysics Data System (ADS)

    Huang, Hung-Lung; Tung, Jen-Chuan; Guo, Guang-Yu

    2015-04-01

    Co-based Heusler compounds are ferromagnetic with a high Curie temperature and a large magnetization density, and thus are promising for spintronic applications. In this paper, we perform a systematic ab initio study of two principal spin-related phenomena, namely, anomalous Hall effect and current spin polarization, in Co2-based Heusler compounds Co2Fe X (X =Al , Ga , In , Si , Ge , Sn ) in the cubic L2 1 structure within the density functional theory with the generalized gradient approximation (GGA). The accurate all-electron full-potential linearized augmented plane-wave method is used. First, we find that the spin polarization of the longitudinal current (PL) in Co2Fe X (X =Al , Ga , In , Al0.5Si0.5 , and Sn ) is ˜100 % even though that of the electronic states at the Fermi level (PD) is not. Further, the other compounds also have a high current spin polarization with PL>85 %. This indicates that all the Co2Fe X compounds considered are promising for spin-transport devices. Interestingly, PD is negative in Co2Fe X (X =Si , Ge , and Sn ), differing in sign from the PL as well as that from the transport experiments. Second, the calculated anomalous Hall conductivities (AHCs) are moderate, being within 200 S/cm, and agree well with the available experiments on a highly L2 1 ordered Co2FeSi specimen although they differ significantly from the reported experiments on other compounds where the B2 antisite disorders were present. Surprisingly, the AHC in Co2FeSi decreases and then changes sign when Si is replaced by Ge and finally by Sn. Third, the calculated total magnetic moments agree well with the corresponding experimental ones in all the studied compounds except Co2FeSi where a difference of 0.3 μB/f .u . exists. We also perform the GGA plus on-site Coulomb interaction U calculations in the GGA + U scheme. We find that including the U affects the calculated total magnetic moment, spin polarization and AHC significantly, and in most cases, unfortunately

  7. Progress Report 2011: Understanding compound phase transitions in Heusler alloy giant magnetocaloric materials

    SciTech Connect

    Stadler, Shane

    2011-12-13

    Our goal is to gain insight into the fundamental physics that is responsible for magnetocaloric effects (MCE) and related properties at the atomic level. We are currently conducting a systematic study on the effects of atomic substitutions in Ni2MnGa-based alloys, and also exploring related full- and half-Heusler alloys, for example Ni-Mn-X (X=In, Sn, Sb), that exhibit a wide variety of interesting and potentially useful physical phenomena. It is already known that the magnetocaloric effect in the Heusler alloys is fundamentally connected to other interesting phenomena such as shape-memory properties. And the large magnetic entropy change in Ni2Mn0.75Cu0.25Ga has been attributed to the coupling of the first-order, martensitic transition with the second-order ferromagnetic paramagnetic (FM-PM) transition. Our research to this point has focused on understanding the fundamental physics at the origin of these complex, compound phase transitions, and the novel properties that emerge. We synthesize the materials using a variety of techniques, and explore their material properties through structural, magnetic, transport, and thermo-magnetic measurements.

  8. Direct observation of half-metallicity in the Heusler compound Co2MnSi.

    PubMed

    Jourdan, M; Minár, J; Braun, J; Kronenberg, A; Chadov, S; Balke, B; Gloskovskii, A; Kolbe, M; Elmers, H J; Schönhense, G; Ebert, H; Felser, C; Kläui, M

    2014-05-30

    Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of (93(-11)(+7)) % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure.

  9. Direct observation of half-metallicity in the Heusler compound Co2MnSi

    PubMed Central

    Jourdan, M.; Minár, J.; Braun, J.; Kronenberg, A.; Chadov, S.; Balke, B.; Gloskovskii, A.; Kolbe, M.; Elmers, H.J.; Schönhense, G.; Ebert, H.; Felser, C.; Kläui, M.

    2014-01-01

    Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of () % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure. PMID:24875774

  10. Rapid Microwave Preparation of Thermoelectric TiNiSn and TiCoSb Half-Heusler Compounds

    SciTech Connect

    Birkel, Christina S.; Zeier, Wolfgang G.; Douglas, Jason E.; Lettiere, Bethany R.; Mills, Carolyn E.; Seward, Gareth; Birkel, Alexander; Snedaker, Matthew L.; Zhang, Yichi; Snyder, G. Jeffrey; Pollock, Tresa M.; Seshadri, Ram; Stucky, Galen D.

    2012-10-25

    The 18-electron ternary intermetallic systems TiNiSn and TiCoSb are promising for applications as high-temperature thermoelectrics and comprise earth-abundant, and relatively nontoxic elements. Heusler and half-Heusler compounds are usually prepared by conventional solid state methods involving arc-melting and annealing at high temperatures for an extended period of time. Here, we report an energy-saving preparation route using a domestic microwave oven, reducing the reaction time significantly from more than a week to one minute. A microwave susceptor material rapidly heats the elemental starting materials inside an evacuated quartz tube resulting in near single phase compounds. The initial preparation is followed by a densification step involving hot-pressing, which reduces the amount of secondary phases, as verified by synchrotron X-ray diffraction, leading to the desired half-Heusler compounds, demonstrating that hot-pressing should be treated as part of the preparative process. For TiNiSn, high thermoelectric power factors of 2 mW/mK{sup 2} at temperatures in the 700 to 800 K range, and zT values of around 0.4 are found, with the microwave-prepared sample displaying somewhat superior properties to conventionally prepared half-Heuslers due to lower thermal conductivity. The TiCoSb sample shows a lower thermoelectric figure of merit when prepared using microwave methods because of a metallic second phase.

  11. Influence of main-group element on half-metallic properties in half-Heusler compound

    NASA Astrophysics Data System (ADS)

    Liu, Hongyan; Li, Yushan; Tian, Fuyang; Li, Getian

    2016-04-01

    We investigate the band structure, magnetism and density of states of half-Heusler compounds CoCrZ (Z = Si,Ge,P,As) based on the first-principle calculations. Combined with molecular orbital hybridization theory, we discuss the influence of the main-group element on half-metallic properties of CoCrZ. It is found that the replacement of Ge for Si in CoCrSi can adjust the position of the Fermi level, and while it has no impact on the energy gap width and magnetic structure. However, the substitution of P for Si can effectively adjust the magnetism without disrupting its half-metallicity. Our results demonstrate that the electronic structure of CoCrZ is mainly dependent on the number of valence electrons of the main-group element.

  12. Integration of antiferromagnetic Heusler compound Ru2MnGe into spintronic devices

    NASA Astrophysics Data System (ADS)

    Balluff, Jan; Huminiuc, Teodor; Meinert, Markus; Hirohata, Atsufumi; Reiss, Günter

    2017-07-01

    We report on the integration of an antiferromagnetic Heusler compound acting as a pinning layer into magnetic tunneling junctions (MTJs). The antiferromagnet Ru2MnGe is used to pin the magnetization direction of a ferromagnetic Fe layer in MgO based thin film tunneling magnetoresistance stacks. The samples were prepared using magnetron co-sputtering. We investigate the structural properties by X-ray diffraction and reflection, as well as atomic force and high-resolution transmission electron microscopy. We find an excellent crystal growth quality with a low interface roughnesses of 1-3 Å, which is crucial for the preparation of working tunneling barriers. Using Fe as a ferromagnetic electrode material, we prepared magnetic tunneling junctions and measured the magnetoresistance. We find a sizeable maximum magnetoresistance value of 135%, which is comparable to other common Fe based MTJ systems.

  13. Stability of Weyl points in magnetic half-metallic Heusler compounds

    NASA Astrophysics Data System (ADS)

    Chadov, Stanislav; Wu, Shu-Chun; Felser, Claudia; Galanakis, Iosif

    2017-07-01

    We employ ab initio fully relativistic electronic structure calculations to study the stability of the Weyl points in the momentum space within the class of the half-metallic ferromagnetic full Heusler materials, by focusing on Co2TiAl as a well-established prototype compound. Here we show that both the number of the Weyl points together with their k -space coordinates can be controlled by the orientation of the magnetization. This alternative degree of freedom, which is absent in other topological materials (e.g., in Weyl semimetals), introduces functionalities that are specific for the class of half-metallic ferromagnets. Of special interest are crossing points which are preserved irrespective of any arbitrary rotation of the magnetization axis.

  14. Conditions for spin-gapless semiconducting behavior in Mn{sub 2}CoAl inverse Heusler compound

    SciTech Connect

    Galanakis, I.; Özdoğan, K.; Şaşıoğlu, E.; Blügel, S.

    2014-03-07

    Employing ab initio electronic structure calculations, we investigate the conditions for spin-gapless semiconducting (SGS) behavior in the inverse Mn{sub 2}CoAl Heusler compound. We show that tetragonalization of the lattice, which can occur during films growth, keeps the SGS character of the perfect cubic compound. On the contrary, atomic swaps even between sites with different local symmetry destroy the SGS character giving rise to a half-metallic state. Furthermore, the occurrence of Co-surplus leads also to half-metallicity. Thus, we propose that in order to achieve SGS behavior during the growth of Mn{sub 2}CoAl (and similar SGS Heusler compounds) thin films, one should minimize the occurrence of defects, while small deformations of the lattice, due to the lattice mismatch with the substrate, do not play a crucial role.

  15. Band structure and transport studies of half Heusler compound DyPdBi: An efficient thermoelectric material

    NASA Astrophysics Data System (ADS)

    Krishnaveni, S.; Sundareswari, M.; Deshmukh, P. C.; Valluri, S. R.; Roberts, Ken

    2016-05-01

    The discovery of Heusler alloys has revolutionized the research field of intermetallics due to the ease with which one can derive potential candidates for multifunctional applications. During recent years, many half Heusler alloys have been investigated for their thermoelectric properties. The f electron based rare earth ternary half Heusler compound DyPdBi has its f energy levels located close to the Fermi energy level. Other research efforts have emphasized that such materials have good thermoelectric capabilities. We have explored using first principles the electronic band structure of DyPdBi by use of different exchange correlation potentials in the density functional theoretical framework. Transport coefficients that arise in the study of thermoelectric properties of DyPdBi have been calculated and illustrate its potential as an efficient thermoelectric material. Both the theoretically estimated Seebeck coefficient and the power factor agree well with the available experimental results. Our calculations illustrate that it is essential to include spin-orbit coupling in these models of f electron half Heusler materials.

  16. Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds

    NASA Astrophysics Data System (ADS)

    Steil, Daniel; Schmitt, Oliver; Fetzer, Roman; Kubota, Takahide; Naganuma, Hiroshi; Oogane, Mikihiko; Ando, Yasuo; Rodan, Steven; Blum, Christian G. F.; Balke, Benjamin; Wurmehl, Sabine; Aeschlimann, Martin; Cinchetti, Mirko

    2015-04-01

    Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics. Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 - xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the

  17. A new class of topological insulators from I-III-IV half-Heusler compounds with strong band inversion strength

    NASA Astrophysics Data System (ADS)

    Zhang, X. M.; Xu, G. Z.; Du, Y.; Liu, E. K.; Liu, Z. Y.; Wang, W. H.; Wu, G. H.

    2014-02-01

    In this paper, by first principle calculations, we investigate systematically the band topology of a new half-Heusler family with composition of I(A)-III(A)-IV(A). The results clearly show that many of the I-III-IV half-Heusler compounds are in fact promising to be topological insulator candidates. The characteristic feature of these new topological insulators is the naturally strong band inversion strength (up to -2 eV) without containing heavy elements. Moreover, we found that both the band inversion strength and the bulk insulating gap of the compounds can be tailored through strain engineering, and therefore would be grown epitaxially in the form of thin films, and useful in spintronics and other applications.

  18. Ferrimagnetism and disorder of epitaxial Mn2-xCoxVAl Heusler compound thin films

    SciTech Connect

    Meinert, Markus; Schmalhorst, Jan-Michael; Reiss, Gunter; Arenholz, Elke

    2011-01-29

    The quaternary full Heusler compound Mn{sub 2-x}Co{sub x}VAl with x = 1 is predicted to be a half-metallic antiferromagnet. Thin films of the quaternary compounds with x = 0-2 were prepared by dc and RF magnetron co-sputtering on heated MgO (0 0 1) substrates. The magnetic structure was examined by x-ray magnetic circular dichroism and the chemical disorder was characterized by x-ray diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn{sub 2}VAl (x = 0). For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to Mn. The observed reduced magnetic moments are interpreted with the help of band structure calculations in the coherent potential approximation. Mn{sub 2}VAl is very sensitive to disorder involving Mn, because nearest-neighbour Mn atoms couple antiferromagnetically. Co{sub 2}VAl has B2 order and has reduced magnetization. In the cases with x {ge} 0.9 conventional ferromagnetism was observed, closely related to the atomic disorder in these compounds.

  19. Structural, electronic, mechanical, and thermoelectric properties of a novel half Heusler compound HfPtPb

    NASA Astrophysics Data System (ADS)

    Kaur, Kulwinder; Rai, D. P.; Thapa, R. K.; Srivastava, Sunita

    2017-07-01

    We explore the structural, electronic, mechanical, and thermoelectric properties of a new half Heusler compound HfPtPb, an all metallic heavy element, recently proposed to be stable [Gautier et al., Nat. Chem. 7, 308 (2015)]. In this work, we employ density functional theory and semi-classical Boltzmann transport equations with constant relaxation time approximation. The mechanical properties, such as shear modulus, Young's modulus, elastic constants, Poisson's ratio, and shear anisotropy factor, have been investigated. The elastic and phonon properties reveal that this compound is mechanically and dynamically stable. Pugh's ratio and Frantsevich's ratio demonstrate its ductile behavior, and the shear anisotropic factor reveals the anisotropic nature of HfPtPb. The band structure predicts this compound to be a semiconductor with a band gap of 0.86 eV. The thermoelectric transport parameters, such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity, and lattice thermal conductivity, have been calculated as a function of temperature. The highest value of Seebeck coefficient is obtained for n-type doping at an optimal carrier concentration of 1.0 × 1020 e/cm3. We predict the maximum value of figure of merit (0.25) at 1000 K. Our investigation suggests that this material is an n-type semiconductor.

  20. Direct evidence for minority spin gap in the C o2MnSi Heusler compound

    NASA Astrophysics Data System (ADS)

    Andrieu, Stéphane; Neggache, Amina; Hauet, Thomas; Devolder, Thibaut; Hallal, Ali; Chshiev, Mairbek; Bataille, Alexandre M.; Le Fèvre, Patrick; Bertran, François

    2016-03-01

    Half metal magnets are of great interest in the field of spintronics because of their potential full spin polarization at the Fermi level (EF) and low magnetization damping. The high Curie temperature and the predicted 0.7 eV minority spin gap make the C o2MnSi Heusler compound very promising for applications. We investigated the half-metallic magnetic character of this compound using spin-resolved photoemission, ab initio calculation, and ferromagnetic resonance. At the surface of C o2MnSi , a gap in the minority spin channel is observed, leading to 100 % spin polarization. However, this gap is 0.3 eV below EF, and a minority spin state is observed at EF. We show that a minority spin gap at EF can nevertheless be recovered either by changing the chemical composition of the compound or by covering the surface by Mn, MnSi, or MgO. This spin-gap recovery results in extremely small damping coefficients, reaching values as low as 7 ×10-4 .

  1. Full-scale computation for all the thermoelectric property parameters of half-Heusler compounds

    PubMed Central

    Hong, A. J.; Li, L.; He, R.; Gong, J. J.; Yan, Z. B.; Wang, K. F.; Liu, J. -M.; Ren, Z. F.

    2016-01-01

    The thermoelectric performance of materials relies substantially on the band structures that determine the electronic and phononic transports, while the transport behaviors compete and counter-act for the power factor PF and figure-of-merit ZT. These issues make a full-scale computation of the whole set of thermoelectric parameters particularly attractive, while a calculation scheme of the electronic and phononic contributions to thermal conductivity remains yet challenging. In this work, we present a full-scale computation scheme based on the first-principles calculations by choosing a set of doped half-Heusler compounds as examples for illustration. The electronic structure is computed using the WIEN2k code and the carrier relaxation times for electrons and holes are calculated using the Bardeen and Shockley’s deformation potential (DP) theory. The finite-temperature electronic transport is evaluated within the framework of Boltzmann transport theory. In sequence, the density functional perturbation combined with the quasi-harmonic approximation and the Klemens’ equation is implemented for calculating the lattice thermal conductivity of carrier-doped thermoelectric materials such as Ti-doped NbFeSb compounds without losing a generality. The calculated results show good agreement with experimental data. The present methodology represents an effective and powerful approach to calculate the whole set of thermoelectric properties for thermoelectric materials. PMID:26947395

  2. Full-scale computation for all the thermoelectric property parameters of half-Heusler compounds

    DOE PAGES

    Hong, A. J.; Li, L.; He, R.; ...

    2016-03-07

    The thermoelectric performance of materials relies substantially on the band structures that determine the electronic and phononic transports, while the transport behaviors compete and counter-act for the power factor PF and figure-of-merit ZT. These issues make a full-scale computation of the whole set of thermoelectric parameters particularly attractive, while a calculation scheme of the electronic and phononic contributions to thermal conductivity remains yet challenging. In this work, we present a full-scale computation scheme based on the first-principles calculations by choosing a set of doped half- Heusler compounds as examples for illustration. The electronic structure is computed using the WIEN2k codemore » and the carrier relaxation times for electrons and holes are calculated using the Bardeen and Shockley’s deformation potential (DP) theory. The finite-temperature electronic transport is evaluated within the framework of Boltzmann transport theory. In sequence, the density functional perturbation combined with the quasi-harmonic approximation and the Klemens’ equation is implemented for calculating the lattice thermal conductivity of carrier-doped thermoelectric materials such as Tidoped NbFeSb compounds without losing a generality. The calculated results show good agreement with experimental data. Lastly, the present methodology represents an effective and powerful approach to calculate the whole set of thermoelectric properties for thermoelectric materials.« less

  3. Realization of spin gapless semiconductors: the Heusler compound Mn2CoAl.

    PubMed

    Ouardi, Siham; Fecher, Gerhard H; Felser, Claudia; Kübler, Jürgen

    2013-03-08

    Recent studies have reported an interesting class of semiconductor materials that bridge the gap between semiconductors and half-metallic ferromagnets. These materials, called spin gapless semiconductors, exhibit a band gap in one of the spin channels and a zero band gap in the other and thus allow for tunable spin transport. Here, we report the first experimental verification of the spin gapless magnetic semiconductor Mn(2)CoAl, an inverse Heusler compound with a Curie temperature of 720 K and a magnetic moment of 2 μ(B). Below 300 K, the compound exhibits nearly temperature-independent conductivity, very low, temperature-independent carrier concentration, and a vanishing Seebeck coefficient. The anomalous Hall effect is comparatively low, which is explained by the symmetry properties of the Berry curvature. Mn(2) CoAl is not only suitable material for room temperature semiconductor spintronics, the robust spin polarization of the spin gapless semiconductors makes it very promising material for spintronics in general.

  4. Full-scale computation for all the thermoelectric property parameters of half-Heusler compounds

    SciTech Connect

    Hong, A. J.; Li, L.; He, R.; Gong, J. J.; Yan, Z. B.; Wang, K. F.; Liu, J. -M.; Ren, Z. F.

    2016-03-07

    The thermoelectric performance of materials relies substantially on the band structures that determine the electronic and phononic transports, while the transport behaviors compete and counter-act for the power factor PF and figure-of-merit ZT. These issues make a full-scale computation of the whole set of thermoelectric parameters particularly attractive, while a calculation scheme of the electronic and phononic contributions to thermal conductivity remains yet challenging. In this work, we present a full-scale computation scheme based on the first-principles calculations by choosing a set of doped half- Heusler compounds as examples for illustration. The electronic structure is computed using the WIEN2k code and the carrier relaxation times for electrons and holes are calculated using the Bardeen and Shockley’s deformation potential (DP) theory. The finite-temperature electronic transport is evaluated within the framework of Boltzmann transport theory. In sequence, the density functional perturbation combined with the quasi-harmonic approximation and the Klemens’ equation is implemented for calculating the lattice thermal conductivity of carrier-doped thermoelectric materials such as Tidoped NbFeSb compounds without losing a generality. The calculated results show good agreement with experimental data. Lastly, the present methodology represents an effective and powerful approach to calculate the whole set of thermoelectric properties for thermoelectric materials.

  5. Full-scale computation for all the thermoelectric property parameters of half-Heusler compounds.

    PubMed

    Hong, A J; Li, L; He, R; Gong, J J; Yan, Z B; Wang, K F; Liu, J-M; Ren, Z F

    2016-03-07

    The thermoelectric performance of materials relies substantially on the band structures that determine the electronic and phononic transports, while the transport behaviors compete and counter-act for the power factor PF and figure-of-merit ZT. These issues make a full-scale computation of the whole set of thermoelectric parameters particularly attractive, while a calculation scheme of the electronic and phononic contributions to thermal conductivity remains yet challenging. In this work, we present a full-scale computation scheme based on the first-principles calculations by choosing a set of doped half-Heusler compounds as examples for illustration. The electronic structure is computed using the WIEN2k code and the carrier relaxation times for electrons and holes are calculated using the Bardeen and Shockley's deformation potential (DP) theory. The finite-temperature electronic transport is evaluated within the framework of Boltzmann transport theory. In sequence, the density functional perturbation combined with the quasi-harmonic approximation and the Klemens' equation is implemented for calculating the lattice thermal conductivity of carrier-doped thermoelectric materials such as Ti-doped NbFeSb compounds without losing a generality. The calculated results show good agreement with experimental data. The present methodology represents an effective and powerful approach to calculate the whole set of thermoelectric properties for thermoelectric materials.

  6. Full-scale computation for all the thermoelectric property parameters of half-Heusler compounds

    NASA Astrophysics Data System (ADS)

    Hong, A. J.; Li, L.; He, R.; Gong, J. J.; Yan, Z. B.; Wang, K. F.; Liu, J.-M.; Ren, Z. F.

    2016-03-01

    The thermoelectric performance of materials relies substantially on the band structures that determine the electronic and phononic transports, while the transport behaviors compete and counter-act for the power factor PF and figure-of-merit ZT. These issues make a full-scale computation of the whole set of thermoelectric parameters particularly attractive, while a calculation scheme of the electronic and phononic contributions to thermal conductivity remains yet challenging. In this work, we present a full-scale computation scheme based on the first-principles calculations by choosing a set of doped half-Heusler compounds as examples for illustration. The electronic structure is computed using the WIEN2k code and the carrier relaxation times for electrons and holes are calculated using the Bardeen and Shockley’s deformation potential (DP) theory. The finite-temperature electronic transport is evaluated within the framework of Boltzmann transport theory. In sequence, the density functional perturbation combined with the quasi-harmonic approximation and the Klemens’ equation is implemented for calculating the lattice thermal conductivity of carrier-doped thermoelectric materials such as Ti-doped NbFeSb compounds without losing a generality. The calculated results show good agreement with experimental data. The present methodology represents an effective and powerful approach to calculate the whole set of thermoelectric properties for thermoelectric materials.

  7. μSR and NMR study of the superconducting Heusler compound YPd2Sn

    NASA Astrophysics Data System (ADS)

    Saadaoui, H.; Shiroka, T.; Amato, A.; Baines, C.; Luetkens, H.; Pomjakushina, E.; Pomjakushin, V.; Mesot, J.; Pikulski, M.; Morenzoni, E.

    2013-09-01

    We report on muon-spin rotation and relaxation (μSR) and 119Sn nuclear magnetic resonance (NMR) measurements to study the microscopic superconducting and magnetic properties of the Heusler compound with the highest superconducting transition temperature, YPd2Sn (Tc=5.4 K). Measurements in the vortex state provide the temperature dependence of the effective magnetic penetration depth λ(T) and the field dependence of the superconducting gap Δ(0). The results are consistent with a very dirty s-wave BCS superconductor with a gap Δ(0)=0.85(3) meV, λ(0)=212(1) nm, and a Ginzburg-Landau coherence length ξGL(0)≅23 nm. In spite of its very dirty character, the effective density of condensed charge carriers is high compared to that in the normal state. The μSR data in a broad range of applied fields are well reproduced by taking into account a field-related reduction of the effective superconducting gap. Zero-field μSR measurements, sensitive to the possible presence of very small magnetic moments, do not show any indications of magnetism in this compound.

  8. Observation of strong ferromagnetism in the half-Heusler compound CoTiSb system

    NASA Astrophysics Data System (ADS)

    Sedeek, K.; Hantour, H.; Makram, N.; Said, Sh. A.

    2016-06-01

    Strong ferromagnetism has been detected in the semiconducting half-Heusler CoTiSb compound. The synthesis process was carried out by direct fusion of highly pure Co, Ti, and Sb in an evacuated quartz tube. The structural, micro structural and magnetic properties were investigated. The crystal structure was refined from X-ray powder diffraction data by the Rietveld method. Applying the search match program, three nano-crystalline phases of CoTiSb, Ti3Sb and CoTi2 (50%, 33.3% and 16.7% respectively) were identified for the prepared system. The term "phase" is used to address the co-existence of different stable chemical composition for the same half-Heusler alloy. The scanning electron microscope SEM and the high resolution transmission electron microscope HR-TEM were applied to characterize the morphology, size, shape, crystallinity and lattice spacing. A mixture of ordered and disordered arrangement was detected. Well defined nano-crystalline structure with an average interatomic distance equals 0.333 nm and sharp diffraction spots were measured. Contrary to this, the HR-TEM and electron diffraction image shows distorted structured planes and smeared halo surrounded by weak rings. Thermo-magnetic measurements (M-T) have been measured between 640 °K and 920 °K. Clear magnetic phase transition is detected above 900 °K (Tc), in addition to a second possible phase transition (TFF) around 740 °K. The latter is clarified by plotting ΔM/ΔT vs. T. To determine the type of the detected phase transitions, the field dependence of magnetization was measured at 300 °K and 740 °K. Arrot plots (M2-H/M) confirm the ferromagnetic character at both temperatures. It may be reasonable to assume the TFF transition as an additional ferromagnetic contribution stemming from some sort of exchange interactions. A tentative magnetic phase diagram is given. Overall, the present results suggest that the prepared multiphases CoTiSb system does not obey the 18 valence electron/unit cell

  9. High-field magnetization of Heusler compound Fe2Mn1 -xVxSi

    NASA Astrophysics Data System (ADS)

    Hiroi, Masahiko; Tazoko, Tomoya; Sano, Hiroaki; Shigeta, Iduru; Koyama, Keiichi; Kondo, Akihiro; Kindo, Koich; Manaka, Hirotaka; Terada, Norio

    2017-01-01

    Fe2MnSi exhibits a ferromagnetic transition at TC˜230 K and another transition to a phase with antiferromagnetic components at TA˜60 K. By substituting V for Mn, so as to obtain Fe2Mn1 -xVxSi , TA is revealed to decrease with x and then vanish around x ˜0.2 . In this study, the phase boundary of the transition at TA in the high-field range is found for 0 ≤x ≤0.15 with pulsed fields up to ˜70 T. The magnetization of Fe2Mn1 -xVxSi slowly increases even at the highest field of ˜70 T, though it occurs more gradually as x increases. We compare the magnetization for 0 ≤x ≤0.20 at 62 T with the Slater-Pauling rule, which holds when a Heusler compound is a half-metal, and find fairly good agreement. This suggests an intimate relation between the high-field phase and the half-metallic electronic structure, and that at the high-field limit the phase approaches the half-metallic state, which has been predicted by band-structure calculations.

  10. Magneto-optical spectroscopy of Co{sub 2}FeSi Heusler compound

    SciTech Connect

    Veis, M. Beran, L.; Antos, R.; Legut, D.; Hamrle, J.; Pistora, J.; Sterwerf, Ch.; Meinert, M.; Schmalhorst, J.-M.; Kuschel, T.; Reiss, G.

    2014-05-07

    Magneto-optical and electronic properties of the Co{sub 2}FeSi Heusler compound were studied by polar Kerr magneto-optical spectroscopy and ab-initio calculations. The thin-film samples were grown by dc/rf magnetron co-sputtering on MgO(100) substrates. A Cr seed layer was deposited prior to the Co{sub 2}FeSi layer to achieve its epitaxial growth. The magneto-optical spectroscopy was carried out using generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.4 to 5.5 eV with an applied magnetic field of up to 1.2 T. The polar Kerr spectra showed a smooth spectral behavior up to 5.5 eV indicating nearly free charge carriers. Experimental data were compared with ab-initio calculations based on density functional theory employing the full-potential linearized augmented plane wave method.

  11. Ru2NbGa: A Heusler-type compound with semimetallic characteristics

    NASA Astrophysics Data System (ADS)

    Kuo, C. N.; Lee, H. W.; Wei, C.-M.; Lin, Y. H.; Kuo, Y. K.; Lue, C. S.

    2016-11-01

    The Heusler-type compound of Ru2NbGa has been successfully synthesized. X-ray analysis confirms that Ru2NbGa crystallizes in a cubic L 21 structure. The electronic properties of Ru2NbGa have been characterized by means of the transport and nuclear magnetic resonance (NMR) measurements. The temperature dependence of the electrical resistivity exhibits a typical semimetallic behavior. The NMR spin-lattice relaxation rate shows activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap. We have also deduced a low Fermi-level density of states (DOS), being consistent with the semimetallic characteristic for Ru2NbGa . In addition, we have performed first-principles total-energy calculations including G0W0 and G W0 corrections for band gaps to investigate the electronic band structure of Ru2NbGa . The calculated result reveals an indirect overlap between electron and hole pockets that leads to a residual DOS at the Fermi level, providing a consistent explanation for the experimental observations.

  12. Theoretical investigations of electronic structure and magnetism in Zr2CoSn full-Heusler compound

    NASA Astrophysics Data System (ADS)

    Birsan, A.; Kuncser, V.

    2015-08-01

    The half-metallic properties of a new and promising full-Heusler compound, Zr2CoSn, are investigated by means of ab initio calculations within the Density Functional Theory framework. It was shown that the ferromagnetic ordered Hg2CuTi-type crystal structure is energetically the most favorable for this compound. The total magnetic moment is 3 μB/f.u. and follows a typical Slater-Pauling dependence. The half metallicity disappears if the unit cell volume is contracted by more than 5%.

  13. Growth and Electronic Structure of Heusler Compounds for Use in Electron Spin Based Devices

    NASA Astrophysics Data System (ADS)

    Patel, Sahil Jaykumar

    Spintronic devices, where information is carried by the quantum spin state of the electron instead of purely its charge, have gained considerable interest for their use in future computing technologies. For optimal performance, a pure spin current, where all electrons have aligned spins, must be generated and transmitted across many interfaces and through many types of materials. While conventional spin sources have historically been elemental ferromagnets, like Fe or Co, these materials pro duce only partially spin polarized currents. To increase the spin polarization of the current, materials like half-metallic ferromagnets, where there is a gap in the minority spin density of states around the Fermi level, or topological insulators, where the current transport is dominated by spin-locked surface states, show promise. A class of materials called Heusler compounds, with electronic structures that range from normal metals, to half metallic ferromagnets, semiconductors, superconductors and even topological insulators, interfaces well with existing device technologies, and through the use of molecular beam epitaxy (MBE) high quality heterostructures and films can be grown. This dissertation examines the electronic structure of surfaces and interfaces of both topological insulator (PtLuSb-- and PtLuBi--) and half-metallic ferromagnet (Co2MnSi-- and Co2FeSi--) III-V semiconductor heterostructures. PtLuSb and PtLuBi growth by MBE was demonstrated on Alx In1--xSb (001) ternaries. PtLuSb (001) surfaces were observed to reconstruct with either (1x3) or c(2x2) unit cells depending on Sb overpressure and substrate temperature. viii The electronic structure of these films was studied by scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission spectroscopy. STS measurements as well as angle resolved photoemission spectropscopy (ARPES) suggest that PtLuSb has a zero-gap or semimetallic band structure. Additionally, the observation of linearly dispersing surface

  14. Robust tunability of magnetoresistance in half-Heusler RPtBi (R=Gd, Dy, Tm, and Lu) compounds

    DOE PAGES

    Mun, Eundeok; Bud'ko, Sergey L.; Canfield, Paul C.

    2016-03-15

    We present the magnetic field dependencies of transport properties for RPtBi (R = Gd, Dy, Tm, and Lu) half-Heusler compounds. Temperature- and field-dependent resistivity measurements of high-quality RPtBi single crystals reveal an unusually large, nonsaturating magnetoresistance (MR) up to 300 K under a moderate magnetic field of H = 140 kOe. At 300 K, the large MR effect decreases as the rare earth is traversed from Gd to Lu and the magnetic field dependence of MR shows a deviation from the conventional H2 behavior. The Hall coefficient (RH) for R = Gd indicates a sign change around 120 K, whereasmore » RH curves for R = Dy, Tm, and Lu remain positive for all measured temperatures. At 300 K, the Hall resistivity reveals a deviation from the linear field dependence for all compounds. Thermoelectric power measurements on this family show strong temperature and magnetic field dependencies which are consistent with resistivity measurements. A highly enhanced thermoelectric power under applied magnetic field is observed as high as ~100 μV/K at 140 kOe. Furthermore, analysis of the transport data in this series reveals that the rare-earth-based half-Heusler compounds provide opportunities to tune MR effect through lanthanide contraction and to elucidate the mechanism of nontrivial MR.« less

  15. Robust tunability of magnetoresistance in half-Heusler R PtBi (R =Gd , Dy, Tm, and Lu) compounds

    NASA Astrophysics Data System (ADS)

    Mun, Eundeok; Bud'ko, Sergey L.; Canfield, Paul C.

    2016-03-01

    We present the magnetic field dependencies of transport properties for R PtBi (R =Gd , Dy, Tm, and Lu) half-Heusler compounds. Temperature- and field-dependent resistivity measurements of high-quality R PtBi single crystals reveal an unusually large, nonsaturating magnetoresistance (MR) up to 300 K under a moderate magnetic field of H =140 kOe. At 300 K, the large MR effect decreases as the rare earth is traversed from Gd to Lu and the magnetic field dependence of MR shows a deviation from the conventional H2 behavior. The Hall coefficient (RH) for R =Gd indicates a sign change around 120 K, whereas RH curves for R =Dy , Tm, and Lu remain positive for all measured temperatures. At 300 K, the Hall resistivity reveals a deviation from the linear field dependence for all compounds. Thermoelectric power measurements on this family show strong temperature and magnetic field dependencies which are consistent with resistivity measurements. A highly enhanced thermoelectric power under applied magnetic field is observed as high as ˜100 μ V /K at 140 kOe. Analysis of the transport data in this series reveals that the rare-earth-based half-Heusler compounds provide opportunities to tune MR effect through lanthanide contraction and to elucidate the mechanism of nontrivial MR.

  16. Atomic origin of the spin-polarization of the Co2FeAl Heusler compound

    NASA Astrophysics Data System (ADS)

    Liang, Jaw-Yeu; Lam, Tu-Ngoc; Lin, Yan-Cheng; Chang, Shu-Jui; Lin, Hong-Ji; Tseng, Yuan-Chieh

    2016-02-01

    Using synchrotron x-ray techniques, we studied the Co2FeAl spin-polarization state that generates the half-metallicity of the compound during an A2 (low-spin)  →  B2 (high-spin) phase transition. Given the advantage of element specificity of x-ray techniques, we could fingerprint the structural and magnetic cross-reactions between Co and Fe within a complex Co2FeAl structure deposited on a MgO (0 0 1) substrate. X-ray diffraction and extended x-ray absorption fine structure investigations determined that the Co atoms preferably populate the (1/4,1/4,1/4) and (3/4,3/4,3/4) sites during the development of the B2 phase. X-ray magnetic spectroscopy showed that although the two magnetic elements were ferromagnetically coupled, they interacted in a competing manner via a charge-transfer effect, which enhanced Co spin polarization at the expense of Fe spin polarization during the phase transition. This means that the spin-polarization of Co2FeAl was electronically dominated by Fe in A2 whereas the charge transfer turned the dominance to Co upon B2 formation. Helicity-dependent x-ray absorption spectra also revealed that only the minority state of Co/Fe was involved in the charge-transfer effect whereas the majority state was independent of it. Despite an overall increase of Co2FeAl magnetization, the charge-transfer effect created an undesired trade-off during the Co-Fe exchange interactions, because of the presence of twice as many X sites (Co) as Y sites (Fe) in the Heusler X 2 YZ formula. This suggests that the spin-polarization of Co2FeAl is unfortunately regulated by compromising the enhanced X (Co) sites and the suppressed Y (Fe) sites, irrespective of the development of the previously known high-spin-polarization phase of B2. This finding provides a possible cause for the limited half-metallicity of Co2FeAl discovered recently. Electronic tuning between the X and Y sites is necessary to further increase the spin-polarization, and likely the half

  17. Effects of Ga substitution on the structural and magnetic properties of half metallic Fe2MnSi Heusler compound

    NASA Astrophysics Data System (ADS)

    Pedro, S. S.; Caraballo Vivas, R. J.; Andrade, V. M.; Cruz, C.; Paixão, L. S.; Contreras, C.; Costa-Soares, T.; Caldeira, L.; Coelho, A. A.; Carvalho, A. Magnus G.; Rocco, D. L.; Reis, M. S.

    2015-01-01

    The so-called half-metallic magnets have been proposed as good candidates for spintronic applications due to the feature of exhibiting a hundred percent spin polarization at the Fermi level. Such materials follow the Slater-Pauling rule, which relates the magnetic moment with the valence electrons in the system. In this paper, we study the bulk polycrystalline half-metallic Fe2MnSi Heusler compound replacing Si by Ga to determine how the Ga addition changes the magnetic, the structural, and the half-metal properties of this compound. The material does not follow the Slater-Pauling rule, probably due to a minor structural disorder degree in the system, but a linear dependence on the magnetic transition temperature with the valence electron number points to the half-metallic behavior of this compound.

  18. Spin transport and accumulation in n{sup +}-Si using Heusler compound Co{sub 2}FeSi/MgO tunnel contacts

    SciTech Connect

    Ishikawa, Mizue Sugiyama, Hideyuki; Inokuchi, Tomoaki; Saito, Yoshiaki; Hamaya, Kohei

    2015-08-31

    We investigate spin transport and accumulation in n{sup +}-Si using Heusler compound Co{sub 2}FeSi/MgO/Si on insulator (SOI) devices. The magnitudes of the non-local four- and three-terminal Hanle effect signals when using Heusler compound Co{sub 2}FeSi/MgO/SOI devices are larger than when using CoFe/MgO/SOI devices, whereas the preparation methods of MgO layers on SOI are exactly same in both devices. Different bias voltage dependencies on the magnitude of spin accumulation signals are also observed between these devices. Especially, Co{sub 2}FeSi/MgO/SOI devices show large spin accumulation signals compared with CoFe/MgO/SOI devices in the low bias voltage region less than ∼1000 mV in which the increase of the spin polarization is expected from the estimation of the density of states in Heusler compound Co{sub 2}FeSi and CoFe under spin extraction conditions. These results indicate that the species of ferromagnetic material definitely affects the magnitude and behavior of the spin signals. The use of highly polarized ferromagnets such as Heusler compounds would be important for improving the spin polarization and the magnitude of spin signals through Si channels.

  19. Ferromagnetism in half-metallic quaternary FeVTiAl Heusler compound

    NASA Astrophysics Data System (ADS)

    Bhat, Tahir Mohiuddin; Bhat, Idris Hamid; Yousuf, Saleem; Gupta, Dinesh C.

    2016-05-01

    The electronic structure and magnetic properties of FeVTiAl quaternary Heusler alloy have been investigated within the density functional theory framework. The material was found completely spin-polarized half-metallic Ferromagnet in the ground state with F-43m structure. The structural stability was further confirmed by calculating different elastic constants in the cubic phase. Present study predicts an energy band gap of 0.72 eV calculated in localized minority spin channel at an equilibrium lattice parameter of 6.0Å. The calculated total spin magnetic moment of 2 µB/f.u. is in agreement with the Slater-Pauling rule for full Heusler alloys.

  20. Ferromagnetism in half-metallic quaternary FeVTiAl Heusler compound

    SciTech Connect

    Bhat, Tahir Mohiuddin; Bhat, Idris Hamid; Yousuf, Saleem; Gupta, Dinesh C.

    2016-05-23

    The electronic structure and magnetic properties of FeVTiAl quaternary Heusler alloy have been investigated within the density functional theory framework. The material was found completely spin-polarized half-metallic Ferromagnet in the ground state with F-43m structure. The structural stability was further confirmed by calculating different elastic constants in the cubic phase. Present study predicts an energy band gap of 0.72 eV calculated in localized minority spin channel at an equilibrium lattice parameter of 6.0Å. The calculated total spin magnetic moment of 2 µ{sub B}/f.u. is in agreement with the Slater-Pauling rule for full Heusler alloys.

  1. Design of L21-type antiferromagnetic semiconducting full-Heusler compounds: A first principles DFT + GW study

    NASA Astrophysics Data System (ADS)

    Tas, M.; Şaşıoǧlu, E.; Friedrich, C.; Blügel, S.; Galanakis, I.

    2017-02-01

    Antiferromagnetic spintronics is an on-going growing field of research. Employing both standard density functional theory and the GW approximation within the framework of the full-potential linearized augmented-plane-wave method, we study the electronic and magnetic properties of seven potential antiferromagnetic semiconducting Heusler compounds with 18 (or 28 when Zn is present) valence electrons per unit cell. We show that in these compounds G-type antiferromagnetism is the ground state and that they are all either semiconductors (Cr2ScP, Cr2TiZn, V2ScP, V2TiSi, and V3Al) or semimetals (Mn2MgZn and Mn2NaAl). The many-body corrections have a minimal effect on the electronic band structure with respect to the standard electronic structure calculations.

  2. Computational prediction of high thermoelectric performance in p-type half-Heusler compounds with low band effective mass.

    PubMed

    Fang, Teng; Zheng, Shuqi; Zhou, Tian; Yan, Lei; Zhang, Peng

    2017-02-08

    Half-Heusler (HH) compounds are important high temperature thermoelectric (TE) materials and have gained ever-increasing popularity. In recent years, p-type FeNbSb-based heavy-band HH compounds have attracted considerable attention with the record-high zT value of 1.5. Here, we use first-principles based methods to predict a very high zT value of 1.54 at 1200 K in p-type RuTaSb alloys. The high band degeneracy and low band effective mass contribute to a high power factor. Although the electrical thermal conductivity is high due to the high carrier mobility and hence electrical conductivity, the total thermal conductivity is moderate because of the low lattice thermal conductivity. The predicted high zT demonstrates that the p-type RuTaSb HH alloys are promising as TE materials for high temperature power generation.

  3. High-Tc ferromagnetic semiconductor-like behavior and unusual electrical properties in compounds with a 2×2×2 superstructure of the half-Heusler phase.

    PubMed

    Xiong, Ding-Bang; Okamoto, Norihiko L; Waki, Takeshi; Zhao, Yufeng; Kishida, Kyosuke; Inui, Haruyuki

    2012-02-27

    Heusler phases, including the full- and half-Heusler families, represent an outstanding class of multifunctional materials on account of their great tunability in compositions, valence electron counts (VEC), and properties. Here we demonstrate a systematic design of a series of new compounds with a 2×2×2 superstructure of the half-Heusler unit cell in X-Y-Z (X=Fe, Ru, Co, Rh, Ir; Y=Zn, Mn; Z=Sn, Sb) systems. Their structures were solved by using both powder and single-crystal X-ray diffraction, and also directly observed by using high-angle annular dark-field imaging in a scanning transmission electron microscope (HAADF-STEM). The VEC values of these new compounds span a wide and continuous range comparable to those for the full- and half-Heusler families, thereby implying tunability in compositions and physical properties in the superstructure. In fact, we observed abnormal electrical properties and a ferromagnetic semiconductor-like behavior with a high and tunable Curie temperature in these superstructures. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  4. Sorting Stable versus Unstable Hypothetical Compounds: The Case of Multi-Functional ABX Half-Heusler Filled Tetrahedral Structures

    SciTech Connect

    Zhang, X.; Yu, L.; Zakutayev, A.; Zunger, A.

    2012-04-10

    Electronic structure theory has recently been used to propose hypothetical compounds in presumed crystal structures, seeking new useful functional materials. In some cases, such hypothetical materials are metastable, albeit with technologically useful long lifetimes. Yet, in other cases, suggested hypothetical compounds may be significantly higher in energy than their lowest-energy crystal structures or competing phases, making their synthesis and eventual device-stability questionable. By way of example, the focus here is on the family of 1:1:1 compounds ABX called 'filled tetrahedral structure' (sometimes called Half-Heusler) in the four groups with octet electron count: I-I-VI (e.g., CuAgSe), I-II-V (e.g., AgMgAs), I-III-IV (e.g., LiAlSi), and II-II-IV (e.g., CaZnSn). First-principles thermodynamics is used to sort the lowest-energy structure and the thermodynamic stability of the 488 unreported hypothetical ABX compounds, many of which were previously proposed to be useful technologically. It is found that as many as 235 of the 488 are unstable with respect to decomposition (hence, are unlikely to be viable technologically), whereas other 235 of the unreported compounds are predicted to be thermodynamically stable (hence, potentially interesting new materials). 18 additional materials are too close to determine. The electronic structures of these predicted stable compounds are evaluated, seeking potential new material functionalities.

  5. Dirac cone and pseudogapped density of states in the topological half-Heusler compound YPtBi

    NASA Astrophysics Data System (ADS)

    Kronenberg, A.; Braun, J.; Minár, J.; Elmers, H.-J.; Kutnyakhov, D.; Zaporozhchenko, A. V.; Wallauer, R.; Chernov, S.; Medjanik, K.; Schönhense, G.; Kläui, M.; Chadov, S.; Ebert, H.; Jourdan, M.

    2016-10-01

    Topological insulators (TIs) are exciting materials, which exhibit unprecedented properties, such as helical spin-momentum locking, which leads to large torques for magnetic switching and highly efficient spin current detection. Here we explore the compound YPtBi, an example from the class of half-Heusler materials, for which the typical band inversion of topological insulators was predicted. We prepared this material as thin films by conventional cosputtering from elementary targets. By in situ time-of-flight momentum microscopy, a Dirac conelike surface state with a Dirac point ≃300 meV below the Fermi energy was observed, in agreement with electronic structure-photoemission calculations. Only little additional spectral weight due to other states was observed at EF, which corroborates the identification of the topologically protected surface state and is highly relevant for spintronics applications.

  6. First-principles study of mechanical, exchange interactions and the robustness in Co2MnSi full Heusler compounds

    NASA Astrophysics Data System (ADS)

    Akriche, A.; Bouafia, H.; Hiadsi, S.; Abidri, B.; Sahli, B.; Elchikh, M.; Timaoui, M. A.; Djebour, B.

    2017-01-01

    In this work we report the results of ab-initio studies of structural, mechanical, electronic and magnetic properties of Co based Co2MnSi Heusler compound in stoichiometric composition. All of which are accurately calculated by the full-potential (FP-LMTO) program combined with the spin polarized generalized gradient approximation in the density functional formalism (DFT). The total energy calculations clearly favor the ferromagnetic ground state. The lattice parameter, elastic constants and their related parameters were also evaluated and compared to experimental and theoretical values whenever possible. In this paper, the electronic properties are treated with GGA+U approach. The magnetic exchange constants temperature has been calculated using a mean field-approximation (MFA). The half-metal to metal transition was observed around 40 GPa. Increasing pressure has no impact on the total magnetic moment or the overall shape of the band structure that indicates the robustness of the electronic structure of this system.

  7. Half-metallic ferromagnetism with unexpectedly small spin splitting in the Heusler compound Co2FeSi.

    PubMed

    Bombor, Dirk; Blum, Christian G F; Volkonskiy, Oleg; Rodan, Steven; Wurmehl, Sabine; Hess, Christian; Büchner, Bernd

    2013-02-08

    Half-metallic ferromagnetism stands for the technologically sought-after metallicity with 100% spin polarization. Electrical transport should, in principle, sensitively probe half-metallic ferromagnetism, since electron-magnon scattering processes are expected to be absent, with clear-cut consequences for the resistivity and the magnetoresistance. Here we present electrical transport data for single-crystalline Co(2)FeSi, a candidate half-metallic ferromagnet Heusler compound. The data reveal a textbooklike exponential suppression of the electron-magnon scattering rate with decreasing temperature which provides strong evidence that this material indeed possesses perfect spin polarization at low temperature. However, the energy scale for thermally activated spin-flip scattering is relatively low (activation gap Δ≈100 K) which has decisive influence on the magnetoresistance and the anomalous Hall effect, which exhibit strong qualitative changes when crossing T≈100 K.

  8. Half-metallicity and magnetism of quaternary Heusler compounds CoRuTiZ (Z=Si, Ge, and Sn)

    NASA Astrophysics Data System (ADS)

    Bahramian, S.; Ahmadian, F.

    2017-02-01

    First-principle calculations based on the density functional theory for new quaternary Heusler compounds CoRuTiZ (Z=Si, Ge, and Sn) were performed. It was found that all three compounds were stable at YI structure in ferromagnetic state. The CoRuTiSi, CoRuTiGe, and CoRuTiSn were half-metal with integer magnetic moments of 1.00 μB per formula unit and half-metallic gaps of 0.13, 0.10, and 0.01 eV at their equilibrium volume, respectively. The density of states (DOSs) and band structures of these compounds were studied and the origin of half-metallicity was discussed. The CoRuTiSi, CoRuTiGe, and CoRuTiSn compounds showed half-metallic characteristics at lattice constants ranges of 5.77-6.36 Å, 5.66-6.16 Å, and 5.83-6.23 Å, indicating the lattice distortion did not affect the half-metallic properties of these compounds which makes them interesting materials in the spintronic field.

  9. Structural, electronic, elastic, optical, and vibrational properties of HfXSb (X = Co, Rh, Ru) half-Heusler compounds: an ab initio study

    NASA Astrophysics Data System (ADS)

    Çoban, C.; Çiftçi, Y. Ö.; Çolakoğlu, K.

    2016-11-01

    Structural, electronic, elastic, optical, and vibrational properties of ternary half-Heusler compounds HfXSb (X = Co, Rh, Ru) were studied with means of ab initio calculations based on the density functional theory. The calculated lattice constants were in good agreement with the available data. The electronic structure and corresponding density of states (DOS) were also calculated. Indirect band gaps were observed for HfCoSb and HfRhSb. Due to some valence bands crossing the Fermi level, HfRuSb has metallic character. In addition to the electronic structure, elastic and optical properties, phonon dispersion curves and phonon DOS were calculated. A detailed comparison was made between these three half-Heusler compounds.

  10. First-Principles study of quaternary Heusler compounds CoX'FeSi (X' = 4d transition elements) for spintronics applications

    NASA Astrophysics Data System (ADS)

    Ghosh, Srikrishna; Kundu, Ashish; Ghosh, Subhradip

    2017-05-01

    Using Density Functional Theory (DFT) calculation, we have studied structural, electronic and magnetic properties of Quaternary Heusler alloys (QHA) CoX'FeSi where X' is a transition metal with 4d electrons. On the basis of structural properties, electronic structure (DOS) and magnetic moments we investigate these materials in detail to search new materials for spintronics applications and to have a better understanding of the reasons which make these compounds suitable for such potential applications. From our study, CoZrFeSi is found to be half-metal with 100% spin polarization while CoNbFeSi seems to be a nearly half-metal. It is also divulged that half-metallicity in these compounds intricately allied with arrangements of magnetic atoms in the Heusler lattice.

  11. Termination layer compensated tunnelling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy.

    PubMed

    Jeong, Jaewoo; Ferrante, Yari; Faleev, Sergey V; Samant, Mahesh G; Felser, Claudia; Parkin, Stuart S P

    2016-01-18

    Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn-Ge termination layers that are oppositely magnetized to the higher moment Mn-Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.

  12. Magnetocaloric effect in “reduced” dimensions: Thin films, ribbons, and microwires of Heusler alloys and related compounds: Magnetocaloric effect in “reduced” dimensions

    SciTech Connect

    Khovaylo, Vladimir V.; Rodionova, Valeria V.; Shevyrtalov, Sergey N.; Novosad, Val

    2014-08-19

    Room temperature magnetic refrigeration is an energy saving and environmentally-friendly technology which has developed rapidly from a basic idea to prototype devices. The performance of magnetic refrigerators crucially depends on the magnetocaloric properties and the geometry of the employed refrigerants. Here we review the magnetocaloric properties of Heusler alloys and related compounds with a high surface to volume ratio such as films, ribbons and microwires, and compare them with their bulk counterparts.

  13. First principles study on half-metallic properties of Heusler compounds Ti2VZ (Z=Al, Ga, and In)

    NASA Astrophysics Data System (ADS)

    Galehgirian, S.; Ahmadian, F.

    2015-01-01

    First principles calculations using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method in the framework of density functional theory (DFT) were performed to study the electronic structures and magnetic properties of new full-Heusler compounds Ti2VZ (Z=Al, Ga, and In). Electronic structure calculations showed that Ti2VZ (Z=Al, Ga, and In) compounds in AlCu2Mn-type are conventional ferrimagnets. The Ti2VAl, Ti2VGa, and Ti2VIn compounds in the CuHg2Ti-type structure have half-metallic characteristics with a respective majority band gap of 0.52, 0.51, and 0.59 eV at the equilibrium lattice parameter. The origin of half-metallicity in these compounds was also discussed. The total magnetic moments of Ti2VZ (Z=Al, Ga, and In) compounds in the CuHg2Ti-type structures were 2 μB per formula unit which were in agreement with Slater-Pauling rule (Mtot=18-Ztot). The Ti2VAl, Ti2VGa, and Ti2VIn compounds in the CuHg2Ti-type structure respectively showed half-metallic characteristics at lattice constants ranges of 6.12-7.17 Å, 5.99-7.12 Å, and 6.31-7.06 Å, indicating the lattice distortion did not affect the half-metallic properties of these compounds which makes them interesting materials in the spintronics field.

  14. Ab initio prediction of ferrimagnetism, exchange interactions and Curie temperatures in Mn₂TiZ Heusler compounds.

    PubMed

    Meinert, M; Schmalhorst, J-M; Reiss, G

    2011-01-26

    The Heusler compounds Mn(2)TiZ (Z = Al, Ga, In, Si, Ge, Sn, P, As, Sb) are of great interest due to their potential ferrimagnetic properties and high spin polarization. Here, we present calculations of the structural and magnetic properties of these materials. Their magnetic moment follows the Slater-Pauling rule m = N(V) - 24. None of them is actually a perfect half-metallic ferrimagnet, but some exhibit more than 90% spin polarization and Curie temperatures well above room temperature. The exchange interactions are complex; direct and indirect exchange contributions are identified. The Curie temperature scales with the total magnetic moment, and it has a positive pressure dependence. The role of the Z element is investigated: it influences the properties of the compounds mainly via its valence electron number and its atomic radius, which determines the lattice parameter. Based on these results, Mn(2)TiSi, Mn(2)TiGe, and Mn(2)TiSn are proposed as candidates for spintronic applications.

  15. Half-metallic magnetism of Co 2CrX (X=As, Sb) Heusler compounds: An ab initio study

    NASA Astrophysics Data System (ADS)

    Kanbur, Ulvi; Gökoğlu, Gökhan

    2011-05-01

    In this study, we present the electronic, magnetic, and structural properties of two novel half-metallic full-Heusler compounds, Co 2CrAs and Co 2CrSb, in cubic L2 1 geometry. The calculations are based on the density functional theory within plane-wave pseudopotential method and spin-polarized generalized gradient approximation of the exchange-correlation functional. The electronic band structures and density of states of the systems indicate half-metallic behavior with vanishing electronic density of states of minority spins at Fermi level, which yields perfect spin polarization. The calculated magnetic moments of both systems in L2 1 structure are 5.00 μB, which are largely localized on the chromium site. The energy gaps in minority spin states are restricted by the 3d-states of cobalt atoms on two different sublattices. The formation enthalpies for both structures are negative indicating stability of these systems against decomposition into stable solid compounds.

  16. Termination layer compensated tunnelling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy

    PubMed Central

    Jeong, Jaewoo; Ferrante, Yari; Faleev, Sergey V.; Samant, Mahesh G.; Felser, Claudia; Parkin, Stuart S. P.

    2016-01-01

    Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn–Ge termination layers that are oppositely magnetized to the higher moment Mn–Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible. PMID:26776829

  17. Prospective high thermoelectric performance of the heavily p-doped half-Heusler compound CoVSn

    DOE PAGES

    Shi, Hongliang; Ming, Wenmei; Parker, David S.; ...

    2017-05-11

    The electronic structure and transport properties of the half-Heusler compound CoVSn are studied in this paper systematically by combining first-principles electronic structure calculations and Boltzmann transport theory. The band structure at the valence-band edge is complex with multiple maxima derived from hybridized transition element d states. The result is a calculated thermopower larger than 200 μV /Κ within a wide range of doping concentrations and temperatures for heavily doped p-type CoVSn. The thermoelectric properties additionally benefit from the corrugated shapes of the hole pockets in our calculated isoenergy surfaces. Our calculated power factor S2σ/τ (with respect to an average unknownmore » scattering time) of CoVSn is comparable to that of FeNbSb. A smaller lattice thermal conductivity can be expected from the smaller group velocities of acoustical modes compared to FeNbSb. Finally, overall, good thermoelectric performance for CoVSn can be expected by considering the electronic transport and lattice thermal conductivity.« less

  18. Calculated magneto-optical Kerr spectra of the half-Heusler compounds AuMnX (X = In, Sn, Sb).

    PubMed

    Amft, M; Oppeneer, P M

    2007-08-08

    The ferromagnetic ground states of the half-Heusler compounds AuMnX (X = In, Sn, Sb) have been calculated in the framework of the local spin-density approximation (LSDA) to density functional theory (DFT). AuMnSn is computed to be a half-metallic ferromagnet, whereas AuMnIn and AuMnSb are not half-metallic, due to their different band filling. The computed relativistic electronic structures served as inputs to calculate the magneto-optical Kerr rotations and ellipticities for all three materials. In the case of AuMnSn the largest, zero-temperature, polar Kerr rotation has been found to be -0.45° at about 1 eV photon energy. The computed MOKE spectra of AuMnSn are in qualitative agreement with recent experiments. The largest Kerr rotations of AuMnIn and AuMnSb have been calculated to be +0.64° at 4.3 eV and -0.85° at 0.9 eV, respectively.

  19. Ab-initio calculations on half-Heusler NiXSn (X = Zr, Hf) compounds: electronic and optical properties under pressure

    NASA Astrophysics Data System (ADS)

    Ozisik, H. B.; Ateser, E.; Ozisik, H.; Colakoglu, K.; Deligoz, E.

    2017-02-01

    In this study, we have investigated the electronic and optical properties of half-Heusler NiXSn (X = Zr, Hf) compounds under pressure by means of first principles calculations. The generalized gradient approximation is used to model exchange-correlation effects. We have estimated a transition from indirect band gap to direct band gap at 50 and 127 GPa for NiZrSn and NiHfSn, respectively. We have also plotted the static dielectric constant versus pressure for both compounds. The obtained results are in agreement with the available experimental and theoretical data.

  20. Ab-initio calculations on half-Heusler NiXSn (X = Zr, Hf) compounds: electronic and optical properties under pressure

    NASA Astrophysics Data System (ADS)

    Ozisik, H. B.; Ateser, E.; Ozisik, H.; Colakoglu, K.; Deligoz, E.

    2017-07-01

    In this study, we have investigated the electronic and optical properties of half-Heusler NiXSn (X = Zr, Hf) compounds under pressure by means of first principles calculations. The generalized gradient approximation is used to model exchange-correlation effects. We have estimated a transition from indirect band gap to direct band gap at 50 and 127 GPa for NiZrSn and NiHfSn, respectively. We have also plotted the static dielectric constant versus pressure for both compounds. The obtained results are in agreement with the available experimental and theoretical data.

  1. Thermoelectric properties and electronic structure of substituted Heusler compounds: NiTi0.3-xScxZr0.35Hf0.35Sn

    NASA Astrophysics Data System (ADS)

    Ouardi, Siham; Fecher, Gerhard H.; Balke, Benjamin; Schwall, Michael; Kozina, Xeniya; Stryganyuk, Gregory; Felser, Claudia; Ikenaga, Eiji; Yamashita, Yoshiyuki; Ueda, Shigenori; Kobayashi, Keisuke

    2010-12-01

    The effect of Ti substitution by Sc on the thermoelectric properties of the Heusler compounds NiTi0.3-xScxZr0.35Hf0.35Sn (where 0compound NiTi0.3Zr0.35Hf0.35Sn showed n-type conductivity with a Seebeck coefficient of -288 μV/K at 350 K, while under Sc substitution the system switched to p-type behavior. A maximum Seebeck coefficient of +230 μV/K (350 K) was obtained by 4% Sc substitution, which is the highest value for p-type thermoelectric compounds based on Heusler alloys. The electronic structure was studied by photoelectron spectroscopy excited by hard x-ray synchrotron radiation. Massive in gap states are observed for the parent compound. This proves that the electronic states close to the Fermi energy play a key role on the behavior of the transport properties. Especially, they are responsible for the high, negative Seebeck coefficient of the parent compound.

  2. First principle study of structural, electronic and magnetic properties of half-Heusler IrCrZ (Z=Ge, As, sn and sb) compounds

    NASA Astrophysics Data System (ADS)

    Allaf Behbahani, Marzieh; Moradi, Mahmood; Rostami, Mohammad; Davatolhagh, Saeed

    2016-05-01

    First-principle calculations based on the density functional theory for new half-Heusler IrCrZ (Z=Ge, As, Sn and Sb) alloys are performed. It is found that the half-Heusler IrCrGe and IrCrSn compounds have an antiferromagnetic ground state while the ferromagnetic state is more stable than the antiferromagnetic and non-magnetic states for both IrCrAs and IrCrSb compounds. IrCrAs and IrCrSb exhibit half-metallic property with integer magnetic moments of 2.00 μB per formula unit and half-metallic gaps of 0.28 and 0.27 eV at their equilibrium volume, respectively. In addition, the density of states (DOSs) and band structures of IrCrAs and IrCrSb compounds are studied and the origin of their half-metallic gaps are discussed in detail. The estimation of Curie temperatures of IrCrAs and IrCrSb compounds is performed within the mean field approximation (MFA). The Curie temperatures of IrCrAs and IrCrSb are estimated to be 1083 and 1470 K, respectively. The stability of the half-metallicity in IrCrAs and IrCrSb compounds with the variation of lattice constant are also investigated.

  3. Ground state properties and thermoelectric behavior of Ru2VZ (Z=Si, ge, sn) half-metallic ferromagnetic full-Heusler compounds

    NASA Astrophysics Data System (ADS)

    Yalcin, Battal Gazi

    2016-06-01

    The ground state properties namely structural, mechanical, electronic and magnetic properties and thermoelectric behavior of Ru2VZ (Z=Si, Ge and Sn) half-metallic ferromagnetic full-Heusler compounds are systematically investigated. These compounds are ferromagnetic and crystallize in the Heusler type L21 structure (prototype: Cu2MnAl, Fm-3m 225). This result is confirmed for Ru2VSi and Ru2VSn by experimental work reported by Yin and Nash using high temperature direct reaction calorimetry. The studied materials are half-metallic ferromagnets with a narrow direct band gap in the minority spin channel that amounts to 31 meV, 66 meV and 14 meV for Ru2VSi, Ru2VGe, and Ru2VSn, respectively. The total spin magnetic moment (Mtot) of the considered compounds satisfies a Slater-Pauling type rule for localized magnetic moment systems (Mtot=(NV-24)μB), where NV=25 is the number of valence electrons in the primitive cell. The Curie temperature within the random phase approximation (RPA) is found to be 23 K, 126 K and 447 K for Ru2VSi, Ru2VGe and Ru2VSn, respectively. Semi-classical Boltzmann transport theories have been used to obtain thermoelectric constants, such as Seebeck coefficient (S), electrical (σ/τ) and thermal conductivity (κ/τ), power factor (PF) and the Pauli magnetic susceptibility (χ). ZTMAX values of 0.016 (350 K), 0.033 (380 K) and 0.063 (315 K) are achieved for Ru2VSi, Ru2VGe and Ru2VSn, respectively. It is expected that the obtained results might be a trigger in future experimentally interest in this type of full-Heusler compounds.

  4. Exploration of new multifunctional magnetic materials based on a variety of Heusler alloys and rare-earth compounds

    NASA Astrophysics Data System (ADS)

    Pathak, Arjun Kumar

    2011-12-01

    Magnetic, magnetocaloric, magnetotransport and magnetoelastic properties of Ni-Mn-X (X = In, and Ga) Heusler alloys and La-Fe-Si based rare earth compounds have been synthesized and investigated by x-ray diffraction, magnetization, strain, and electrical resistivity measurements. The phase transitions, magnetic, magnetocaloric, magnetotransport and magnetoelastic properties strongly depend on the composition of these systems. In Ni50Mn50-xInx with x = 13.5, magnetocaloric and magnetotransport properties associated with the paramagnetic martensitic to paramagnetic austenitic transformation were studied. It was shown that magnetic entropy changes (DeltaSM) and magnetoresistance (MR) associated with this transformation are larger and the hysteresis effect is significantly lower when compared to that associated with paramagnetic-ferromagnetic transitions or ferromagnetic-antiferromagnetic/paramagnetic transitions in other systems. The Hall resistivity and the Hall angle shows unusual behavior in the vicinity of the martensitic phase transition for Ni50Mn 50-xInx with x = 15.2. The observed Hall resistivity and Hall angle are 50 μO·cm and tan-1 0.5, respectively. It was observed that the presence of Ge, Al and Si atoms on the In sites strongly affects the crystal structure, and the electric and magnetic behaviors of Ni50Mn35In15. It was found that the partial substitution of In atoms by Si in Ni50Mn35In15 results in an increase in the magnetocaloric effect, exchange bias and shape memory effect. In Ni50Mn35In15-xSi x, the peak values of positive DeltaSM for magnetic field changes H = 5 T were found to depend on composition and vary from 82 J·kg -1·K-1 for x = 1 (at T = 275 K) to 124 J·kg -1·K-1 for x = 3 (at T = 239 K). The partial substitution of Ni by Co in Ni50Mn35In15 significantly improves the magnetocaloric effect and MR in the vicinity of martensitic transition. In addition, significantly large inverse DeltaS M and MR were observed at the inverse

  5. Electronic and magnetic properties of X2YZ and XYZ Heusler compounds: a comparative study of density functional theory with different exchange-correlation potentials

    NASA Astrophysics Data System (ADS)

    Rai, D. P.; Sandeep; Shankar, A.; Pradhan Sakhya, Anup; Sinha, T. P.; Khenata, R.; Ghimire, M. P.; Thapa, R. K.

    2016-07-01

    The electronic and magnetic properties of Heusler compounds X2YZ and XYZ (X = Co, Ni, Pt, Fe; Y = Mn, Cr, Vi; Z = Al, Sb, Ga) are investigated by using the density functional theory with generalized gradient approximation (GGA), GGA plus U (LSDA+U), and modified Becke-Johnson (mBJ) exchange potential. It is found that the half-metallic gaps are generally widened reasonably by LSDA+U and mBJ as compared to the conventional GGA. For the Co-based Heusler compounds the inclusion of U in GGA leads to a larger minority band gap while it is destroyed for Fe2VAl and NiMnSb. The magnetic properties of Co2VSi and Co2VSn are well defined within LSDA+U and mBJ with an exact integer value of magnetic moment. The band gaps of Fe2VAl and CoMnSb given by mBJ are in good agreement with the available experimental data of x-ray absorption spectroscopy. Except for the reasonably larger band gap, the mBJ band structure is almost same as that of GGA but is remarkably different from that of LSDA+U.

  6. Effects of Ga substitution on the structural and magnetic properties of half metallic Fe{sub 2}MnSi Heusler compound

    SciTech Connect

    Pedro, S. S. Caraballo Vivas, R. J.; Andrade, V. M.; Cruz, C.; Paixão, L. S.; Contreras, C.; Costa-Soares, T.; Rocco, D. L.; Reis, M. S.; Caldeira, L.; Coelho, A. A.; Carvalho, A. Magnus G.

    2015-01-07

    The so-called half-metallic magnets have been proposed as good candidates for spintronic applications due to the feature of exhibiting a hundred percent spin polarization at the Fermi level. Such materials follow the Slater-Pauling rule, which relates the magnetic moment with the valence electrons in the system. In this paper, we study the bulk polycrystalline half-metallic Fe{sub 2}MnSi Heusler compound replacing Si by Ga to determine how the Ga addition changes the magnetic, the structural, and the half-metal properties of this compound. The material does not follow the Slater-Pauling rule, probably due to a minor structural disorder degree in the system, but a linear dependence on the magnetic transition temperature with the valence electron number points to the half-metallic behavior of this compound.

  7. First-Principles Prediction of Electronic, Magnetic, and Optical Properties of Co2MnAs Full-Heusler Half-Metallic Compound

    NASA Astrophysics Data System (ADS)

    Bakhshayeshi, A.; Sarmazdeh, M. Majidiyan; Mendi, R. Taghavi; Boochani, A.

    2016-12-01

    Electronic, magnetic, and optical properties of Co2MnAs full-Heusler compound have been calculated using a first-principles approach with the full-potential linearized augmented plane-wave (FP-LAPW) method and generalized gradient approximation plus U (GGA + U). The results are compared with various properties of Co2MnZ (Z = Si, Ge, Al, Ga, Sn) full-Heusler compounds. The results of our calculations show that Co2MnAs is a half-metallic ferromagnetic compound with 100% spin polarization at the Fermi level. The total magnetic moment and half-metallic gap of Co2MnAs compound are found to be 6.00μ B and 0.43 eV, respectively. It is also predicted that the spin-wave stiffness constant and Curie temperature of Co2MnAs compound are about 3.99 meV nm2 and 1109 K, respectively. The optical results show that the dominant behavior, at energy below 2 eV, is due to interactions of free electrons in the system. Interband optical transitions have been calculated based on the imaginary part of the dielectric function and analysis of critical points in the second energy derivative of the dielectric function. The results show that there is more than one plasmon energy for Co2MnAs compound, with the highest occurring at 25 eV. Also, the refractive index variations and optical reflectivity for radiation at normal incidence are calculated for Co2MnAs. Because of its high magnetic moment, high Curie temperature, and 100% spin polarization at the Fermi level as well as its optical properties, Co2MnAs is a good candidate for use in spintronic components and magnetooptical devices.

  8. First-Principles Prediction of Electronic, Magnetic, and Optical Properties of Co2MnAs Full-Heusler Half-Metallic Compound

    NASA Astrophysics Data System (ADS)

    Bakhshayeshi, A.; Sarmazdeh, M. Majidiyan; Mendi, R. Taghavi; Boochani, A.

    2017-04-01

    Electronic, magnetic, and optical properties of Co2MnAs full-Heusler compound have been calculated using a first-principles approach with the full-potential linearized augmented plane-wave (FP-LAPW) method and generalized gradient approximation plus U (GGA + U). The results are compared with various properties of Co2Mn Z ( Z = Si, Ge, Al, Ga, Sn) full-Heusler compounds. The results of our calculations show that Co2MnAs is a half-metallic ferromagnetic compound with 100% spin polarization at the Fermi level. The total magnetic moment and half-metallic gap of Co2MnAs compound are found to be 6.00 μ B and 0.43 eV, respectively. It is also predicted that the spin-wave stiffness constant and Curie temperature of Co2MnAs compound are about 3.99 meV nm2 and 1109 K, respectively. The optical results show that the dominant behavior, at energy below 2 eV, is due to interactions of free electrons in the system. Interband optical transitions have been calculated based on the imaginary part of the dielectric function and analysis of critical points in the second energy derivative of the dielectric function. The results show that there is more than one plasmon energy for Co2MnAs compound, with the highest occurring at 25 eV. Also, the refractive index variations and optical reflectivity for radiation at normal incidence are calculated for Co2MnAs. Because of its high magnetic moment, high Curie temperature, and 100% spin polarization at the Fermi level as well as its optical properties, Co2MnAs is a good candidate for use in spintronic components and magnetooptical devices.

  9. Completely compensated ferrimagnetism and sublattice spin crossing in the half-metallic Heusler compound Mn1.5FeV0.5Al

    NASA Astrophysics Data System (ADS)

    Stinshoff, Rolf; Nayak, Ajaya K.; Fecher, Gerhard H.; Balke, Benjamin; Ouardi, Siham; Skourski, Yurii; Nakamura, Tetsuya; Felser, Claudia

    2017-02-01

    The Slater-Pauling rule states that L 21 Heusler compounds with 24 valence electrons never exhibit a total spin magnetic moment. In the case of strongly localized magnetic moments at one of the atoms (here Mn) they will exhibit a fully compensated half-metallic ferrimagnetic state instead, in particular, when symmetry does not allow for antiferromagnetic order. With the aid of magnetic and anomalous Hall effect measurements, it is experimentally demonstrated that Mn1.5V0.5FeAl follows such a scenario. The ferrimagnetic state is tuned by the composition. A small residual magnetization, which arises due to a slight mismatch of the magnetic moments in the different sublattices, results in a pronounced change of the temperature dependence of the ferrimagnet. A compensation point is confirmed by observation of magnetic reversal and sign change of the anomalous Hall effect. Theoretical models are presented that correlate the electronic structure and the compensation mechanisms of the different half-metallic ferrimagnetic states in the Mn-V-Fe-Al Heusler system.

  10. Half-metallic ferromagnetism in full-Heusler compounds ACaX{sub 2} (A = K and Rb; X = N and O)

    SciTech Connect

    Umamaheswari, R. Vijayalakshmi, D. Yogeswari, M. Kalpana, G.

    2014-04-24

    Electronic structure and magnetic properties of hypothetical ACaX{sub 2} (A = K and Rb; X= N and O) compounds in full-Heusler phase have been investigated based on density functional theory (DFT) within the local density approximation (LDA). The electronic band structures and density of states of these compounds show that the spin-down electrons have metallic, and the spin-up electrons have a semi conducting gap resulting in stable half-metallic ferromagnetic behaviour. The strong spin polarization of 2p states of N and O atoms is found to be the origin of ferromagnetism which results in a total magnetic moment of 3{sub μB} and 1{sub μB} respectively.

  11. Exchange-spring like magnetic behavior of the tetragonal Heusler compound Mn2FeGa as a candidate for spin-transfer torque

    NASA Astrophysics Data System (ADS)

    Gasi, Teuta; Nayak, Ajaya K.; Winterlik, Jürgen; Ksenofontov, Vadim; Adler, Peter; Nicklas, Michael; Felser, Claudia

    2013-05-01

    We report structural, magnetic, and Mössbauer studies of the Heusler compound Mn2FeGa. Theoretical calculations predict that a tetragonal phase in Mn2FeGa could be an interesting candidate for spin torque transfer applications due to the presence of perpendicular magnetic anisotropy. Experimentally, we found that Mn2FeGa crystallizes in a tetragonal structure after annealing at low temperatures (≤400 °C), whereas, it becomes pseudocubic for higher annealing temperatures. The sample annealed at 400 °C shows a high Curie temperature of 650 K and a hard-magnetic behavior. We observed a nonsaturating and exchange-spring type of hysteresis loops, which indicates that the sample contains two different magnetic states. The Mössbauer measurements clearly support the structural and magnetic data. All these properties make the material a potential candidate for spintronic devices, especially in thin films with perpendicular magnetic anisotropy.

  12. Thermoelectric Figures of Merit of Zn4Sb3 and Zrnisn-based Half-heusler Compounds Influenced by Mev Ion-beam Bombardments

    NASA Astrophysics Data System (ADS)

    Budak, S.; Guner, S.; Muntele, C. I.; Ila, D.

    Semiconducting β-Zn4Sb3 and ZrNiSn-based half-Heusler compound thin films with applications as thermoelectric (TE) materials were prepared using ion beam assisted deposition (IBAD). High-purity solid zinc (Zn) and antimony (Sb) were evaporated by electron beam to grow the β-Zn4Sb3 thin film while high-purity zirconium (Zr) powder and nickel (Ni) tin (Sn) powders were evaporated by electron beam to grow the ZrNiSn-based half-Heusler compound thin film. Rutherford backscattering spectrometry (RBS) was used to analyze the composition of the thin films. The grown thin films were subjected to 5 MeV Si ions bombardment for generation of nanostructures in the films. We measured the thermal conductivity, Seebeck coefficient, and electrical conductivity of these two systems before and after 5 MeV Si ions beam bombardment. The two material systems have been identified as promising TE materials for the application of thermal-to-electrical energy conversion, but the efficiency still limits their applications. The electronic energy deposited due to ionization in the track of MeV ion beam couldcause localized crystallization. The nanostructures produced by MeV ion beam can cause significant change in both the electrical and the thermal conductivity of thin films, thereby improving the efficiency. We used the 3ω-method (3rd harmonic) measurement system to measure the cross-plane thermal conductivity, the van der Pauw measurement system to measure the electrical conductivity, and the Seebeck-coefficient measurement system to measure the cross-plane Seebeck coefficient. The thermoelectric figures of merit of the two material systems were then derived by calculations using the measurement results. The MeV ion-beam bombardment was found to decrease the thermal conductivity of thin films and increase the efficiency of thermal-to-electrical energy conversion.

  13. Ab Initio Prediction of the Structural, Electronic, Elastic, and Thermoelectric Properties of Half-Heusler Ternary Compounds TiIrX (X = As and Sb)

    NASA Astrophysics Data System (ADS)

    Chibani, S.; Arbouche, O.; Zemouli, M.; Amara, K.; Benallou, Y.; Azzaz, Y.; Belgoumène, B.; Bentayeb, A.; Ameri, M.

    2017-08-01

    The structural, electronic, elastic, and thermoelectric properties of TiIrX (X = As and Sb) half-Heusler compounds with 18 valence electrons were studied using density functional theory. The generalized gradient approximation of Perdew-Burke and Ernzerhof used for calculation of the structural parameters and elastic properties of TiIrAs and TiIrSb denotes that the computed lattice constants were in excellent agreement with the available experimental data and previous theoretical works. Furthermore, the calculated elastic constants for both compounds satisfy the Born criteria indicating their mechanical stabilities. The modified Becke-Johnson potential (TB-mBJ) was used to provide a better description of the electronic structures, which indicate that both compounds are narrow-gap semiconductors. Additionally, the investigations of thermoelectric performance were carried out using the results of ab initio band-structure calculations and the semi-classical Boltzmann theory within the constant relaxation time approximations. The predicted values of the figure of merit ZT e are close to unity at room temperature. This reveals that TiIrAs and TiIrSb compounds are excellent candidates for practical applications in the thermoelectric devices.

  14. Structural, electronic, elastic, thermoelectric and thermodynamic properties of the NbMSb half heusler (M=Fe, Ru, Os) compounds with first principle calculations

    NASA Astrophysics Data System (ADS)

    Abid, O. Miloud; Menouer, S.; Yakoubi, A.; Khachai, H.; Omran, S. Bin; Murtaza, G.; Prakash, Deo; Khenata, R.; Verma, K. D.

    2016-05-01

    The structural, electronic, elastic, thermoelectric and thermodynamic properties of NbMSb (M = Fe, Ru, Os) half heusler compounds are reported. The full-potential linearized augmented plane wave (FP-LAPW) plus local orbital (lo) method, based on the density functional theory (DFT) was employed for the present study. The equilibrium lattice parameter results are in good compliance with the available experimental measurements. The electronic band structure and Boltzmann transport calculations indicated a narrow indirect energy band gap for the compound having electronic structure favorable for thermoelectric performance as well as with substantial thermopowers at temperature ranges from 300 K to 800 K. Furthermore, good potential for thermoelectric performance (thermopower S ≥ 500 μeV) was found at higher temperature. In addition, the analysis of the charge density, partial and total densities of states (DOS) of three compounds demonstrate their semiconducting, ionic and covalent characters. Conversely, the calculated values of the Poisson's ratio and the B/G ratio indicate their ductile makeup. The thermal properties of the compounds were calculated by quasi-harmonic Debye model as implemented in the GIBBS code.

  15. Superconducting properties of Zr1+xNi2-xGa and Zr1-xNi2+xGa Heusler compounds

    NASA Astrophysics Data System (ADS)

    Alzahrani, Saad; Khan, Mahmud

    2017-05-01

    The superconducting properties of a series of Zr1+xNi2-xGa and Zr1-xNi2+xGa compounds have been investigated by x-ray diffraction, electrical resistivity, dc magnetization, and ac susceptibility measurements. While the parent compound, ZrNi2Ga, exhibited the cubic L21 Heusler structure, multiple non-cubic structures formed in the Zr and Ni rich doped materials. For x ≤ 0.3, all Zr1-xNi2+xGa compounds demonstrated superconducting behavior, but no superconductivity was observed in the Zr1+xNi2-xGa alloys for x > 0.2. The magnetization data revealed that all materials in both Zr1+xNi2-xGa and Zr1-xNi2+xGa series exhibited type-II superconductivity. With increasing doping concentration x, the paramagnetic ordering were enhanced in both systems while the superconducting properties were found to weaken. The observations are discussed considering the structural disorders in the systems.

  16. Electronic structure and thermoelectric properties of half-Heusler compounds with eight electron valence count—KScX (X = C and Ge)

    SciTech Connect

    Ciftci, Yasemin O.; Mahanti, Subhendra D.

    2016-04-14

    Electronic band structure and structural properties of two representative half-Heusler (HH) compounds with 8 electron valence count (VC), KScC and KScGe, have been studied using first principles methods within density functional theory and generalized gradient approximation. These systems differ from the well studied class of HH compounds like ZrNiSn and ZrCoSb which have VC = 18 because of the absence of d electrons of the transition metal atoms Ni and Co. Electronic transport properties such as Seebeck coefficient (S), electrical conductivity (σ), electronic thermal conductivity (κ{sub e}) (the latter two scaled by electronic relaxation time), and the power factor (S{sup 2}σ) have been calculated using semi-classical Boltzmann transport theory within constant relaxation time approximation. Both the compounds are direct band gap semiconductors with band extrema at the X point. Their electronic structures show a mixture of heavy and light bands near the valance band maximum and highly anisotropic conduction and valence bands near the band extrema, desirable features of good thermoelectric. Optimal p- or n-type doping concentrations have been estimated based on thermopower and maximum power factors. The optimum room temperature values of S are ∼1.5 times larger than that of the best room temperature thermoelectric Bi{sub 2}Te{sub 3}. We also discuss the impact of the band structure on deviations from Weidemann-Franz law as one tunes the chemical potential across the band gap.

  17. Electronic structure and thermoelectric properties of half-Heusler compounds with eight electron valence count—KScX (X = C and Ge)

    NASA Astrophysics Data System (ADS)

    Ciftci, Yasemin O.; Mahanti, Subhendra D.

    2016-04-01

    Electronic band structure and structural properties of two representative half-Heusler (HH) compounds with 8 electron valence count (VC), KScC and KScGe, have been studied using first principles methods within density functional theory and generalized gradient approximation. These systems differ from the well studied class of HH compounds like ZrNiSn and ZrCoSb which have VC = 18 because of the absence of d electrons of the transition metal atoms Ni and Co. Electronic transport properties such as Seebeck coefficient (S), electrical conductivity (σ), electronic thermal conductivity (κe) (the latter two scaled by electronic relaxation time), and the power factor (S2σ) have been calculated using semi-classical Boltzmann transport theory within constant relaxation time approximation. Both the compounds are direct band gap semiconductors with band extrema at the X point. Their electronic structures show a mixture of heavy and light bands near the valance band maximum and highly anisotropic conduction and valence bands near the band extrema, desirable features of good thermoelectric. Optimal p- or n-type doping concentrations have been estimated based on thermopower and maximum power factors. The optimum room temperature values of S are ˜1.5 times larger than that of the best room temperature thermoelectric Bi2Te3. We also discuss the impact of the band structure on deviations from Weidemann-Franz law as one tunes the chemical potential across the band gap.

  18. First Principles Investigation of the Elastic, Optoelectronic and Thermal Properties of XRuSb: (X = V, Nb, Ta) Semi-Heusler Compounds Using the mBJ Exchange Potential

    NASA Astrophysics Data System (ADS)

    Bencherif, K.; Yakoubi, A.; Della, N.; Miloud Abid, O.; Khachai, H.; Ahmed, R.; Khenata, R.; Bin Omran, S.; Gupta, S. K.; Murtaza, G.

    2016-07-01

    Semi-Heusler materials are intensively investigated due to their potential use in diverse applications, such as in spintronics and green energy applications. In this work, we employ the density functional theory to calculate the structural, electronic, elastic, thermal and optical properties of the VRuSb, NbRuSb and TaRuSb semi-Heusler compounds. The calculated results for the lattice constants, bulk moduli and their corresponding pressure derivative values are in fairly good agreement with previous works. In addition, besides the local density approximation, the modified Becke-Johnson exchange potential is also used to improve the value of the band gaps. The bonding nature reveals a mixture of covalent and ionic bonding character of the VRuSb, NbRuSb and TaRuSb compounds. Furthermore, the elastic constants ( C ij) and the related elastic moduli confirm their stability in the cubic phase and demonstrate their ductile nature. We also analyze the influence of the pressure and temperature on the primitive cell volume, heat capacity, volume expansion coefficient, and Debye temperature of the semi-Heusler compounds. Additionally, we investigate the optical properties, such as the complex dielectric function, refractive index, reflectivity, and the energy loss function.

  19. Quantitative analysis of anisotropic magnetoresistance in Co{sub 2}MnZ and Co{sub 2}FeZ epitaxial thin films: A facile way to investigate spin-polarization in half-metallic Heusler compounds

    SciTech Connect

    Sakuraba, Y. Hirayama, Y.; Furubayashi, T.; Sukegawa, H.; Li, S.; Takahashi, Y. K.; Hono, K.; Kokado, S.

    2014-04-28

    Anisotropic magnetoresistance (AMR) effect has been systematically investigated in various Heusler compounds Co{sub 2}MnZ and Co{sub 2}FeZ (Z = Al, Si, Ge, and Ga) epitaxial films and quantitatively summarized against the total valence electron number N{sub V}. It was found that the sign of AMR ratio is negative when N{sub V} is between 28.2 and 30.3, and turns positive when N{sub V} becomes below 28.2 and above 30.3, indicating that the Fermi level (E{sub F}) overlaps with the valence or conduction band edges of half-metallic gap at N{sub V} ∼ 28.2 or 30.3, respectively. We also find out that the magnitude of negative AMR ratio gradually increases with shifting of E{sub F} away from the gap edges, and there is a clear positive correlation between the magnitude of negative AMR ratio and magnetoresistive output of the giant magnetoresistive devices using the Heusler compounds. This indicates that AMR can be used as a facile way to optimize a composition of half-metallic Heusler compounds having a high spin-polarization at room temperature.

  20. Quaternary Heusler compounds Co(2-x)Rh(x)MnZ (Z = Ga, Sn, Sb): crystal structure, electronic structure, and magnetic properties.

    PubMed

    Alijani, Vajiheh; Winterlik, Juergen; Fecher, Gerhard H; Naghavi, S Shahab; Chadov, Stanislav; Gruhn, Thomas; Felser, Claudia

    2012-02-01

    Within the huge family of Heusler compounds only a few quaternary derivatives are known that crystallize in the F43m space group. In this work, the yet unreported compounds CoRhMnZ (Z = Ga, Sn, Sb) and the alloy Co(0.5)Rh(1.5)MnSb were investigated in detail by experimental techniques and theoretical methods. The ab initio calculations predict the CoRhMnZ compounds to be half-metallic ferromagnets or to be close to the half-metallic ferromagnetic state. Calculations of the elastic constants show that the cubic structure is stable in compounds containing Mn. Both calculations and experiment reveal that Mn cannot be exchanged by Fe (CoRhFeGa). The low temperature magnetization of the compounds is in the range of 3.4-5.5 μ(B) depending on the composition. The best agreement between experiment and calculation has been achieved for CoRhMnSn (5 μ(B)). The other compounds are also cubic but tend to anti-site disorder. Compared to Co(2)MnSn it is interesting to note that the magnetic properties and half-metallicity are preserved when replacing one of the 'magnetic' Co atoms by a 'non-magnetic' Rh atom. This allows us to increase the spin-orbit interaction at one of the lattice sites while keeping the properties as a precondition for applications and physical effects relying on a large spin-orbit interaction. The Curie temperatures were determined from measurements in induction fields of up to 1 T by applying molecular field fits respecting the applied field. The highest Curie temperature was found for CoRhMnSn (620 K) that makes it, together with the other well defined properties, attractive for above room temperature spintronic applications.

  1. First-principles study on the half-metallic properties of the d0 quaternary Heusler compounds: KCaCBr and KCaCI

    NASA Astrophysics Data System (ADS)

    Du, Jiangtao; Dong, Shengjie; Wang, X. T.; Zhao, Hui; Wang, L. Y.; Feng, L. F.

    2016-10-01

    With ab initio calculations, we studied the structural, electronic, and magnetic properties of quaternary Heusler compounds KCaCX (X = Br and I) adopted stable and metastable phases. We found that the most stable structure is type-3 atomic arrangement configuration where K, Ca, C, and X atoms occupy (0, 0, 0), (0.5, 0.5, 0.5), (0.25, 0.25, 0.25) and (0.75, 0.75, 0.75) positions, respectively. The metastable one is type-1 configuration where K, C, Ca and X occupy (0, 0, 0), (0.5, 0.5, 0.5), (0.25, 0.25, 0.25) and (0.75, 0.75, 0.75) positions, respectively. The results show that they are half-metallic ferromagnets with integer magnetic moments of 2.0 μB at their equilibrium lattice constants. It is also found that the spin-polarization is mainly from the C-2p states. In addition, KCaCBr compound with type-1 configuration shows an electronic transition from ordinary half-metallic state to zero-gap half-metallic state with the changing the lattice parameter.

  2. Half-metallicity and magnetism of the full-Heusler compounds KYX2 (Y=Ti, V, and Cr; X=C, N, and O)

    NASA Astrophysics Data System (ADS)

    Dehghanzadeh, M.; Ahmadian, F.

    2017-02-01

    The electronic structure and half-metallic (HM) properties of new alloys KYX2 (Y=Ti, V, and Cr; X=C, N, and O) containing transition metals and sp elements were investigated within the density functional theory (DFT) using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method. It was found that these new compounds can be experimentally synthesized because of their negative formation energies. The total energy calculations showed that in all compounds, the stable state structure was a ferromagnetic AlCu2Mn-type structure except for KTiC2 and KTiN2 which were stable in a nonmagnetic (NM) AlCu2Mn-type structure. The KTiO2 in both structures, KCrO2 in AlCu2Mn-type structure, and KVO2 in CuHg2Ti-type structure were half-metallic ferromagnets. KVO2 in AlCu2Mn-type structure was a special case with a ferromagnetic semiconducting behavior. The origin of minority band gaps for KTiO2 in both structures was also studied using the band structure calculations. The total magnetic moments of HM compounds were integer values which were in agreement with Slater-Pauling rule (Mtot=Ztot-12). Furthermore, the regions of half-metallictiy in HM compounds were considerably wider than those of Heusler compounds including transition metals, indicating the high robustness of half-metallicity with variation of lattice constants.

  3. Engineering half-Heusler thermoelectric materials using Zintl chemistry

    NASA Astrophysics Data System (ADS)

    Zeier, Wolfgang G.; Schmitt, Jennifer; Hautier, Geoffroy; Aydemir, Umut; Gibbs, Zachary M.; Felser, Claudia; Snyder, G. Jeffrey

    2016-06-01

    Half-Heusler compounds based on XNiSn and XCoSb (X = Ti, Zr or Hf) have rapidly become important thermoelectric materials for converting waste heat into electricity. In this Review, we provide an overview on the electronic properties of half-Heusler compounds in an attempt to understand their basic structural chemistry and physical properties, and to guide their further development. Half-Heusler compounds can exhibit semiconducting transport behaviour even though they are described as ‘intermetallic’ compounds. Therefore, it is most useful to consider these systems as rigid-band semiconductors within the framework of Zintl (or valence-precise) compounds. These considerations aid our understanding of their properties, such as the bandgap and low hole mobility because of interstitial Ni defects in XNiSn. Understanding the structural and bonding characteristics, including the presence of defects, will help to develop different strategies to improve and design better half-Heusler thermoelectric materials.

  4. Pressure-induced electronic, magnetic, half-metallic, and mechanical properties of half-Heusler compound CoCrBi

    NASA Astrophysics Data System (ADS)

    Huang, H. M.; Luo, S. J.; Xiong, Y. C.

    2017-09-01

    The structural, electronic, magnetic, half-metallic and mechanical properties of half-Heusler alloy CoCrBi under pressure have been studied by first-principle calculations. CoCrBi is found to be half-metallic under the uniform pressure from 7.45 to 295 GPa, and the total magnetic moment is 2.00 μB per formula unit. The transitions from metal to half-metal and from half-metal to metal occur at 7.45 and 295 GPa, respectively. The calculations show that the uniform pressure has an important influence on the minority-spin states, leading to a slight reconstruction of the minority-spin states with a shift of Fermi level. The calculated elastic constants revel that the mechanical instability will occur if the pressure is greater than 47.6 GPa. Two kinds of tetragonal distortions under non-uniform pressure are also studied. It is found that the change of c/a ratio has no obvious influence on the electronic structures and magnetic properties, whereas, compressing the lattice constant in one direction can also realize the transition from metallic to half-metallic properties for CoCrBi.

  5. Validity of Rigid-Band Approximation in the Study of Thermoelectric Properties of p-Type FeNbSb-Based Half-Heusler Compounds

    NASA Astrophysics Data System (ADS)

    Fang, Teng; Zheng, Shuqi; Zhou, Tian; Chen, Hong; Zhang, Peng

    2016-11-01

    Recently, we calculated the thermoelectric properties of p-type FeNbSb half-Heusler compounds by employing the rigid-band approximation (RBA) (Fang et al., RSC Adv 6:10507-10512, 2016). Traditionally, the RBA is used to understand and guide doping in semiconductors. It is therefore important to verify its reliability. To this end, we have investigated the validity of the RBA in heavily doped p-type FeNbSb by calculating the electronic structure and Seebeck coefficient of pure and Ti-, Zr-, Hf-, and Ce-doped FeNbSb using ab initio calculations. The results confirm that Ti, Zr, and Hf doping at Nb site shows rigid-band-like behavior, unlike Ce doping, which changes the density of states. We also calculated the electrical transport properties of the doped systems, indicating that the power factor of Ce-doped FeNbSb is lower than those of Ti-, Zr-, and Hf-doped FeNbSb.

  6. Electronic structure, transport properties, and excited states in CoTiSb, CoZrSb, and CoHfSb half-Heusler compounds

    NASA Astrophysics Data System (ADS)

    Janotti, Anderson; Gui, Zhigang; Kawasaki, Jason; Palmstrom, Chris; Himmetoglu, Burak

    CoTiSb is a member of a large family of half-Heusler compounds with 18 valence electrons. CoTiSb is semiconductor material with a band gap a little over 1 eV, and it has been considered promising for thermoelectric applications. It can be grown on conventional III-V semiconductors, and could potentially be integrated in III-V devices. Here we present results of first-principles calculations of electronic structure, transport properties, and excited states in CoTiSb, as well as CoZrSb and CoHfSb. Electronic structures are studied using density functional theory within the local density approximation, hybrid functional and quasiparticle GW methods. Both room-temperature Seebeck coefficient and carrier mobility are calculated from first-principles. We also determine the band alignments to III-V semiconductors, and all the results are presented and discussed in the light of available experimental data. This work was supported by the DOE.

  7. First principles study of a new half-metallic ferrimagnets Mn2-based full Heusler compounds: Mn2ZrSi and Mn2ZrGe

    NASA Astrophysics Data System (ADS)

    Abada, A.; Amara, K.; Hiadsi, S.; Amrani, B.

    2015-08-01

    Half-metallic properties of new predicted Mn2-based full Heusler alloys Mn2ZrSi and Mn2ZrGe have been studied by first-principles full-potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method based on density functional theory (DFT). Our investigation is focused on the structural, elastic, electronic and magnetic properties of these compounds. The AlCu2Mn-type structure is found to be energetically more favorable than the CuHg2Ti-type structure for both compounds and are half-metallic ferrimagnets (HMFIs) with total magnetic moments of 2.000μB per formula unit, well consistent with Slater-Pauling rule (Mtot=(24-Ztot)μB). Calculations show that both the alloys have an indirect band gaps, in the majority-spin channel, with values of 0.505 eV and 0.278 eV for Mn2ZrSi and Mn2ZrGe, respectively. It was found that Mn2ZrSi and Mn2ZrGe preserved their half-metallicity for lattice constants range of 5.85-6.38 Å and 6.05-6.38 Å, respectively, and kept a 100% of spin polarization at the Fermi level. Moreover, the calculated formation energies and elastic constants confirm that these compounds are stable chemically and mechanically, and the good crystallographic compatibility with the lattice of semiconductors used industrially makes them promising magnetic materials in spintronic applications.

  8. CO2-based Flows on Ancient and Modern Mars.

    NASA Astrophysics Data System (ADS)

    Hoffman, N.

    2002-12-01

    Outburst flood channels from the Hesperian and Amazonian Epochs of Mars have conventionally been interpreted as evidence for catastrophic release of groundwater and surface floods akin to jokulhlaups. The Channeled Scablands of Washington state, USA are a type example of this mechanism. However, on Mars there are problems with storage of the large volumes of water in the subsurface, and little evidence for surface impoundments. To explain the volume of erosion requires multiple floods from each source area, which leads to problems of recharge on a cryogenic planet. An alternative model for the floods has been developed in the last few years that explains the outbursts as the violent escape of pressurized liquid CO2, rather than liquid water. The CO2 is trapped underground beneath frozen icy regolith (permafrost) up to 1 km thick, which provides an effective topseal. When the outburst begins, explosive degassing generates a debris cloud akin to a volcanic pyroclastic flow, but at cryogenic temperature. The cloud flows downhill as a density flow, and could potentially erode the observed channels on Mars. Other terrestrial analogues include submarine density flows, which display considerable morphological similarities to Martian channels. There remain some significant problems with CO2-based flow models. To date, no numerical flow model has been offered to support the intuitive conceptual model, and the degree of erosion vs deposition does not match expectation from small-scale flows on Earth. Progress on a numerical flow model will be discussed briefly, as well as scaling relationships that may explain the degree of erosion seen in the channels of Mars. Acknowledging these shortcomings, we nonetheless suggest that the implications of a cold, dry, CO2-based flow model are so significant that the model deserves more attention from the geophysical and planetary science communities. If the model is sustainable, then the implications for the volatile history and thermal

  9. First principle investigations of the structural, electronic and magnetic properties of predicted new zirconium based full-Heusler compounds, Zr2MnZ (Z=Al, Ga and In)

    NASA Astrophysics Data System (ADS)

    Birsan, A.; Kuncser, V.

    2016-05-01

    The crystal structure, electronic and magnetic properties of predicted new full-Heusler compounds Zr2MnZ (Z=Al, Ga, In) were studied within the density functional theory (DFT) framework. These materials exhibit unique properties that connect the spin gapless semiconducting character with the completely compensated ferrimagnetism. Magnetically ordered Zr2MnZ (Z=Al, Ga, In) compounds crystallize in inverse Heusler structure are stable against decomposition and have zero magnetic moment per formula unit, in agreement with Slater-Pauling rule. The Zr2MnAl compound presents semiconducting properties with an energy band gap of 0.41 eV in the majority spin channel and a zero band gap in the minority spin channel. By substituting completely the Al in Zr2MnAl via Ga and In elements, semiconducting pseudo band gaps are formed in the majority spin channels due to different neighborhoods around the manganese atoms, which decreases the energy of Mn triple degenerated anti-bonding states.

  10. Half-Heusler semiconductors as piezoelectrics.

    PubMed

    Roy, Anindya; Bennett, Joseph W; Rabe, Karin M; Vanderbilt, David

    2012-07-20

    We use a first-principles rational-design approach to demonstrate the potential of semiconducting half-Heusler compounds as a previously unrecognized class of piezoelectric materials. We perform a high-throughput scan of a large number of compounds, testing for insulating character and calculating structural, dielectric, and piezoelectric properties. Our results provide guidance for the experimental realization and characterization of high-performance materials in this class that may be suitable for practical applications.

  11. Ultralow Thermal Conductivity in Full Heusler Semiconductors

    NASA Astrophysics Data System (ADS)

    He, Jiangang; Amsler, Maximilian; Xia, Yi; Naghavi, S. Shahab; Hegde, Vinay I.; Hao, Shiqiang; Goedecker, Stefan; OzoliĆš, Vidvuds; Wolverton, Chris

    2016-07-01

    Semiconducting half and, to a lesser extent, full Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full Heusler compounds with ten valence electrons (X2Y Z , X =Ca , Sr, and Ba; Y =Au and Hg; Z =Sn , Pb, As, Sb, and Bi) through high-throughput ab initio screening. These new compounds exhibit ultralow lattice thermal conductivity κL close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials.

  12. Ultralow Thermal Conductivity in Full Heusler Semiconductors.

    PubMed

    He, Jiangang; Amsler, Maximilian; Xia, Yi; Naghavi, S Shahab; Hegde, Vinay I; Hao, Shiqiang; Goedecker, Stefan; Ozoliņš, Vidvuds; Wolverton, Chris

    2016-07-22

    Semiconducting half and, to a lesser extent, full Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full Heusler compounds with ten valence electrons (X_{2}YZ, X=Ca, Sr, and Ba; Y=Au and Hg; Z=Sn, Pb, As, Sb, and Bi) through high-throughput ab initio screening. These new compounds exhibit ultralow lattice thermal conductivity κ_{L} close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials.

  13. Superconductivity and Shubnikov-de Haas oscillations in the noncentrosymmetric half-Heusler compound YPtBi

    NASA Astrophysics Data System (ADS)

    Pavlosiuk, Orest; Kaczorowski, Dariusz; Wiśniewski, Piotr

    2016-07-01

    Recent findings of possible topological superconductivity in YPtBi motivated us to grow and investigate single crystals of this material. The compound was studied by means of ac magnetic susceptibility, electrical resistivity, Hall resistivity, and heat capacity measurements, performed in wide ranges of temperature and magnetic field. The superconductivity below the critical temperature Tc=0.97 K was clearly reflected in the magnetic and electronic transport data. The upper critical field Bc 2 was estimated at 1.84 T. However, no obvious anomaly near Tc was found in the temperature dependence of the heat capacity. In the normal state YPtBi appears semimetallic but its electrical conductance can be described by two components corresponding to parallel channels: metallic and semiconducting, with the latter becoming negligible at temperatures below 40 K. Shubnikov-de Haas oscillations were observed at temperatures below 10 K. Nontrivial Berry phase together with weak antilocalization effect strongly support the presence of Dirac fermions in YPtBi.

  14. High pressure effect on structural, electronic and elastic properties of topological half-Heusler LaPdBi compound

    NASA Astrophysics Data System (ADS)

    Shabara, Reham M.

    2017-08-01

    The elastic, electronic and structural properties of LaPdBi compound were investigated by first-principles density functional theory based on density functional theory (DFT). The lattice constant, energy gap, density of states, band structure, bulk modulus and its first pressure derivative are calculated using the generalized gradient (GGA) approximation. There is a good agreement between our results and available experimental and theoretical values. The lattice constant and energy gap of LaPdBi using the GGA approximation are 6.98 Ǻ and 0.315 eV respectively in fair agreement with experimental results. We calculated bulk modulus and its first pressure-derivative to be 72.25 GPa and 4.43 respectively. We have studied the effect of applying high pressure up, to 80 GPa, on both of the energy gap and bulk modulus of LaPdBi. For example they reached 0.425 eV and 520 GPa respectively at 80 GPa. The effect of high pressure on the density of states is included in this study.

  15. Robust tunability of magnetoresistance in half-Heusler RPtBi (R=Gd, Dy, Tm, and Lu) compounds

    SciTech Connect

    Mun, Eundeok; Bud'ko, Sergey L.; Canfield, Paul C.

    2016-03-15

    We present the magnetic field dependencies of transport properties for RPtBi (R = Gd, Dy, Tm, and Lu) half-Heusler compounds. Temperature- and field-dependent resistivity measurements of high-quality RPtBi single crystals reveal an unusually large, nonsaturating magnetoresistance (MR) up to 300 K under a moderate magnetic field of H = 140 kOe. At 300 K, the large MR effect decreases as the rare earth is traversed from Gd to Lu and the magnetic field dependence of MR shows a deviation from the conventional H2 behavior. The Hall coefficient (RH) for R = Gd indicates a sign change around 120 K, whereas RH curves for R = Dy, Tm, and Lu remain positive for all measured temperatures. At 300 K, the Hall resistivity reveals a deviation from the linear field dependence for all compounds. Thermoelectric power measurements on this family show strong temperature and magnetic field dependencies which are consistent with resistivity measurements. A highly enhanced thermoelectric power under applied magnetic field is observed as high as ~100 μV/K at 140 kOe. Furthermore, analysis of the transport data in this series reveals that the rare-earth-based half-Heusler compounds provide opportunities to tune MR effect through lanthanide contraction and to elucidate the mechanism of nontrivial MR.

  16. Impact of excess iron on the calculated electronic and magnetic properties of Co{sub 3–x}Fe{sub x}Si Heusler-compound

    SciTech Connect

    Debbichi, M.; Hamad, B.

    2014-09-14

    Ab initio density functional calculations are performed on the Co{sub 3–x}Fe{sub x}Si alloys with variable iron composition (1≤x≤3). The evolution of the structural, electronic, and magnetic properties is investigated within different levels of approximations. These alloys crystallize in cubic Heusler structures, which evolve from the regular L2₁ structure for Co₂FeSi to the inverse X structure for x≥2. Using on-site Coulomb interactions of U{sub eff}(Co)=3.07 eV and U{sub eff}(Fe)=3.4 eV is found to describe consistently the experimental properties for x≤2. A good agreement between calculated and experimental magnetic moments is found for the cubic inverse Heusler phases without the addition of Hubbard-model. The spin polarization is found to range from 1 for Fe concentrations of x≤2, indicating the half-metallic character, to –0.29 at x=3.

  17. Surface half-metallicity of half-Heusler compound FeCrSe and interface half-metallicity of FeCrSe/GaP

    NASA Astrophysics Data System (ADS)

    Khalaf Al-zyadi, Jabbar M.; Jolan, Mudhahir H.; Yao, Kai-Lun

    2016-04-01

    Recent studies showed that half-Heusler FeCrSe exhibits half-metallic ferromagnetism (Huang et al. [20]). In this paper, we investigate extensively the electronic, magnetic, and half-metallic properties of the half-Heusler alloy FeCrSe (111) and (001) surfaces and the interface with GaP (111) substrate by using the first-principles calculations within the density functional theory. The atomic density of states demonstrates that the half-me tallicity verified in the bulk FeCrSe is maintained at the CrSe-terminated (001) and Se-terminated (111) surfaces, but lost at both Cr- and Fe-terminated (111) surfaces and the Fe-terminated (001) surface. Alternatively, for the interface of FeCrSe/GaP (111), the bulk half-metallicity is destroyed at Se-P configuration while Se-Ga interface and subinterface show nearly 100% spin polarization. Moreover, the calculated interfacial adhesion energies exhibit that Se-Ga shape is more stable than the Se-P one. The calculated magnetic moments of Se, Ga at the Se-Ga (111) interface and P at the Se-P (111) interface increase with respect to the corresponding bulk values while the atomic magnetic moment of Se atom at the Se-P (111) interface decreases. We also notice that the magnetic moments of subinterface Fe at both Se-Ga and Se-P (111) interfaces decrease compared to the bulk values.

  18. CO2-based in-line phase contrast imaging of small intestine in mice

    NASA Astrophysics Data System (ADS)

    Tang, Rongbiao; Li, Wei-Xia; Huang, Wei; Yan, Fuhua; Chai, Wei-Min; Yang, Guo-Yuan; Chen, Ke-Min

    2013-07-01

    The objective of this study was to explore the potential of CO2 single contrast in-line phase contrast imaging (PCI) for pre-clinical small intestine investigation. The absorption and phase contrast images of CO2 gas production were attained and compared. A further increase in image contrast was observed in PCI. Compared with CO2-based absorption contrast imaging (ACI), CO2-based PCI significantly enhanced the detection of mucosal microstructures, such as pits and folds. The CO2-based PCI could provide sufficient image contrast for clearly showing the intestinal mucosa in living mice without using barium. We concluded that CO2-based PCI might be a novel and promising imaging method for future studies of gastrointestinal disorders.

  19. Disorder scattering effect on the high-temperature lattice thermal conductivity of TiCoSb-based half-Heusler compounds

    SciTech Connect

    Zhou Min; Chen Lidong; Zhang Wenqing; Feng Chude

    2005-07-01

    The lattice thermal conductivities of TiCoSb-based half-Heusler alloys are presented in the temperature range between 300 and 900 K. A phenomenological model calculation of the high-temperature lattice thermal conductivities of these alloys was derived based on the Klemens-Callaway theory [Phys. Rev. 119, 507 (1960); ibid. 113, 1046 (1959)]. Good agreement was obtained between the calculated and the experimental data for TiCoSb, TiCo{sub 0.5}Rh{sub 0.5}Sb, and Ti{sub 0.5}Zr{sub 0.5}CoSb. Furthermore, the model predicts that simultaneously isoelectronic alloying on both Ti and Co sublattices could reduce the lattice thermal conductivity, and a {kappa}{sub L} value of 0.3 W/m K is predicted for Ti{sub 0.5}Zr{sub 0.5}Co{sub 0.5}Rh{sub 0.5}Sb at 900 K.

  20. International Round-Robin Study of the Thermoelectric Transport Properties of an n-Type Half-Heusler Compound from 300 K to 773 K

    NASA Astrophysics Data System (ADS)

    Wang, Hsin; Bai, Shengqiang; Chen, Lidong; Cuenat, Alexander; Joshi, Giri; Kleinke, Holger; König, Jan; Lee, Hee Woong; Martin, Joshua; Oh, Min-Wook; Porter, Wallace D.; Ren, Zhifeng; Salvador, James; Sharp, Jeff; Taylor, Patrick; Thompson, Alan J.; Tseng, Y. C.

    2015-11-01

    International transport property-measurement round-robins have been conducted by the thermoelectric annex under the International Energy Agency (IEA) Implementing Agreement on Advanced Materials for Transportation (AMT). Two previous round-robins used commercially available bismuth telluride as the test material, with the objectives of understanding measurement issues and developing standard testing procedures. This round-robin extended the measurement temperature range to 773 K. It was designed to meet the increasing demands for reliable transport data for thermoelectric materials used for power-generation applications. Eleven laboratories from six IEA-AMT member countries participated in the study. A half-Heusler ( n-type) material prepared by GMZ Energy was selected for the round-robin. The measured transport properties had a narrower distribution of uncertainty than previous round-robin results. The study intentionally included multiple testing methods and instrument types. Over the full temperature range, the measurement discrepancies for the figure of merit, ZT, in this round-robin were ±11.5 to ±16.4% from the averages.

  1. Half-metallic ferromagnetic features in d0 quaternary-Heusler compounds KCaCF and KCaCCl: A first-principles description

    NASA Astrophysics Data System (ADS)

    Du, Jiangtao; Dong, Shengjie; Lu, Yi-Lin; Zhao, Hui; Feng, Liefeng; Wang, L. Y.

    2017-04-01

    The electronic structures and magnetic properties of quaternary Heusler alloys KCaCF and KCaCCl have been analyzed by means of first-principles calculations on the basis of density functional theory. We found that type-3 structure is the most stable configuration where C occupies (0, 0, 0) site, K (0.25, 0.25, 0.25), F/Cl (0.5, 0.5, 0.5), and Ca (0.75, 0.75, 0.75). Type-1 arrangement is the metastable structure in which K, Ca, C, and X occupy (0, 0, 0), (0.25, 0.25, 0.25), (0.5, 0.5, 0.5), and (0.75, 0.75, 0.75) sites, respectively. Both of them are half metals with equilibrium volume. The spin polarization is predominantly from C 2p states. With the variation of the lattice constant, spin-gapless semiconducting characteristic is achieved for type-1 KCaCCl as volume increases.

  2. Investigations of the half-metallic behavior and the magnetic and thermodynamic properties of half-Heusler CoMnTe and RuMnTe compounds: A first-principles study

    NASA Astrophysics Data System (ADS)

    Djaafri, T.; Djaafri, A.; Elias, A.; Murtaza, G.; Khenata, R.; R., Ahmed; Bin Omran, S.; Rached, D.

    2014-08-01

    First-principles spin-polarized density functional theory (DFT) investigations of the structural, electronic, magnetic, and thermodynamics characteristics of the half-Heusler, CoMnTe and RuMnTe compounds are carried out. Calculations are accomplished within a state of the art full-potential (FP) linearized (L) augmented plane wave plus a local orbital (APW + lo) computational approach framed within DFT. The generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE) is implemented as an exchange correlation functional as a part of the total energy calculation. From the analysis of the calculated electronic band structure as well as the density of states for both compounds, a strong hybridization between d states of the higher valent transition metal (TM) atoms (Co, Ru) and lower valent TM atoms of (Mn) is observed. Furthermore, total and partial density of states (PDOS) of the ground state and the results of spin magnetic moments reveal that these compounds are both stable and ideal half-metallic ferromagnetic. The effects of the unit cell volume on the magnetic properties and half-metallicity are crucial. It is worth noting that our computed results of the total spin magnetic moments, for CoMnTe equal to 4 μB and 3 μB per unit cell for RuMnTe, nicely follow the rule μtot = Zt - 18. Using the quasi-harmonic Debye model, which considers the phononic effects, the effecs of pressure P and temperature T on the lattice parameter, bulk modulus, thermal expansion coefficient, Debye temperature, and heat capacity for these compounds are investigated for the first time.

  3. Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films

    NASA Astrophysics Data System (ADS)

    Logan, J. A.; Patel, S. J.; Harrington, S. D.; Polley, C. M.; Schultz, B. D.; Balasubramanian, T.; Janotti, A.; Mikkelsen, A.; Palmstrøm, C. J.

    2016-06-01

    The discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi2Se3, has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic devices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolved photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. This experimental verification of topological behaviour is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic devices.

  4. Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films.

    PubMed

    Logan, J A; Patel, S J; Harrington, S D; Polley, C M; Schultz, B D; Balasubramanian, T; Janotti, A; Mikkelsen, A; Palmstrøm, C J

    2016-06-27

    The discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi2Se3, has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic devices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolved photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. This experimental verification of topological behaviour is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic devices.

  5. Electronic structure of Zr-Ni-Sn systems: role of clustering and nanostructures in half-Heusler and Heusler limits.

    PubMed

    Do, Dat T; Mahanti, S D; Pulikkoti, Jiji J

    2014-07-09

    Half-Heusler and Heusler compounds have been of great interest for several decades for thermoelectric, magnetic, half-metallic and many other interesting properties. Among these systems, Zr-Ni-Sn compounds are interesting thermoelectrics which can go from semiconducting half-Heusler (HH) limit, ZrNiSn, to metallic Heusler (FH) limit, ZrNi2Sn. Recently Makongo et al (2011 J. Am. Chem. Soc. 133 18843) found that dramatic improvement in the thermoelectric power factor of HH can be achieved by putting excess Ni into the system. This was attributed to an energy filtering mechanism due to the presence of FH nanostructures in the HH matrix. Using density functional theory we have investigated clustering and nanostructure formation in ZrNi1+xSn (0 ⩽ x ⩽ 1) systems near the HH (x = 0) and FH (x = 1) ends and have found that excess Ni atoms in HH tend to stay close to each other and form nanoclusters. On the other hand, there is competing interaction between Ni-vacancies occupying different sites in FH which prevents them from forming vacancy nanoclusters. Effects of nano-inclusions on the electronic structure near HH and FH ends are discussed.

  6. Doping effects on thermoelectric properties of the off-stoichiometric Heusler compounds Fe{sub 2−x}V{sub 1+x}Al

    SciTech Connect

    Nishino, Y. Tamada, Y.

    2014-03-28

    The thermoelectric properties of Heusler-type Fe{sub 2−x}V{sub 1+x}Al{sub 1−y}Si{sub y} and Fe{sub 2−x}V{sub 1+x−y}Ti{sub y}Al alloys have been investigated to clarify which off-stoichiometric alloy, i.e., V-rich (x > 0) or V-poor (x < 0), is more effective in enhancing the Seebeck coefficient when doped by Si and Ti, while retaining a low electrical resistivity. Large Seebeck coefficients of −182 μV/K and 110 μV/K at 300 K are obtained for n-type Fe{sub 1.95}V{sub 1.05}Al{sub 0.97}Si{sub 0.03} and p-type Fe{sub 2.04}V{sub 0.93}Ti{sub 0.03}Al, respectively. When the Seebeck coefficient is plotted as a function of valence electron concentration (VEC), the VEC dependence for the doped off-stoichiometric alloys falls on characteristic curves depending on the off-stoichiometric composition x. It is concluded that a larger Seebeck coefficient with a negative sign can be obtained for the V-rich alloys rather than the V-poor alloys, whilst good p-type materials are always derived from the V-poor alloys. Substantial enhancements in the Seebeck coefficient for the off-stoichiometric alloys could be achieved by a favorable modification in the electronic structure around the Fermi level through the antisite V or Fe defect formation.

  7. Activation of microcrystalline cellulose in a CO(2)-based switchable system.

    PubMed

    Zhang, Qinghua; Oztekin, Nermin Simge; Barrault, Joël; De Oliveira Vigier, Karine; Jérôme, François

    2013-04-01

    MCC Hammer: Microcrystalline cellulose (MCC) is dissolved and subsequently regenerated in a CO2 -based switchable solvent system, drastically reducing its crystallinity index and thereby its recalcitrance to hydrolysis. Among the investigated systems, based on cheap, widely available chemicals, is one capable of dissolving up to 15 wt % MCC within 1 h at room temperature. Break it down!

  8. Transforming a Liability Into An Asset-Creating a Market for CO2-based Products

    NASA Astrophysics Data System (ADS)

    David, B. J.

    2016-12-01

    This session will discuss converting CO2 from a liability into an asset. It will specifically discuss how at least 25 products can be created using CO2 as a feedstock and deployed in the market at large scale. Focus will be on products that can both achieve scale from a market standpoint as well as climate significance in use of CO2 as a feedstock. The session will describe the market drivers supporting and inhibiting commercial deployment of CO2-based products. It will list key barriers and risks in the various CO2-based product segments. These barriers/risks could occur across technology, policy, institutional, economic, and other dimensions. The means to mitigate each barrier and the likelihood for such means to be deployed will be discussed.

  9. Numerical analysis of radial inward flow turbine for CO2 based closed loop Brayton cycle

    NASA Astrophysics Data System (ADS)

    Kisan, Jadhav Amit; Govardhan, M.

    2017-06-01

    Last few decades have witnessed a phenomenal growth in the demand for power, which has driven the suppliers to find new sources of energy and increase the efficiency of power generation process. Power generation cycles are either steam based Rankine cycle or closed loop Brayton cycles providing an efficiency of 30 to 40%. An upcoming technology in this regard is the CO2 based Brayton cycle operating near the critical region which has applications in vast areas. Power generation of CO2 based Brayton cycle can vary from few kilowatts for waste heat recovery to hundreds of megawatts in sodium cooled fast reactors. A CO2 based Brayton cycle is being studied for power generation especially in mid-sized concentrated solar power plants by numerous research groups around the world. One of the main components of such a setting is its turbine. Simulating the flow conditions inside the turbine becomes very crucial in order to accurately predict the performance of the system. The flow inside radial inflow turbine is studied at various inlet temperatures and mass flow rates in order to predict the behavior of the turbine under various boundary conditions. The performance investigation of the turbine system is done on the basis of parameters such as total efficiency, pressure ratio, and power coefficient. Effect of different inlet stagnation temperature and exit mass flow rates on these parameters is also studied. Results obtained are encouraging for the use of CO2 as working fluid in Brayton cycle.

  10. A Feasibility Study of CO2-Based Rankine Cycle Powered by Solar Energy

    NASA Astrophysics Data System (ADS)

    Zhang, Xin-Rong; Yamaguchi, Hiroshi; Fujima, Katsumi; Enomoto, Masatoshi; Sawada, Noboru

    An experiment study was carried out in order to investigate feasibility of CO2-based Rankine cycle powered by solar energy. The proposed cycle is to achieve a cogeneration of heat and power, which consists of evacuated solar tube collectors, power generating turbine, heat recovery system, and feed pump. The Rankine cycle of the system utilizes solar collectors to convert CO2 into high-temperature supercritical state, used to drive a turbine and produce electrical power. The cycle also recovers thermal energy, which can be used for absorption refrigerator, air conditioning, hot water supply so on for a building. A set of experimental set-up was constructed to investigate the performance of the CO2-based Rankine cycle. The results show the cycle can achieve production of heat and power with reasonable thermodynamics efficiency and has a great potential of the application of the CO2-based Rankine cycle powered by solar energy. In addition, some research interests related to the present study will also be discussed in this paper.

  11. Spin torque ferromagnetic resonance in Heusler based magnetic tunnel junctions

    NASA Astrophysics Data System (ADS)

    Zhang, Jie; Phung, Timothy; Pushp, Aakash; Jeong, Jaewoo; Ferrante, Yari; Rettner, Charles; Hughes, Brian P.; Yang, See-Hun; Parkin, Stuart S. P.

    Heusler compounds are of interest as electrode materials for use in magnetic tunnel junctions (MTJs) due to their half metallic character, which leads to high spin polarization and high tunneling magnetoresistance. Whilst much work has focused on the influence of the half metallic character of the Heusler compounds on the magnetoresistance of MTJs, there is much less work investigating the influence of this electronic structure on the spin transfer torque. Here, we investigate the bias dependence of the anti-damping like and field-like spin transfer torque components as a function of the bias voltage in symmetric (CoMnSi/MgO/CoMnSi) and asymmetric (CoMnSi/MgO/CoFe) structure magnetic tunnel junctions using spin transfer torque ferromagnetic resonance. Lastly, we report on the effect of asymmetric bias dependence of the differential conductance on the spin transfer torque.

  12. Synthesis and Characterization of Novel Magnetic Heusler Semiconductors for Device and Materials Applications

    NASA Astrophysics Data System (ADS)

    Jamer, Michelle E.

    Spintronic devices for magnetic memory applications control the magnetic properties of the materials by manipulating the spin and magnetic moment of the electrons. Present devices use ferromagnetic materials that have magnetic fringing fields that interfere with other components of the device. The main focus of this research is investigating low-moment ferrimagnetic inverse Heusler materials that could be used in spintronic devices thereby eliminating the external fringing magnetic field. The challenge of this research is that while hundreds of inverse Heusler materials have been predicted for possible uses in devices, many of these compounds have a positive formation energy indicating that they are not likely to form and will decompose into other compounds. The magnetic and structural properties of several inverse Heusler systems were studied. X-ray diffraction was used to determine the phase and ordering of the crystal structure. SQUID magnetometry and X-ray magnetic circular dichroism determined the bulk magnetic properties and the atom-specific magnetic moments. This thesis outlines the first synthesis of Heusler-type V3Al, which was discovered to be an antiferromagnet. Cr2CoAl was found to exist in a Heusler phase with antiferromagnetically coupled Cr and Co atomic moments. In addition, Mn2CoAl, Cr2CoGa, and Mn3Al were grown as thin films on desorbed GaAs substrates by molecular beam epitaxy. This thesis demonstrated the successful synthesis and characterization of several Heusler compounds that could be used in future devices. These are the seminal results of inverse Heusler synthesis, which are proposed in devices such as spin-FETs and nonvolatile magnetic memory.

  13. Half-metallic fully compensated ferrimagnetism in C1b-type half Heusler compounds Mn2Si1-xGex

    NASA Astrophysics Data System (ADS)

    Zhang, Y. J.; Liu, Z. H.; Liu, G. D.; Ma, X. Q.

    2015-08-01

    First-principles electronic structure calculations have been performed for the 18-valence electrons compounds Mn2Si1-xGex(x=0, 0.25, 0.5, 0.75, 1). The results suggest that Mn2Si1-xGex compounds in C1b structure are half-metallic fully compensated ferrimagnets. Furthermore, the size of the half-metallic band gap, the position of the Fermi level, and the magnetic moment of Mn atoms can be manipulated by changing x from 0 to 1 without destroying the half-metallic fully compensated ferrimagnetic property.

  14. Perovskite- and Heusler based materials for thermoelectric converters

    NASA Astrophysics Data System (ADS)

    Weidenkaff, Anke

    2015-03-01

    The broad application of thermoelectric converters in future energy technologies requires the development of active, stable, low cost and sustainable materials. Semiconductors based on perovskite and heusler structures show substantial potential for thermoelectric energy conversion processes. Their good performance can be explained based on their suitable band structure, adjusted charge carrier density, mass and mobility, limited phonon transport, electron filtering possibilities, strongly correlated electronic systems, etc. These properties are widely tuneable by following theoretical concepts and a deep composition-structure-property understanding to change the composition, structure and size of the crystallites in innovative scalable synthesis procedures. Improved thermoelectric materials are developed, synthesised and tested in diverse high temperature applications to improve the efficiency and energy density of the thermoelectric conversion process. The lecture will provide a summary on the field of advanced perovskite-type ceramics and Heusler compounds gaining importance for a large number of future energy technologies.

  15. Performance analysis of STT-RAM with cross shaped free layer using Heusler alloys

    NASA Astrophysics Data System (ADS)

    Bharat Kumary, Tangudu; Ghosh, Bahniman; Awadhiya, Bhaskar; Verma, Ankit Kumar

    2016-01-01

    We have investigated the performance of a spin transfer torque random access memory (STT-RAM) cell with a cross shaped Heusler compound based free layer using micromagnetic simulations. We have designed a free layer using a Cobalt based Heusler compound. Simulation results clearly show that the switching time from one state to the other state has been reduced, also it has been found that the critical switching current density (to switch the magnetization of the free layer of the STT RAM cell) is reduced.

  16. Prediction of phase transition, mechanical and electronic properties of inverse Heusler compound Y2RuPb, via FP-LMTO method

    NASA Astrophysics Data System (ADS)

    Labair, M.; Rached, H.; Rached, D.; Benalia, S.; Abidri, B.; Khenata, R.; Ahmed, R.; Omran, S. Bin; Bouhemadou, A.; Syrotyuk, S. V.

    2016-03-01

    Topological insulators (TI) are immensely investigated due to their promising characteristics for spintronics and quantum computing applications. In this regard, although bismuth, telluride, selenide and antimony containing compounds are typically considered as topological insulators, materials with Hg2CuTi-type structure have also shown their potential for TIs as well. Here, we present first principles study of the Y2RuPb compound, pertaining to its structural, mechanical, electrical and the optical properties. Calculations are executed at the level of the parameterized Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), employing the full-potential (FP) linearized muffin-tin orbital (LMTO) approach, as designed within the density functional theory (DFT). The study is carried out on the Hg2CuTi-type and Cu2MnAl-type structures of the Y2RuPb compound. From our structural calculations, it is found that Y2RuPb is more stable in its Hg2CuTi-type structure; however, the analysis of the mechanical properties reveals its stability in both phases against any kind of elastic deformation. Similarly, Dirac cone shaped surface energy levels found in the predicted electronic band structure of the Y2RuPb compound, and good agreement of the obtained results with Zhang et al., demonstrates that it is a topological insulating material. Additionally, the real and imaginary parts of the dielectric function ɛ (ω) and refractive index n (ω), for an energy range up to 14eV, are analyzed as well.

  17. Ab Initio Calculations on the Structural, Mechanical, Electronic, Dynamic, and Optical Properties of Semiconductor Half-Heusler Compound ZrPdSn

    NASA Astrophysics Data System (ADS)

    Çiftci, Yasemin Ö.; Çoban, Cansu

    2016-02-01

    The structural, mechanical, electronic, dynamic, and optical properties of the ZrPdSn compound crystallising into the MgAgAs structure are investigated by the ab initio calculations based on the density functional theory. The lattice constant, bulk modulus, and first derivative of bulk modulus were obtained by fitting the calculated total energy-atomic volume results to the Murnaghan equation of state. These results were compared to the previous data. The band structure and corresponding density of states (DOS) were also calculated and discussed. The elastic properties were calculated by using the stress-strain method, which shows that the MgAgAs phase of this compound is mechanically stable. The presented phonon dispersion curves and one-phonon DOS confirms that this compound is dynamically stable. In addition, the heat capacity, entropy, and free energy of ZrPdSn were calculated by using the phonon frequencies. Finally, the optical properties, such as dielectric function, reflectivity function, extinction coefficient, refractive index, and energy loss spectrum, were obtained under different pressures.

  18. A first-principle investigation of spin-gapless semiconductivity, half-metallicity, and fully-compensated ferrimagnetism property in Mn2ZnMg inverse Heusler compound

    NASA Astrophysics Data System (ADS)

    Wang, Xiaotian; Cheng, Zhenxiang; Khenata, Rabah; Rozale, Habib; Wang, Jianli; Wang, Liying; Guo, Ruikang; Liu, Guodong

    2017-02-01

    Recently, spin-gapless semiconductors (SGSs) and half-metallic materials (HMMs) have received considerable interest in the fields of materials sciences and solid-state physics because they can provide a high degree of spin polarization in electron transport. The results on band structure calculations reveal that the metallic fully-compensated ferrimagnet (M-FCF) Mn2ZnMg becomes half-metallic fully-compensated ferrimagnet (HM-FCF), fully-compensated ferrimagnetic semiconductor (FCF-S) and fully-compensated ferrimagnetic spin-gapless semiconductor (FCF-SGS) if the uniform strain applied. However, the metallic fully-compensated ferrimagnetism property of the Mn2ZnMg is robust to the tetragonalization. The structure stability based on the calculations of the cohesion energy and the formation energy of this compound has been tested. Furthermore, a magnetic state transition from antiferromagentic (AFM) state to non-magnetic (NM) state can be observed at the lattice constant of 5.20 Å.

  19. Large magnetocrystalline anisotropy in tetragonally distorted Heuslers: a systematic study

    NASA Astrophysics Data System (ADS)

    Matsushita, Y.-I.; Madjarova, G.; Dewhurst, J. K.; Shallcross, S.; Felser, C.; Sharma, S.; Gross, E. K. U.

    2017-03-01

    With a view to the design of hard magnets without rare earths we explore the possibility of large magnetocrystalline anisotropy energies in Heusler compounds that are unstable with respect to a tetragonal distortion. We consider the Heusler compounds Fe2YZ with Y  =  (Ni, Co, Pt), and Co2YZ with Y  =  (Ni, Fe, Pt) where, in both cases, Z  =  (Al, Ga, Ge, In, Sn). We find that for the Co2NiZ, Co2PtZ, and Fe2PtZ families the cubic phase is always, at T  =  0, unstable with respect to a tetragonal distortion, while, in contrast, for the Fe2NiZ and Fe2CoZ families this is the case for only 2 compounds—Fe2NiGe and Fe2NiSn. For all compounds in which a tetragonal distortion occurs we calculate the magnetocrystalline anisotropy energy (MAE) finding remarkably large values for the Pt containing Heuslers, but also large values for a number of the other compounds (e.g. Co2NiGa has an MAE of  ‑2.38 MJ m‑3). The tendency to a tetragonal distortion we find to be strongly correlated with a high density of states (DOS) at the Fermi level in the cubic phase. As a corollary to this fact we observe that upon doping compounds for which the cubic structure is stable such that the Fermi level enters a region of high DOS, a tetragonal distortion is induced and a correspondingly large value of the MAE is then observed.

  20. Half-Heusler topological insulators: A first-principles study with the Tran-Blaha modified Becke-Johnson density functional

    SciTech Connect

    Feng, wanxiang; Xiao, Di; Zhang, Ying; Yao, yugui

    2010-01-01

    We systematically investigate the topological band structures of half-Heusler compounds using first- principles calculations. The modified Becke-Johnson exchange potential together with local-density approxi- mation for the correlation potential MBJLDA has been used here to obtain accurate band inversion strength and band order. Our results show that a large number of half-Heusler compounds are candidates for three- dimensional topological insulators. The difference between band structures obtained using the LDA and MB- JLDA potential is also discussed.

  1. Thermoelectric Properties of Half-Heusler Heterostructures from Ab Initio Calculations

    NASA Astrophysics Data System (ADS)

    Fiedler, Gregor; Kratzer, Peter

    2016-03-01

    Semiconducting half-Heusler alloys have recently emerged as a class of thermoelectric materials with outstanding performance in the medium- to high-temperature range. Heterostructures promise a further reduction of thermal conductivity as a result of phonon scattering at interfaces. Here, both the electronic and phononic spectra of half-Heusler compounds based on Ti, Zr, and Hf are calculated using density functional theory. With this input, thermoelectric properties are obtained, and the thermal conductivity of a heterostructure superlattice is estimated by extending the diffuse mismatch model of interface conductance. We find that a high power factor σ S^2 can be retained in a short-period superlattice, while thermal conductivity is reduced compared to that in single-phase half-Heusler crystals.

  2. Electronic structure of Zr-Ni-Sn systems: the role of nanostructures and clustering in Half-Heusler and Heusler limits

    NASA Astrophysics Data System (ADS)

    Do, Dat; Mahanti, S. D.

    2014-03-01

    Half-Heusler and Heusler compounds have been of great interest for several decades for thermoelectric, magnetic, half-metallic and many other interesting properties. Among these systems, Zr-Ni-Sn compounds are interesting thermoelectrics which can go from semiconducting half-Heusler (HH) limit, ZrNiSn, to metallic Heusler limit (FH), ZrNi2Sn. Recently Makogo et al. [J. Am. Chem. Soc. 133, 18843 (2011)] found that dramatic improvement in the thermoelectric power factor of HH can be achieved by putting excess Ni into the system. This was attributed to an energy filtering mechanism due to the formation of FH nanostructures in the HH matrix. Using density functional theory we have investigated clustering and nanostructure formation in HH1-xFHx systems near the HH and FH ends. These results and the effects of nanostructures on electronic structure and thermoelectric properties will be discussed in this talk. This work was supported by the Center for Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

  3. Growth of Cr2CoGa and inverse Heusler thin films using Molecular Beam Epitaxy

    NASA Astrophysics Data System (ADS)

    Jamer, Michelle; Decapua, Matthew; Player, Gabriel; Heiman, Don

    Theoretical calculations have predicted the existence of inverse Heusler compounds that exhibit zero-moment magnetization while retaining their half-metallicity. These unique compounds have been labeled spin gapless semiconductors (SGS), where the density of states (DOS) can behave as a half-metal or gapless semiconductor. There is a special interest for zero-moment SGS compounds since traditional antiferromagnets cannot be spin-polarized. Such compounds are experimentally attractive for future spintronic devices due to their large magnetic transition temperature (400-800 K). This work focuses on zero-moment inverse Heusler compounds including Cr2CoGa and Mn3Al. Thin films have been grown using MBE and their magnetic, structural, and electrical properties of these compounds have been characterized by various techniques, including XMCD and magnetometry. The atomic moments are found to be large, but significant cancellations lead to small average moments. Supported by NSF Grant ECCS-1402738.

  4. Experimental study of heat pump thermodynamic cycles using CO2 based mixtures - Methodology and first results

    NASA Astrophysics Data System (ADS)

    Bouteiller, Paul; Terrier, Marie-France; Tobaly, Pascal

    2017-02-01

    The aim of this work is to study heat pump cycles, using CO2 based mixtures as working fluids. Since adding other chemicals to CO2 moves the critical point and generally equilibrium lines, it is expected that lower operating pressures as well as higher global efficiencies may be reached. A simple stage pure CO2 cycle is used as reference, with fixed external conditions. Two scenarios are considered: water is heated from 10 °C to 65 °C for Domestic Hot Water scenario and from 30 °C to 35 °C for Central Heating scenario. In both cases, water at the evaporator inlet is set at 7 °C to account for such outdoor temperature conditions. In order to understand the dynamic behaviour of thermodynamic cycles with mixtures, it is essential to measure the fluid circulating composition. To this end, we have developed a non intrusive method. Online optical flow cells allow the recording of infrared spectra by means of a Fourier Transform Infra Red spectrometer. A careful calibration is performed by measuring a statistically significant number of spectra for samples of known composition. Then, a statistical model is constructed to relate spectra to compositions. After calibration, compositions are obtained by recording the spectrum in few seconds, thus allowing for a dynamic analysis. This article will describe the experimental setup and the composition measurement techniques. Then a first account of results with pure CO2, and with the addition of propane or R-1234yf will be given.

  5. A Novel Single-Ion-Conducting Polymer Electrolyte Derived from CO2-Based Multifunctional Polycarbonate.

    PubMed

    Deng, Kuirong; Wang, Shuanjin; Ren, Shan; Han, Dongmei; Xiao, Min; Meng, Yuezhong

    2016-12-14

    This work demonstrates the facile and efficient synthesis of a novel environmentally friendly CO2-based multifunctional polycarbonate single-ion-conducting polymer electrolyte with good electrochemistry performance. The terpolymerizations of CO2, propylene epoxide (PO), and allyl glycidyl ether (AGE) catalyzed by zinc glutarate (ZnGA) were performed to generate poly(propylene carbonate allyl glycidyl ether) (PPCAGE) with various alkene groups contents which can undergo clickable reaction. The obtained terpolymers exhibit an alternating polycarbonate structure confirmed by (1)H NMR spectra and an amorphous microstructure with glass transition temperatures (Tg) lower than 11.0 °C evidenced by differential scanning calorimetry analysis. The terpolymers were further functionalized with 3-mercaptopropionic acid via efficient thiol-ene click reaction, followed by reacting with lithium hydroxide, to afford single-ion-conducting polymer electrolytes with different lithium contents. The all-solid-state polymer electrolyte with the 41.0 mol % lithium containing moiety shows a high ionic conductivity of 1.61 × 10(-4) S/cm at 80 °C and a high lithium ion transference number of 0.86. It also exhibits electrochemical stability up to 4.3 V vs Li(+)/Li. This work provides an interesting design way to synthesize an all-solid-state electrolyte used for different lithium batteries.

  6. Co{sub 2}MnSi Heusler alloy as magnetic electrodes in magnetic tunnel junctions

    SciTech Connect

    Kaemmerer, S.; Thomas, A.; Huetten, A.; Reiss, G.

    2004-07-05

    As a consequence of the growing theoretical predictions of 100% spin-polarized half- and full-Heusler compounds over the past six years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated Co{sub 2}MnSi, as a representative of the full-Heusler compound family, as one magnetic electrode into magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co{sub 2}MnSi at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer, subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co{sub 2}MnSi, and (3) homogenizes the AlO{sub x} barrier. It is shown that the magnetic switching of the ferromagnetic electrodes is well controlled from room temperature down to 10 K. The resulting tunnel magnetoresistance-effect amplitude of the Co{sub 2}MnSi containing magnetic tunnel junctions has been determined as a function of temperature and the spin polarization of the Co{sub 2}MnSi Heusler compound has been estimated to be 61% at 10 K. Thus, the spin polarization of the Co{sub 2}MnSi layer at 10K exceeds that of conventional transition metals.

  7. Geochemical Interaction of Middle Bakken Reservoir Rock and CO2 during CO2-Based Fracturing

    NASA Astrophysics Data System (ADS)

    Nicot, J. P.; Lu, J.; Mickler, P. J.; Ribeiro, L. H.; Darvari, R.

    2015-12-01

    This study was conducted to investigate the effects of geochemical interactions when CO2 is used to create the fractures necessary to produce hydrocarbons from low-permeability Middle Bakken sandstone. The primary objectives are to: (1) identify and understand the geochemical reactions related to CO2-based fracturing, and (2) assess potential changes of reservoir property. Three autoclave experiments were conducted at reservoir conditions exposing middle Bakken core fragments to supercritical CO2 (sc-CO2) only and to CO2-saturated synthetic brine. Ion-milled core samples were examined before and after the reaction experiments using scanning electron microscope, which enabled us to image the reaction surface in extreme details and unambiguously identify mineral dissolution and precipitation. The most significant changes in the reacted rock samples exposed to the CO2-saturated brine is dissolution of the carbonate minerals, particularly calcite which displays severely corrosion. Dolomite grains were corroded to a lesser degree. Quartz and feldspars remained intact and some pyrite framboids underwent slight dissolution. Additionally, small amount of calcite precipitation took place as indicated by numerous small calcite crystals formed at the reaction surface and in the pores. The aqueous solution composition changes confirm these petrographic observations with increase in Ca and Mg and associated minor elements and very slight increase in Fe and sulfate. When exposed to sc-CO2 only, changes observed include etching of calcite grain surface and precipitation of salt crystals (halite and anhydrite) due to evaporation of residual pore water into the sc-CO2 phase. Dolomite and feldspars remained intact and pyrite grains were slightly altered. Mercury intrusion capillary pressure tests on reacted and unreacted samples shows an increase in porosity when an aqueous phase is present but no overall porosity change caused by sc-CO2. It also suggests an increase in permeability

  8. Accelerated discovery of new magnets in the Heusler alloy family

    PubMed Central

    Sanvito, Stefano; Oses, Corey; Xue, Junkai; Tiwari, Anurag; Zic, Mario; Archer, Thomas; Tozman, Pelin; Venkatesan, Munuswamy; Coey, Michael; Curtarolo, Stefano

    2017-01-01

    Magnetic materials underpin modern technologies, ranging from data storage to energy conversion to contactless sensing. However, the development of a new high-performance magnet is a long and often unpredictable process, and only about two dozen magnets are featured in mainstream applications. We describe a systematic pathway to the design of novel magnetic materials, which demonstrates a high throughput and discovery speed. On the basis of an extensive electronic structure library of Heusler alloys containing 236,115 prototypical compounds, we filtered those displaying magnetic order and established whether they can be fabricated at thermodynamic equilibrium. Specifically, we carried out a full stability analysis of intermetallic Heusler alloys made only of transition metals. Among the possible 36,540 prototypes, 248 were thermodynamically stable but only 20 were magnetic. The magnetic ordering temperature, TC, was estimated by a regression calibrated on the experimental TC of about 60 known compounds. As a final validation, we attempted the synthesis of a few of the predicted compounds and produced two new magnets: Co2MnTi, which displays a remarkably high TC in perfect agreement with the predictions, and Mn2PtPd, which is an antiferromagnet. Our work paves the way for large-scale design of novel magnetic materials at potentially high speed. PMID:28439545

  9. Accelerated discovery of new magnets in the Heusler alloy family.

    PubMed

    Sanvito, Stefano; Oses, Corey; Xue, Junkai; Tiwari, Anurag; Zic, Mario; Archer, Thomas; Tozman, Pelin; Venkatesan, Munuswamy; Coey, Michael; Curtarolo, Stefano

    2017-04-01

    Magnetic materials underpin modern technologies, ranging from data storage to energy conversion to contactless sensing. However, the development of a new high-performance magnet is a long and often unpredictable process, and only about two dozen magnets are featured in mainstream applications. We describe a systematic pathway to the design of novel magnetic materials, which demonstrates a high throughput and discovery speed. On the basis of an extensive electronic structure library of Heusler alloys containing 236,115 prototypical compounds, we filtered those displaying magnetic order and established whether they can be fabricated at thermodynamic equilibrium. Specifically, we carried out a full stability analysis of intermetallic Heusler alloys made only of transition metals. Among the possible 36,540 prototypes, 248 were thermodynamically stable but only 20 were magnetic. The magnetic ordering temperature, TC, was estimated by a regression calibrated on the experimental TC of about 60 known compounds. As a final validation, we attempted the synthesis of a few of the predicted compounds and produced two new magnets: Co2MnTi, which displays a remarkably high TC in perfect agreement with the predictions, and Mn2PtPd, which is an antiferromagnet. Our work paves the way for large-scale design of novel magnetic materials at potentially high speed.

  10. Doping designed half-Heusler insulators

    NASA Astrophysics Data System (ADS)

    Yu, Yonggang; Zhang, Xiuwen; Yu, Liping; Yan, Feng; Nagaraja, A.; Mason, T. O.; Zunger, Alex

    2015-03-01

    The 18-valence-electron 1:1:1 compounds of the type III-X-V, IV-X-IV, IV-IX-V and V-IX-IV include thermoelectric materials, topological insulators, and recently a high mobility p-type transparent conductor TaIrGe (arXiv:1406.0872), yet their intrinsic doping trends are poorly known or understood. Using the ``modern theory of doping'' that addresses via DFT and HSE the thermodynamic formation energies and the DFT-corrected transition levels in the gap, we find the following interesting trends: (1) High atomic number compounds such as TaIrGe made of metallic elements can surprisingly have a large band gap (direct) of ~ 2.5 eV. (2) Half-Heusler such as A(IV)B(X)C(IV) is naturally n-type if its DFT calculated chemical stability field resides within the A-rich or B-rich domain of the stability triangle, while it is p-type if it resides within the C-rich domain. Such calculations provide a good metric. (3) When the B atom [at (1/4,1/4,1/4)] is as large as Ir or Pt, the compound prefers p-type because the C-on-A antisite [such as Ge Ta (1 -) ] is a shallow acceptor producing holes yet the hole-killer donor of B-interstitial is unfavorable. (4) When B =Ni or Co, the compound favors n-type due to the dominance of B-interstitial defects (e.g. TiCoSb). We will show the calculated leading defect types and the dependence of carrier concentrations on chemical conditions for newly predicted half-Heulser insulators. This study is supported by DOE, Office of Science, Basic Energy Science, MSE division grant to CU Boulder.

  11. A new n-type half-Heusler thermoelectric material NbCoSb

    SciTech Connect

    Huang, Lihong; He, Ran; Chen, Shuo; Zhang, Hao; Dahal, Keshab; Zhou, Haiqing; Wang, Hui; Zhang, Qinyong; Ren, Zhifeng

    2015-10-15

    Highlights: • Half-Heusler alloy NbCoSb with 19 valence electron count was studied as TE material. • It is surprising that NbCoSb is n-type. • A maximum ZT of ∼0.4 is achieved at 700 °C without optimization. • It opens up a new route to develop new half-Heusler thermoelectric materials. • It is very interesting that a traditionally thought of VEC of 18 is not required. - Abstract: We surprisingly made a new n-type thermoelectric compound NbCoSb with half-Heusler (HH) structure having valence electron count of 19, different from the traditional 18, which opens up a new route to develop new half-Heusler thermoelectric materials not following the traditional valence electron count of 18. The samples are made by arc melting followed by ball milling and hot pressing. The effect of hot pressing temperature on the thermoelectric properties of NbCoSb samples has been studied. A maximum thermoelectric figure-of-merit (ZT) of ∼0.4 is achieved at 700 °C in NbCoSb sample that is hot pressed at 1000 °C. This work add a new member to HH compounds for thermoelectric applications, although the peak ZT of ∼0.4 is still lower than that of the traditional HHs. Moreover, it is very interesting to see that a traditionally thought of valence electron counts of 18 is not required.

  12. Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films

    SciTech Connect

    Logan, J. A.; Patel, S. J.; Harrington, S. D.; Polley, C. M.; Schultz, B. D.; Balasubramanian, T.; Janotti, A.; Mikkelsen, A.; Palmstrøm, C. J.

    2016-06-27

    The discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi2Se3, has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic devices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolved photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. As a result, this experimental verification of topological behavior is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic devices.

  13. Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films

    DOE PAGES

    Logan, J. A.; Patel, S. J.; Harrington, S. D.; ...

    2016-06-27

    The discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi2Se3, has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic devices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolvedmore » photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. As a result, this experimental verification of topological behavior is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic devices.« less

  14. Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films

    PubMed Central

    Logan, J. A.; Patel, S. J.; Harrington, S. D.; Polley, C. M.; Schultz, B. D.; Balasubramanian, T.; Janotti, A.; Mikkelsen, A.; Palmstrøm, C. J.

    2016-01-01

    The discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi2Se3, has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic devices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolved photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. This experimental verification of topological behaviour is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic devices. PMID:27346655

  15. Bias dependence of spin transfer torque in Co2MnSi Heusler alloy based magnetic tunnel junctions

    NASA Astrophysics Data System (ADS)

    Zhang, Jie; Phung, Timothy; Pushp, Aakash; Ferrante, Yari; Jeong, Jaewoo; Rettner, Charles; Hughes, Brian P.; Yang, See-Hun; Jiang, Yong; Parkin, Stuart S. P.

    2017-04-01

    Heusler compounds are of interest as electrode materials for use in magnetic tunnel junctions (MTJs) due to their half metallic character, which leads to 100% spin polarization and high tunneling magnetoresistance. Most work to date has focused on the improvements to tunneling magnetoresistance that can stem from the use of Heusler electrodes, while there is much less work investigating the influence of Heusler electrodes on the spin transfer torque properties of MTJs. Here, we investigate the bias dependence of the anti-damping like and field-like spin transfer torque components in both symmetric (Co2MnSi/MgO/Co2MnSi) and asymmetric (Co2MnSi/MgO/CoFe) structure Heusler based MTJs using spin transfer torque ferromagnetic resonance. We find that while the damping like torque is linear with respect to bias for both MTJ structures, the asymmetric MTJ structure has an additional linear component to the ordinarily quadratic field like torque bias dependence and that these results can be accounted for by a free electron tunneling model. Furthermore, our results suggest that the low damping and low saturation magnetization properties of Heusler alloys are more likely to lead significant improvements to spin torque switching efficiency rather than their half metallic character.

  16. Fast and Green - CO2 Based Extraction, Isolation, and Quantification of Phenolic Styrax Constituents.

    PubMed

    Scheuba, Johanna; Wronski, Valerie-Katharina; Rollinger, Judith M; Grienke, Ulrike

    2017-08-01

    In this study the first supercritical fluid based protocol for the extraction, analysis, and isolation of six polar compounds, i.e., o-vanillin (1), styracin (2), vanillin (3), trans-cinnamic acid (4), vanillic acid (5), and shikimic acid (6), was developed. First, eight styrax resin products (R1-R8) obtained from various Liquidambar tree species, which are known to contain compounds 2-6, were extracted with a 1 : 1 mixture of supercritical CO2 and EtOH. Within 4 minutes, the compounds were successfully baseline separated on an Acquity UPC(2) BEH 2-EP (3.0 × 100 mm, 1.7 µm) column using a mobile phase of supercritical CO2 and MeOH with 0.1 % phosphoric acid. The compounds were quantified and the method was validated according to current ICH guidelines. Scaling up to preparative supercritical fluid chromatography using a Viridis BEH 2-EP (10 × 250 mm, 5 µm) column allowed for a fast separation and isolation of the selected constituents 2 and 4 from R6 within 7 minutes. This supercritical fluid protocol is easily adaptable to compounds of similar polarity. The increase in speed and its environmental friendliness underline its superiority over conventional set-ups. Georg Thieme Verlag KG Stuttgart · New York.

  17. Enhancement of thermoelectric properties in the Nb-Co-Sn half-Heusler/Heusler system through spontaneous inclusion of a coherent second phase

    NASA Astrophysics Data System (ADS)

    Buffon, Malinda L. C.; Laurita, Geneva; Verma, Nisha; Lamontagne, Leo; Ghadbeigi, Leila; Lloyd, Demetrious L.; Sparks, Taylor D.; Pollock, Tresa M.; Seshadri, Ram

    2016-08-01

    Half-Heusler XYZ compounds with an 18 valence electron count are promising thermoelectric materials, being thermally and chemically stable, deriving from relatively earth-abundant components, and possessing appropriate electrical transport properties. The typical drawback with this family of compounds is their high thermal conductivity. A strategy for reducing thermal conductivity is through the inclusion of secondary phases designed to minimize negative impact on other properties. Here, we achieve this through the addition of excess Co to half-Heusler NbCoSn, which introduces precipitates of a semi-coherent NbCo2Sn Heusler phase. A series of NbCo1+xSn materials are characterized here using X-ray and neutron diffraction studies and electron microscopy. Electrical and thermal transport measurements and electronic structure calculations are used to understand property evolution. We find that annealing has an important role to play in determining antisite ordering and properties. Antisite disorder in the as-prepared samples improves thermoelectric performance through the reduction of thermal conductivity, but annealing during the measurement degrades properties to resemble those of the annealed samples. Similar to the more widely studied TiNi1+xSn system, Co addition to the NbCoSn phase results in improved thermoelectric performance through a decrease in thermal conductivity which results in a 20% improvement in the thermoelectric figure of merit, zT.

  18. Enhancement of thermoelectric properties in the Nb–Co–Sn half-Heusler/Heusler system through spontaneous inclusion of a coherent second phase

    SciTech Connect

    Buffon, Malinda L. C. Verma, Nisha; Lamontagne, Leo; Pollock, Tresa M.; Laurita, Geneva; Ghadbeigi, Leila; Sparks, Taylor D.; Lloyd, Demetrious L.; Seshadri, Ram

    2016-08-21

    Half-Heusler XYZ compounds with an 18 valence electron count are promising thermoelectric materials, being thermally and chemically stable, deriving from relatively earth-abundant components, and possessing appropriate electrical transport properties. The typical drawback with this family of compounds is their high thermal conductivity. A strategy for reducing thermal conductivity is through the inclusion of secondary phases designed to minimize negative impact on other properties. Here, we achieve this through the addition of excess Co to half-Heusler NbCoSn, which introduces precipitates of a semi-coherent NbCo{sub 2}Sn Heusler phase. A series of NbCo{sub 1+x}Sn materials are characterized here using X-ray and neutron diffraction studies and electron microscopy. Electrical and thermal transport measurements and electronic structure calculations are used to understand property evolution. We find that annealing has an important role to play in determining antisite ordering and properties. Antisite disorder in the as-prepared samples improves thermoelectric performance through the reduction of thermal conductivity, but annealing during the measurement degrades properties to resemble those of the annealed samples. Similar to the more widely studied TiNi{sub 1+x}Sn system, Co addition to the NbCoSn phase results in improved thermoelectric performance through a decrease in thermal conductivity which results in a 20% improvement in the thermoelectric figure of merit, zT.

  19. Nanoscale structural heterogeneity in Ni-rich half-Heusler TiNiSn

    SciTech Connect

    Douglas, Jason E. Pollock, Tresa M.; Chater, Philip A.; Brown, Craig M.; Seshadri, Ram

    2014-10-28

    The structural implications of excess Ni in the TiNiSn half-Heusler compound are examined through a combination of synchrotron x-ray and neutron scattering studies, in conjunction with first principles density functional theory calculations on supercells. Despite the phase diagram suggesting that TiNiSn is a line compound with no solid solution, for small x in TiNi{sub 1+x}Sn there is indeed an appearance—from careful analysis of the scattering—of some solubility, with the excess Ni occupying the interstitial tetrahedral site in the half-Heusler structure. The analysis performed here would point to the excess Ni not being statistically distributed, but rather occurring as coherent nanoclusters. First principles calculations of energetics, carried out using supercells, support a scenario of Ni interstitials clustering, rather than a statistical distribution.

  20. Computational Discovery of a Novel Semiconductor: A Vacancy-Ordered Fe1.5TiSb Heusler Phase

    NASA Astrophysics Data System (ADS)

    Ishwar Hegde, Vinay; Naghibolashrafi, Nariman; Keshavarz, Sahar; Munira, Kamaram; Butler, William; Leclair, Patrick; Wolverton, Chris

    Many full- and half-Heusler phase compounds are half-metallic ferromagnets, and are attractive for spintronic applications due to their relatively high Curie temperatures. However, while it is known that defects such as vacancies (on the X site of an X2YZ Heusler phase) can lead to a loss of half-metallic character, their effect on the stability and order of these compounds has not been adequately explored. To address this shortcoming, we perform a binary cluster expansion (CE) of Fe and vacancies on the Fe sublattice of the FexVac2-xTiSb Heusler compound. From our CE, we computationally predict the stability of a novel semiconductor phase with an interesting new structure type: R 3 m spacegroup with composition Fe1.5TiSb, i.e., between the full- and half-Heusler compositions. By comparing the electronic structure of all the competing structures at x = 1 . 5 , we find that the gap opened in the minority-spin channel due to vacancies strongly correlates with the stability of the structure. We study the effect of vacancies on the structural order in Fe1.5TiSb by generating special quasi-random structures (SQSs) as approximations to the true disordered state, and find that the material undergoes an order-disorder transition at elevated temperatures of ~1450 K.

  1. Defects-driven appearance of half-metallic ferrimagnetism in Co Mn-based Heusler alloys

    NASA Astrophysics Data System (ADS)

    Özdog˜an, K.; Galanakis, I.; Şaşıog˜lu, E.; Aktaş, B.

    2007-06-01

    Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co 2MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co 1- xMn x] 2MnSi is of particular interest since Mn 3Si is known to crystallize in the Heusler L21 lattice structure of Co 2MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.

  2. Improving the thermoelectric properties of half-Heusler TiNiSn through inclusion of a second full-Heusler phase: microwave preparation and spark plasma sintering of TiNi(1+x)Sn.

    PubMed

    Birkel, Christina S; Douglas, Jason E; Lettiere, Bethany R; Seward, Gareth; Verma, Nisha; Zhang, Yichi; Pollock, Tresa M; Seshadri, Ram; Stucky, Galen D

    2013-05-14

    Half-Heusler thermoelectrics offer the possibility to choose from a variety of non-toxic and earth-abundant elements. TiNiSn is of particular interest and - with its relatively high electrical conductivity and Seebeck coefficient - allows for optimization of its thermoelectric figure of merit, reaching values of up to 1 in heavily-doped and/or phase-segregated systems. In this contribution, we used an energy- and time-efficient process involving solid-state preparation in a commercial microwave oven and a fast consolidation technique, Spark Plasma Sintering, to prepare a series of Ni-rich TiNi1+xSn with small deviations from the half-Heusler composition. Spark Plasma Sintering plays an important role in the process by being a part of the synthesis of the material rather than solely a densification technique. Synchrotron powder X-ray diffraction and microprobe data confirm the presence of a secondary TiNi2Sn full-Heusler phase within the half-Heusler matrix. We observe a clear correlation between the amount of full-Heusler phase and the lattice thermal conductivity of the samples, resulting in decreasing total thermal conductivity with increasing TiNi2Sn fraction. This trend shows that phonons are scattered effectively as a result of the microstructure of the materials with full-Heusler inclusions in the size range of microns to tens of microns. The best performing samples with around 5% of TiNi2Sn phase exhibit maximum figures of merit of almost 0.6 between 750 K and 800 K which is an increase of ca. 35% compared to the zT of the parent compound TiNiSn.

  3. Biphasic thermoelectric materials derived from the half-Heusler/full-Heusler system Ti-Ni-Sn

    NASA Astrophysics Data System (ADS)

    Douglas, Jason Everett

    Among the possible avenues for increasing the efficiency of global energy usage, thermoelectrics are an exciting, solid-state option. Thermoelectric materials, which convert an internal temperature gradient into a voltage and vice versa, have found applications in refrigeration as well as power generation from waste heat. TiNiSn, a semiconductor of the half-Heusler (hH) crystal structure, is of particular interest due to its very favorable electronic transport properties, conductivity (sigma) and Seebeck coefficient ( S), at relevant temperature regimes (between 600 K and 900 K). Unfortunately, its overall efficiency is hampered by a comparatively high thermal conductivity (kappa). In the design of thermoelectric materials, a number of approaches have been taken to increase the thermoelectric figure of merit, ZT = ( S2sigma/kappa)T, where T is temperature. In this work we examine how microstructure can be used to alter these thermoelectric propertiesin a biphasic Ti-Ni-Sn materials containing full-Heusler (fH) TiNi2Sn embedded within hH thermoelectric TiNiSn. We explored a wide range of Ni compositions in TiNi1+xSn--from stoichiometric TiNiSn to high Heusler volume fraction, TiNi1.25Sn--materials prepared by levitation induction melting followed by annealing. Phase distributions and microstructure were characterized using synchrotron x-ray diffraction and optical and electron microscopy. In a sample of the nominal composition TiNi1.15Sn, a significant decrease in thermal conductivity (about 30%) is observed for the biphasic material despite the metallic second-phase particles existing at the micrometer scale; a 50% increase in the electrical conductivity is also measured. These result in a maximum figure of merit, ZT, of 0.44 at 800 K, which is 25% greater than is observed for the x = 0 sample. Density functional theory calculations using hybrid functionals were performed to determine band alignments between the half- and full-Heusler compounds, as well as

  4. Spin-polarization and electronic properties of half-metallic Heusler alloys calculated from first principles.

    PubMed

    Galanakis, I; Mavropoulos, Ph

    2007-08-08

    Half-metallic Heusler alloys are amongst the most promising materials for future magneto-electronic applications. We review some recent results on the electronic properties of these compounds. The origin of the gap in these half-metallic alloys and its connection to the magnetic properties are well understood. Changing the lattice parameter slightly shifts the Fermi level. Spin-orbit coupling induces states within the gap but the alloys keep a very high degree of spin polarization at the Fermi level. Small degrees of doping and disorder as well as defects with low formation energy have little effect on the properties of the gap, while temperature effects can lead to a quick loss of half-metallicity. Finally, we discuss two special issues: the case of quaternary Heusler alloys and the half-metallic ferrimagnets.

  5. Theoretical prediction of half metallic ferromagnetic full-Heusler alloys Cs2CrGe

    NASA Astrophysics Data System (ADS)

    Cherid, S.; Benstaali, W.; Abbad, A.; Bentata, S.; Lantri, T.; Abbar, B.

    2017-07-01

    The structural, electronic and elastic properties of full-Heusler alloys Cs2CrGe are examined in this study using FP-LAPW method based on density functional theory. Results of our calculations predict that the Hg2CuTi-type structure is more stable than the Cu2MnAl-type structure and that the ground state of this alloy is ferromagnetic. The band structure of Cs2CrGe shows half metallic behavior for the two approaches GGA and mBJ-GGA with an indirect band gap. The total magnetic moment calculated is in good agreement with the Slater-Pauling rule for full-Heusler alloys with an important magnetic moment equal to 4 μB. Elastic properties indicate that our compound is ductile, anisotropic and not too rigid.

  6. Thermoelectric properties of half-Heusler topological insulators MPtBi (M = Sc, Y, La) induced by strain

    SciTech Connect

    Ding, Guangqian; Gao, G. Y. Yao, KaiLun; Yu, Li; Ni, Yun

    2016-01-14

    Thermoelectric (TE) materials and topological insulators (TIs) were recently known to exhibit close connection, which offers new prospects in improving the TE performance. However, currently known TE materials from TIs mostly belong to the early Bi{sub 2}Te{sub 3} family. In order to extend TE materials to other classes of TIs, we use the first-principles combined with Boltzmann transport theory to study the electronic and TE properties of experimental half-Heusler compounds MPtBi (M = Sc, Y, La). We find that all MPtBi are topological semimetals at equilibrium lattices while TIs under a stretched uniaxial strain, which is in agreement with previous works. We further predict that comparable TE performance with Bi{sub 2}Te{sub 3} can be realized in half-Heusler TI LaPtBi by an 8% stretched uniaxial strain. We also reveal that the lattice thermal conductivity of LaPtBi is unprecedented low compared with those of traditional half-Heusler compounds (not TIs). These findings indicate the potential of half-Heusler TIs as TE materials.

  7. Thermoelectric properties of half-Heusler topological insulators MPtBi (M = Sc, Y, La) induced by strain

    NASA Astrophysics Data System (ADS)

    Ding, Guangqian; Gao, G. Y.; Yu, Li; Ni, Yun; Yao, KaiLun

    2016-01-01

    Thermoelectric (TE) materials and topological insulators (TIs) were recently known to exhibit close connection, which offers new prospects in improving the TE performance. However, currently known TE materials from TIs mostly belong to the early Bi2Te3 family. In order to extend TE materials to other classes of TIs, we use the first-principles combined with Boltzmann transport theory to study the electronic and TE properties of experimental half-Heusler compounds MPtBi (M = Sc, Y, La). We find that all MPtBi are topological semimetals at equilibrium lattices while TIs under a stretched uniaxial strain, which is in agreement with previous works. We further predict that comparable TE performance with Bi2Te3 can be realized in half-Heusler TI LaPtBi by an 8% stretched uniaxial strain. We also reveal that the lattice thermal conductivity of LaPtBi is unprecedented low compared with those of traditional half-Heusler compounds (not TIs). These findings indicate the potential of half-Heusler TIs as TE materials.

  8. The intrinsic disorder related alloy scattering in ZrNiSn half-Heusler thermoelectric materials.

    PubMed

    Xie, Hanhui; Wang, Heng; Fu, Chenguang; Liu, Yintu; Snyder, G Jeffrey; Zhao, Xinbing; Zhu, Tiejun

    2014-11-03

    The intrinsic structural disorder dramatically affects the thermal and electronic transport in semiconductors. Although normally considered an ordered compound, the half-Heusler ZrNiSn displays many transport characteristics of a disordered alloy. Similar to the (Zr,Hf)NiSn based solid solutions, the unsubstituted ZrNiSn compound also exhibits charge transport dominated by alloy scattering, as demonstrated in this work. The unexpected charge transport, even in ZrNiSn which is normally considered fully ordered, can be explained by the Ni partially filling interstitial sites in this half-Heusler system. The influence of the disordering and defects in crystal structure on the electron transport process has also been quantitatively analyzed in ZrNiSn1-xSbx with carrier concentration nH ranging from 5.0 × 10(19) to 2.3 × 10(21) cm(-3) by changing Sb dopant content. The optimized carrier concentration nH ≈ 3-4 × 10(20) cm(-2) results in ZT ≈ 0.8 at 875K. This work suggests that MNiSn (M = Hf, Zr, Ti) and perhaps most other half-Heusler thermoelectric materials should be considered highly disordered especially when trying to understand the electronic and phonon structure and transport features.

  9. The intrinsic disorder related alloy scattering in ZrNiSn half-Heusler thermoelectric materials

    PubMed Central

    Xie, Hanhui; Wang, Heng; Fu, Chenguang; Liu, Yintu; Snyder, G. Jeffrey; Zhao, Xinbing; Zhu, Tiejun

    2014-01-01

    The intrinsic structural disorder dramatically affects the thermal and electronic transport in semiconductors. Although normally considered an ordered compound, the half-Heusler ZrNiSn displays many transport characteristics of a disordered alloy. Similar to the (Zr,Hf)NiSn based solid solutions, the unsubstituted ZrNiSn compound also exhibits charge transport dominated by alloy scattering, as demonstrated in this work. The unexpected charge transport, even in ZrNiSn which is normally considered fully ordered, can be explained by the Ni partially filling interstitial sites in this half-Heusler system. The influence of the disordering and defects in crystal structure on the electron transport process has also been quantitatively analyzed in ZrNiSn1-xSbx with carrier concentration nH ranging from 5.0×1019 to 2.3×1021 cm−3 by changing Sb dopant content. The optimized carrier concentration nH ≈ 3–4×1020 cm−2 results in ZT ≈ 0.8 at 875K. This work suggests that MNiSn (M = Hf, Zr, Ti) and perhaps most other half-Heusler thermoelectric materials should be considered highly disordered especially when trying to understand the electronic and phonon structure and transport features. PMID:25363573

  10. Modeling of full-Heusler alloys within tight-binding approximation: Case study of Fe2MnAl

    NASA Astrophysics Data System (ADS)

    Azhar, A.; Majidi, M. A.; Nanto, D.

    2017-07-01

    Heusler alloys have been known for about a century, and predictions of magnetic moment values using Slater-Pauling rule have been successful for many such materials. However, such a simple counting rule has been found not to always work for all Heusler alloys. For instance, Fe2CuAl has been found to have magnetic moment of 3.30 µB per formula unit although the Slater-Pauling rule suggests the value of 2 µB. On the other hand, a recent experiment shows that a non-stoichiometric Heusler compound Fe2Mn0.5Cu0.5Al possesses magnetic moment of 4 µB, closer to the Slater-Pauling prediction for the stoichiometric compound. Such discrepancies signify that the theory to predict the magnetic moment of Heusler alloys in general is still far from being complete. Motivated by this issue, we propose to do a theoretical study on a full-Heusler alloy Fe2MnAl to understand the formation of magnetic moment microscopically. We model the system by constructing a density-functional-theory-based tight-binding Hamiltonian and incorporating Hubbard repulsive as well as spin-spin interactions for the electrons occupying the d-orbitals. Then, we solve the model using Green's function approach, and treat the interaction terms within the mean-field approximation. At this stage, we aim to formulate the computational algorithm for the overall calculation process. Our final goal is to compute the total magnetic moment per unit cell of this system and compare it with the experimental data.

  11. The electronic and magnetic properties of quaternary Heusler alloy CoFeMnGe

    NASA Astrophysics Data System (ADS)

    Seema, K.

    2016-05-01

    We present study of quaternary Heusler alloy CoFeMnGe using density functional theory. The compound is half-metallic with half-metallic gap of 0.13 eV. The total magnetic moment of this compound is 3.96 μB which is in close agreement with Slater-Pauling rule. The effect of lattice compression and expansion shows the robustness of half-metallicity. A large value of half-metallic gap and 100% spin-polarization makes this material interesting for spin dependent applications.

  12. Resolving the phase structure of nonstoichiometric Co2FeGa Heusler nanoparticles

    NASA Astrophysics Data System (ADS)

    Wang, Changhai; Casper, Frederick; Guo, Yanzhi; Gasi, Teuta; Ksenofontov, Vadim; Balke, Benjamin; Fecher, Gerhard H.; Felser, Claudia; Hwu, Yeu-Kuang; Lee, Jey-Jau

    2012-12-01

    Heusler nanocrystals, i.e., Heusler compounds with dimensions in nanometer range are promising materials for next-generation spin-related devices. Recently, we have developed a chemical approach to prepare L21 ordered Heusler nanoparticles and characterized their size-related structure and magnetic properties. In this work, effect of precursor composition is investigated in terms of their importance in controlling the phase structure of Co2FeGa nanoparticles. The formation of the L21 ordered Co2FeGa phase is evidenced by combining X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy and Mössbauer spectroscopy measurements. From XRD, precursor compositions of low Co and high Fe are associated with decreasing amounts and even disappearance of fcc Co and fcc Fe impurities. We also find that, even though the XRD data indicate only pure Co2FeGa phase in sample with excess Fe, EXAFS and Mössbauer spectroscopy data unambiguously reveal the co-existence of bcc Fe with Co2FeGa main phase.

  13. Phase separation and antisite defects in the thermoelectric TiNiSn half-Heusler alloys

    SciTech Connect

    Kirievsky, K.; Gelbstein, Y. Fuks, D.

    2013-07-15

    The half-Heusler TiNiSn alloys have recently gained an attention as promising candidates for thermoelectric applications. Improvement of these alloys for such applications can be obtained by both electronic and compositional optimizations. The latter can result in a miscibility gap, allowing a phase separation in the nano-scale and consequently a thermal conductivity reduction. Combination of ab initio calculations and statistical thermodynamics was applied for studying the relative stability of a number of superstructures in TiNiSn based alloys. The quasi-binary phase diagram beyond T=0 K for TiNiSn–TiNi{sub 2}Sn solid solutions was calculated using energy parameters extracted from the total energy calculations for ordered structures in the Ni sublattice. We demonstrated that a decomposition of the off-stoichiometric Ni-rich half-Heusler alloy into the stoichiometric TiNiSn phase and into Ni deficient Heusler TiNi{sub 2}Sn phase occurs at elevated temperatures—an effect which recently had been observed experimentally. Furthermore, favorable energetic conditions for antisite defects formation were deduced, based on calculations of the energy of formation, an effect which was explained as a cooperative process of partial disordering on the Ni sublattice. The influence of these two effects on improvement of the thermoelectric performance of TiNiSn based half Heusler compounds is discussed. - Graphical abstract: Phase separation and antisite defects in the thermoelectric TiNiSn alloy, are covered as methods for nanostructuring and thereby enhancement of the thermoelectric potential. - Highlights: • Ab initio calculations/statistical thermodynamics was applied for studying the TiNiSn system. • The phase diagram for TiNiSn–TiNi{sub 2}Sn solid solutions was calculated. • Decomposition of the Ni-rich HH into TiNiSn and Ni deficient TiNi{sub 2}Sn phases was observed. • Favorable energetic conditions for antisite defects formation were deduced.

  14. Environmental Life Cycle Analysis of Water and CO2-Based Fracturing Fluids Used in Unconventional Gas Production.

    PubMed

    Wilkins, Rodney; Menefee, Anne H; Clarens, Andres F

    2016-12-06

    Many of the environmental impacts associated with hydraulic fracturing of unconventional gas wells are tied to the large volumes of water that such operations require. Efforts to develop nonaqueous alternatives have focused on carbon dioxide as a tunable working fluid even though the full environmental and production impacts of a switch away from water have yet to be quantified. Here we report on a life cycle analysis of using either water or CO2 for gas production in the Marcellus shale. The results show that CO2-based fluids, as currently conceived, could reduce greenhouse gas emissions by 400% (with sequestration credit) and water consumption by 80% when compared to conventional water-based fluids. These benefits are offset by a 44% increase in net energy use when compared to slickwater fracturing as well as logistical barriers resulting from the need to move and store large volumes of CO2. Scenario analyses explore the outlook for CO2, which under best-case conditions could eventually reduce life cycle energy, water, and greenhouse gas (GHG) burdens associated with fracturing. To achieve these benefits, it will be necessary to reduce CO2 sourcing and transport burdens and to realize opportunities for improved energy recovery, averted water quality impacts, and carbon storage.

  15. Anisotropy in layered half-metallic Heusler alloy superlattices

    NASA Astrophysics Data System (ADS)

    Azadani, Javad G.; Munira, Kamaram; Romero, Jonathon; Ma, Jianhua; Sivakumar, Chockalingam; Ghosh, Avik W.; Butler, William H.

    2016-01-01

    We show that when two Heusler alloys are layered in the [001], [110], or [111] directions for various thicknesses to form a superlattice, the Slater-Pauling rule may still be satisfied and the resulting superlattice is often half-metallic with gaps comparable to or larger than those of its constituents. In addition, uniaxial magnetocrystalline anisotropy is induced because of the differences in the electronic structure of the two Heuslers in the superlattice. Various full-full, full-half, and half-half Heusler superlattices are studied, and potential half-metallic superlattices with perpendicular magnetocrystalline anisotropy are identified.

  16. Anisotropy in layered half-metallic Heusler alloy superlattices

    SciTech Connect

    Azadani, Javad G.; Munira, Kamaram; Sivakumar, Chockalingam; Butler, William H.; Romero, Jonathon; Ma, Jianhua; Ghosh, Avik W.

    2016-01-28

    We show that when two Heusler alloys are layered in the [001], [110], or [111] directions for various thicknesses to form a superlattice, the Slater-Pauling rule may still be satisfied and the resulting superlattice is often half-metallic with gaps comparable to or larger than those of its constituents. In addition, uniaxial magnetocrystalline anisotropy is induced because of the differences in the electronic structure of the two Heuslers in the superlattice. Various full-full, full-half, and half-half Heusler superlattices are studied, and potential half-metallic superlattices with perpendicular magnetocrystalline anisotropy are identified.

  17. Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films

    NASA Astrophysics Data System (ADS)

    Logan, John; Patel, Sahil; Harrington, Sean; Polley, Craig; Schultz, Brian; Balasubramanian, T.; Janotti, Anderson; Mikkelsen, Anders; Palmstrøm, Chris

    Topological insulators are a recently discovered new quantum state of matter that has a bulk band gap but also possesses cross-gap surface states which are protected by time-reversal symmetry. The experimental realization of topologically non-trivial surface states (TSSs) in materials such as Bi2Se3 has generated widespread interest in identifying other material systems that exhibit TSSs due to their many uses including spintronic devices. In particular, recent theory calculations suggest that TSSs may be found in certain half-Heusler ternary compounds. If experimentally realizable, this would provide an opportunity for the creation of entirely new heterostructure spintronic devices that make use of the structurally-identical but electronically-varied nature of Heusler compounds. Here, we show the presence of a TSS in the half-Heusler compound PtLuSb. Spin and angle-resolved photoemission spectroscopy reveals a surface state with linear dispersion and a helical tangential spin texture consistent with theoretical predictions and the expectation for a topological insulator.

  18. Static and dynamic magnetic properties of cubic Mn-Co-Ga Heusler films

    SciTech Connect

    Demiray, A. S. Iihama, S.; Naganuma, H.; Oogane, M.; Ando, Y.; Kubota, T.; Mizukami, S. Miyazaki, T.

    2014-05-07

    We investigated the static and dynamic magnetic properties of thin films of Mn-Co-Ga Heusler compound. Gilbert damping and exchange stiffness constants of the films were evaluated by using the ferromagnetic resonance technique in the X-band regime (f = 9.4 GHz). By analyzing the experimental spectra, magnetic parameters of the films such as the line width and the Gilbert damping were deduced, and the exchange stiffness constant was estimated from the perpendicular standing spin-wave resonance. The Gilbert damping constant was estimated to be 0.017 in a specific film composition. The exchange stiffness constant showed a linear dependence on the film composition.

  19. Half-Heusler (TiZrHf)NiSn Unileg Module with High Powder Density.

    PubMed

    Populoh, Sascha; Brunko, Oliver C; Gałązka, Krzysztof; Xie, Wenjie; Weidenkaff, Anke

    2013-03-27

    (TiZrHf)NiSn half-Heusler compounds were prepared by arc melting and their thermoelectric properties characterized in the temperature range between 325 K and 857 K, resulting in a Figure of Merit ZT ≈ 0.45. Furthermore, the prepared samples were used to construct a unileg module. This module was characterized in a homemade thermoelectric module measurement stand and yielded 275 mW/cm² and a maximum volumetric power density of 700 mW/cm³. This was reached using normal silver paint as a contacting material; from an improved contacting, much higher power yields are to be expected.

  20. Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms

    PubMed Central

    Batterman, Stuart

    2017-01-01

    The ventilation rate (VR) is a key parameter affecting indoor environmental quality (IEQ) and the energy consumption of buildings. This paper reviews the use of CO2 as a “natural” tracer gas for estimating VRs, focusing on applications in school classrooms. It provides details and guidance for the steady-state, build-up, decay and transient mass balance methods. An extension to the build-up method and an analysis of the post-exercise recovery period that can increase CO2 generation rates are presented. Measurements in four mechanically-ventilated school buildings demonstrate the methods and highlight issues affecting their applicability. VRs during the school day fell below recommended minimum levels, and VRs during evening and early morning were on the order of 0.1 h−1, reflecting shutdown of the ventilation systems. The transient mass balance method was the most flexible and advantageous method given the low air change rates and dynamic occupancy patterns observed in the classrooms. While the extension to the build-up method improved stability and consistency, the accuracy of this and the steady-state method may be limited. Decay-based methods did not reflect the VR during the school day due to heating, ventilation and air conditioning (HVAC) system shutdown. Since the number of occupants in classrooms changes over the day, the VR expressed on a per person basis (e.g., L·s−1·person−1) depends on the occupancy metric. If occupancy measurements can be obtained, then the transient mass balance method likely will provide the most consistent and accurate results among the CO2-based methods. Improved VR measurements can benefit many applications, including research examining the linkage between ventilation and health. PMID:28165398

  1. Search for effective spin injection heterostructures based on half-metal Heusler alloys/gallium arsenide semiconductors: A theoretical investigation

    NASA Astrophysics Data System (ADS)

    Sivakumar, Chockalingam

    Efficient electrical spin injection from half-metal (HM) electrodes into semiconducting (SC) channel material is a desirable aspect in spintronics, but a challenging one. Half-metals based on the Heusler alloy family are promising candidates as spin sources due to their compatibility with compound SCs, and very high Curie temperatures. Numerous efforts were made in the past two decades to grow atomically abrupt interfaces between HM_Heusler and SC heterostructures. However, diffusion of magnetic impurities into the semiconductor, defects and disorder near the interface, and formation of reacted phases were great challenges. A number of theoretical efforts were undertaken to understand the role of such material defects in destroying the half-metallicity and also to propose promising half-metal/SC heterostructures based on first principles. This dissertation summarizes the investigations undertaken to decode the complexity of, and to understand the various physical properties of, a number of real-world Heusler/SC heterostructure samples, based on the ab initio density functional theory (DFT) approach. In addition, it summarizes various results from the first principles-based search for promising half-metal/SC heterostructures. First, I present results from DFT-based predictive models of actual Co 2MnSi (CMS)/GaAs heterostructures grown in (001) texture. I investigate the physical, chemical, electronic, and magnetic properties to understand the complexity of these structures and to pinpoint the origin of interfacial effects, when present. Based on the investigations of such models, I discuss the utility of those actual samples in spintronic applications. Next, I summarise the results from an ab initio DFT-based survey of 6 half-Heusler half-metal/GaAs heterostructure models in (110) texture, since compound semiconductors such as GaAs have very long spin lifetime in (110) layering. I show 3 half-Heusler alloys (CoVAs, NiMnAs, and RhFeGe), that when interfaced with Ga

  2. Heusler Alloys for CPP-GMR

    NASA Astrophysics Data System (ADS)

    Culbert, C.; Williams, M.; Chshiev, M.; Leclair, P.; Butler, W. H.

    2007-03-01

    Half-Metallic full Heusler alloys of composition X2YZ and structure type L21 have aroused interest because of their potential application in CPP-GMR spin valves for readers in hard disk drives. The X and Y are typically transition metals and the Z is a non-transition metal element. The structure of these alloys can be viewed as a variant of bcc in which (100) atomic layers of X alternate with layers of YZ. The alloys Co2MnSi and Co2MnGe have received particular attention because of their high TC which exceeds 900K. We have performed first-principles calculations using the VASP code in GGA to investigate the properties of these materials. We have found them to be half-metals in bulk in agreement with previous work. We obtained minority gaps at the Fermi energy of 0.36 and 0.51eV for Co2MnGe and Co2MnSi, respectively. We also investigated multilayers consisting of Heusler and various possible spacer materials. Interestingly, we found that for one or two atomic layers of Cr alternating with Co2MnGe along (100), the system remained half-metallic. Cr can actually be used in this way to increase the minority gap. We found that Co2MnGe slabs were typically not half metallic, but slabs terminated in a pure Mn layer retained a minority gap.

  3. Ultrahigh mobility and nonsaturating magnetoresistance in Heusler topological insulators

    NASA Astrophysics Data System (ADS)

    Shekhar, Chandra; Ouardi, Siham; Nayak, Ajaya K.; Fecher, Gerhard H.; Schnelle, Walter; Felser, Claudia

    2012-10-01

    We report the transport properties of the proposed Heusler topological insulators YPtSb, LaPtBi, and LuPdSb. All compounds show ultrahigh hole mobility with value of 4124, 4275, and 1800 cm2 V-1 s-1 for YPtBi, LaPtBi, and LuPdSb, respectively, at 300 K. The temperature dependence of mobility shows strong and weak phonon scattering for LaPtBi and LuPdSb, respectively, while YPtSb shows strong impurity scattering. These ultrahigh values of the mobility are not only due to gaplessness but also to the presence of linear dispersion of the bands close to the Fermi energy, where charge carriers behave like relativistic particles. Furthermore, nonsaturating magnetoresistance (MR) is observed in the temperature range 2-300 K that shows linear behavior at high fields. A tentative relationship between the linear MR and mobility is discussed, which indicates that the mobility controls the linear part of MR.

  4. Recent progress in half-Heusler thermoelectric materials

    SciTech Connect

    Huang, Lihong; Zhang, Qinyong; Yuan, Bo; Lai, Xiang; Yan, Xiao; Ren, Zhifeng

    2016-04-15

    Highlights: • Summarize the recent progress and advances in HH thermoelectric materials. • Preparing nanocomposites could reduce thermal conductivity. • Introducing enhance phonon scattering could further reduce the thermal conductivity. • Forming ternary systems to reducing the cost effectively. • The new class of HHs presents another opportunity to further optimize the HH system. - Abstract: Half-Heusler (HH) thermoelectric (TE) materials have been attracting extensive research interest over the last two decades, owing to their thermal stability, mechanical strength, and moderate ZT. This material system are potential candidates for medium to high temperature applications, which is close to the temperature range of most industrial waste heat sources. In this mini-review article, we briefly summarize the recent progress and advances in HH thermoelectric materials. Some effectively available approaches, such as HH nanocomposites to reduce thermal conductivity, using larger atomic mass and size differences to enhance phonon scattering to further reduce the thermal conductivity, forming ternary systems following the cost effective approach. In addition, new thermoelectric HH members are also discussed in this article, which points out that many new HH compounds may be possible for TE applications.

  5. Optimization of the thermoelectric properties of FeNbSb-based half-Heusler materials

    NASA Astrophysics Data System (ADS)

    Li, Wenfeng; Yang, Gui; Zhang, Jianwei

    2016-05-01

    FeNbSb-based half-Heusler compounds have recently been reported as promising materials for good high-temperature thermoelectric materials with a ZT  >  1. Their electronic structure and thermoelectric properties are investigated based on a first-principles simulation and the semi-classical Boltzmann transport theory. The band structures show not only light and heavy bands but also high band degeneracy near the valence band maximum, which is beneficial for thermoelectric performance. The calculated Seebeck coefficients of p-type FeNbSb at high carrier concentrations exhibit the expected high values, which is consistent with experimental data. The evolution of the electrical conductivity and power factor with carrier concentration at different temperatures is investigated. Our results show that the thermoelectric performance of p-type FeNbSb can be improved by appropriate substitution; for example, by doping Hf on the Nb site, the maximum ZT of the p-type FeNb1-x Hf x Sb can reach ~1.5 at 1200 K. This study can provide some theoretical guidance for experimental research to improve the thermoelectric performance of FeNbSb-based half-Heusler compounds.

  6. Thermodynamic Properties of Heusler Fe2-x C ox M n S i

    NASA Astrophysics Data System (ADS)

    Ito, Masakazu; Furuta, Tatsuya; Kai, Keita; Taira, Atsushi; Onda, Keijiro; Shigeta, Iduru; Hiroi, Masahiko

    2017-04-01

    We investigated the thermodynamic properties of Heusler compounds Fe2-x C ox m n S i (0.00 ≤ x ≤ 2.00). The specific heats CP(T) for compounds with x ≤ 0.1 exhibit a λ-type anomaly arising from spin rearrangements at TR. With increasing x, TR decreases linearly and vanishes at x ∼ 0.169 . The magnetic entropy, STR, derived from the magnetic specific heat, Cm(T), released at TR decreases by increasing x. This means the canting angle of spins from the [111] direction decreases by the substitution of Fe atoms with Co atoms, based on the magnetic structure model of Fe2MnSi proposed by Miles et al. For compounds with 0.5 ≤ x , CP(T) in the low-T range can be reproduced by Debye T3 law. The electronic specific heat coefficient decreases monotonically with x.

  7. A preliminary evaluation of an O2/CO2 based eddy covariance theory at Missouri AmeriFlux site

    NASA Astrophysics Data System (ADS)

    Yan, B.; Gu, L.

    2013-12-01

    The eddy covariance (EC) technique has been widely used at flux sites on every continent, across most ecosystem types and climates to monitor exchanges of momentum, mass and energy between land surface and atmosphere. In an attempt to develop a self-consistent theory for the EC technique, Gu et al. (2012) reformulated the fundamental equations for EC by introducing the concept of constraining gas that has no net ecosystem sink/source. Gu (2013) expanded the theory of Gu et al. (2012) to include paired gases whose ecosystem exchange ratios are stable over an averaging period (e.g. 30 min) and therefore can be used to constrain EC flux measurements of any gases. He proposed that O2 and CO2 are an ideal pair of gases as their biological processes are coupled and their ecosystem exchange ratio (also known as oxidative ratio) is close to 1. Advantages of this new O2/CO2 based EC theory include: 1) avoidance of covariance loss in calculating dry air density induced by spatial separation of measuring instruments and use of multiple indirectly derived variables, 2) the minimum number of assumptions adopted for the derivation of the equation, and 3) avoidance of errors related to linearization of ideal gas law. In this study, we conducted a preliminary evaluation for the basic principle of Gu (2013) EC theory. We crosschecked net ecosystem exchange (NEE) estimations from different, independent methods by using CO2 and H2O as paired constraining gases. Using CO2 and H2O instead of CO2 and O2 as paired constraining gases is not ideal in the framework of Gu (2013); however, no fast response O2 analyzer is currently available. CO2 and H2O are both transported between the inside of plants and canopy air through stomata on leaves in the processes of photosynthesis and transpiration which are known to be closely coupled. However, this close coupling is contaminated by other ecosystem sinks/sources, e.g. respiration of plants and soil for CO2 and evaporation of intercepted and soil

  8. Unconventional superconductivity in half-Heusler semimetals

    NASA Astrophysics Data System (ADS)

    Brydon, Philip; Wang, Limin; Weinert, Michael; Agterberg, Daniel

    We consider the superconductivity of the topological half-Heusler semimetals YPtBi and LuPtBi, where pairing occurs between j = 3 / 2 quasiparticles. This permits Cooper pairs with quintet or septet total angular momentum, in addition to singlet and triplet states. Purely on-site interactions can generate unconventional (quintet) time-reversal symmetry-breaking states with topologically nontrivial point or line nodes. Furthermore, due to the broken inversion symmetry in these materials, the usual s-wave singlet state can mix with a p-wave septet state, also with topologically stable line-nodes. We acknowledge support from Microsoft Station Q, LPS-CMTC, and JQI-NSF-PFC (P.M.R.B), J. Paglione and the U.S. Department of Energy Early Career Award DE-SC-0010605 (L.W.), and the NSF via DMREF-1335215 (D.F.A. and M.W.).

  9. Polarization reduction in half-metallic Heusler alloys: the effect of point defects and interfaces with semiconductors.

    PubMed

    Picozzi, Silvia; Freeman, Arthur J

    2007-08-08

    Half-metallic full-Heusler alloys represent a promising class of materials for spintronic applications. However, (i) intrinsic point defects in Heusler compounds can be detrimental with respect to their predicted 100% spin polarization at the Fermi level and (ii) when joined to mainstream semiconductors the presence of interface states-which destroys half-metallicity-can degrade their performance. Here, we present an overview of recent first-principles calculations performed to explore both these issues. In particular, we focus on ab initio FLAPW calculations performed for Co(2)MnGe and Co(2)MnSi in the presence of intrinsic defects (such as stoichiometric atomic swaps as well as non-stoichiometric antisites) and when interfaced with GaAs and Ge. Our findings show that Mn antisites, due to their low formation energies, can easily occur, in excellent consistency with experimental observations, and that they do not destroy half-metallicity. On the other hand, Co antisites, which also show a modest formation energy, give rise to defect states at the Fermi level. As for the [001]-ordered interfaces, we show that the strong hybridization in proximity to the junction gives rise to rather broad interface states that locally destroy half-metallicity. However, the bulk gaps (both in the minority spin channel for the Heusler alloy and for both spin channels in the semiconducting side) are fully recovered within a few layers away from the junction.

  10. Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias

    NASA Astrophysics Data System (ADS)

    Nayak, Ajaya K.; Nicklas, Michael; Chadov, Stanislav; Khuntia, Panchanana; Shekhar, Chandra; Kalache, Adel; Baenitz, Michael; Skourski, Yurii; Guduru, Veerendra K.; Puri, Alessandro; Zeitler, Uli; Coey, J. M. D.; Felser, Claudia

    2015-07-01

    Rational material design can accelerate the discovery of materials with improved functionalities. This approach can be implemented in Heusler compounds with tunable magnetic sublattices to demonstrate unprecedented magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. Our design approach is also demonstrated on a second material with a magnetic transition above room temperature, Mn-Fe-Ga, exemplifying the universality of the concept and the feasibility of room-temperature applications. These findings may lead to the development of magneto-electronic devices and rare-earth-free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.

  11. Study of angle dependent magnetoresistance in half-Heusler YPtBi

    NASA Astrophysics Data System (ADS)

    Kim, Hyunsoo; Wang, Kefeng; Hodovanets, Halyna; Paglione, Johnpierre

    Semimetallic half-Heusler compounds (RTBi, R = rare earth, T = Pd, Pt) have been attracting much attention because multiple theoretical calculations predicted the topologically non-trivial band structure. However, the detail band structure near the Γ point depends on the specific calculation methods, and also the band structure inferred from experimental results show discrepancy from the theoretical one. Particularly in RPtBi (R=Y, Lu, Dy, Gd), the surface metallic states, which is absent in most of the theoretical results, were evident by recent ARPES measurements, but there has not been any detailed study on the metallic surface states. Moreover, the observation of topological nodal superconductivity in YPtBi makes the knowledge of Fermiology crucial to understand the pairing mechanism in the half-Heusler superconductors. Here, we present experimental results on angular dependence of magnetoresistance at various temperatures in single crystals of YPtBi. Based on observation of the angular dependence of Shubnikov-de Haas quantum oscillations, we discuss possible topologies of the bulk as well as the surface Fermi surfaces.

  12. Half-Heusler thermoelectrics: a complex class of materials.

    PubMed

    Bos, Jan-Willem G; Downie, Ruth A

    2014-10-29

    Half-Heusler thermoelectrics first attracted interest in the late-1990s and are currently undergoing a renaissance. This has been driven by improved synthesis, processing and characterisation methods, leading to increases in the thermoelectric figure of merit and the observation of novel phenomena such as carrier filtering in nanocomposite samples. The difficulty in extracting good thermoelectric performance is at first glance surprising given the relative simplicity of the ideal crystal structure with only site occupancies and lattice parameter as crystallographic variables. However, the observed thermoelectric properties are found to depend sensitively on sample processing. Recent work has shown that prepared ingots can contain a range of inhomogeneities, including interstitials, nano- and micron sized Heusler inclusions and multiple half-Heusler phases. For this reason, the prepared materials are far more complex than initially appreciated and this may offer opportunities to enhance the thermoelectric figure of merit.

  13. Structural and Thermoelectric Properties of Ternary Full-Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Hayashi, K.; Eguchi, M.; Miyazaki, Y.

    2016-09-01

    The thermoelectric properties of ternary full-Heusler alloys, Co2 YZ, which are in a ferromagnetic state up to high temperature above 300 K, were measured and are discussed in terms of the crystal structure and electronic states. Among the full-Heusler alloys studied, the Co2MnSi sample exhibited the highest absolute value of Seebeck coefficient and also the highest electrical conductivity in the temperature range from 300 K to 1023 K. The highest power factor of 2.9 × 10-3 W/m-K2 was obtained for the Co2MnSi sample at 550 K, demonstrating the potential of half-metallic full-Heusler alloys as thermoelectric materials.

  14. First-principles study of new series of quaternary Heusler alloys CsSrCZ (Z=Si, Ge, Sn, P, As, and Sb)

    NASA Astrophysics Data System (ADS)

    Bouabça, A.; Rozale, H.; Amar, A.; Wang, X. T.; Sayade, A.; Chahed, A.

    2016-12-01

    The structural, electronic, magnetic, and thermal properties of new quaternary Heusler alloys CsSrCZ (Z=Si, Ge, Sn, P, As, and Sb) were investigated using the full-potential linearized augmented plane wave (FPLAPW) within the generalized gradient approximation (GGA) and GGA plus modified Becke and Johnson as the exchange correlation. The results showed that all Heusler compounds were stable in Type (I) structure. The CsSrCZ (Z=Si, Ge, Sn) compounds had a nearly HM characteristic, and CsSrCZ (Z=P, As, Sb) compounds were true half-metallic (HM) ferromagnets. The strong spin polarization of p orbital for C, Si, Ge, Sn, P, As, and Sb atoms is found to be the origin of ferromagnetic. The half-metallicity is preserved up to a lattice contraction of 3.45%, 1.69%, 1.69%, 7.16%, 7.16%, and 11.2% for all six quaternary Heusler compounds. We also investigated the thermal effects using the quasi-harmonic Debye model.

  15. Efficient spin injector scheme based on Heusler materials.

    PubMed

    Chadov, Stanislav; Graf, Tanja; Chadova, Kristina; Dai, Xuefang; Casper, Frederick; Fecher, Gerhard H; Felser, Claudia

    2011-07-22

    We present a rational design scheme intended to provide stable high spin polarization at the interfaces of the magnetoresistive junctions by fulfilling the criteria of structural and chemical compatibilities at the interface. This can be realized by joining the semiconducting and half-metallic Heusler materials with similar structures. The present first-principles calculations verify that the interface remains half-metallic if the nearest interface layers effectively form a stable Heusler material with the properties intermediately between the surrounding bulk parts. This leads to a simple rule for selecting the proper combinations.

  16. Etchant solutions for the removal of Cu(0) in a supercritical CO2-based "dry" chemical mechanical planarization process for device fabrication.

    PubMed

    Bessel, Carol A; Denison, Ginger M; DeSimone, Joseph M; DeYoung, James; Gross, Stephen; Schauer, Cynthia K; Visintin, Pamela M

    2003-04-30

    The microelectronics industry is focused on increasing chip complexity, improving the density of electron carriers, and decreasing the dimensions of the interconnects into the sub-0.25 mum regime while maintaining high aspect ratios. Water-based chemical mechanical planarization or polishing (CMP) faces several technical and environmental challenges. Condensed CO2 has significant potential for replacing current CMP solvents as a "dry" etching medium because of its unique properties. In working toward a condensed CO2-based CMP process, we have successfully investigated the oxidation and chelation of solid copper metal in liquid and supercritical CO2 using ethyl peroxydicarbonate and a beta-diketone chelating agent.

  17. Investigation of attractive and repulsive interactions associated with ketones in supercritical CO2, based on Raman spectroscopy and theoretical calculations.

    PubMed

    Kajiya, Daisuke; Saitow, Ken-ichi

    2013-08-07

    Carbonyl compounds are solutes that are highly soluble in supercritical CO2 (scCO2). Their solubility governs the efficiency of chemical reactions, and is significantly increased by changing a chromophore. To effectively use scCO2 as solvent, it is crucial to understand the high solubility of carbonyl compounds, the solvation structure, and the solute-solvent intermolecular interactions. We report Raman spectroscopic data, for three prototypical ketones dissolved in scCO2, and four theoretical analyses. The vibrational Raman spectra of the C=O stretching modes of ketones (acetone, acetophenone, and benzophenone) were measured in scCO2 along the reduced temperature Tr = T∕Tc = 1.02 isotherm as a function of the reduced density ρr = ρ∕ρc in the range 0.05-1.5. The peak frequencies of the C=O stretching modes shifted toward lower energies as the fluid density increased. The density dependence was analyzed by using perturbed hard-sphere theory, and the shift was decomposed into attractive and repulsive energy components. The attractive energy between the ketones and CO2 was up to nine times higher than the repulsive energy, and its magnitude increased in the following order: acetone < acetophenone < benzophenone. The Mulliken charges of the three solutes and CO2 molecules obtained by using quantum chemistry calculations described the order of the magnitude of the attractive energy and optimized the relative configuration between each solute and CO2. According to theoretical calculations for the dispersion energy, the dipole-induced-dipole interaction energy, and the frequency shift due to their interactions, the experimentally determined attractive energy differences in the three solutes were attributed to the dispersion energies that depended on a chromophore attached to the carbonyl groups. It was found that the major intermolecular interaction with the attractive shift varied from dipole-induced dipole to dispersion depending on the chromophore in the ketones

  18. Modelling the phase diagram of magnetic shape memory Heusler alloys

    NASA Astrophysics Data System (ADS)

    Entel, P.; Buchelnikov, V. D.; Khovailo, V. V.; Zayak, A. T.; Adeagbo, W. A.; Gruner, M. E.; Herper, H. C.; Wassermann, E. F.

    2006-03-01

    We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg-Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1-xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume-Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

  19. Achieving perpendicular anisotropy in half-metallic Heusler alloys for spin device applications

    SciTech Connect

    Munira, Kamaram; Romero, Jonathon; Butler, William H.

    2014-05-07

    Various full Heusler alloys are interfaced with MgO and the magnetic properties of the Heusler-MgO junctions are studied. Next to MgO, the cubic Heusler system distorts to a tetragonal one, thereby inducing an anisotropy. The half-metallicity and nature of anisotropy (in-plane or perpendicular) in the Heusler-MgO system is governed mostly by the interface Heusler layers. There is a trend that Mn-O bonding near the MgO-Heusler junction results in perpendicular anisotropy. The ability to remain half-metallic and have perpendicular anisotropy makes some of these alloys potential candidates as free-layers in Spin Transfer Torque Random Access Memory (STT-RAM) devices, particularly, Cr{sub 2}MnAs-MgO system with MnAs interface layers and Co{sub 2}MnSi-MgO system with Mn{sub 2} interface layers.

  20. Spin injection from Heusler alloys into semiconductors: A materials perspective

    NASA Astrophysics Data System (ADS)

    Farshchi, R.; Ramsteiner, M.

    2013-05-01

    The notion of using electron spins as bits for highly efficient computation coupled with non-volatile data storage has driven an intense international research effort over the past decade. Such an approach, known as spin-based electronics or spintronics, is considered to be a promising alternative to charge-based electronics in future integrated circuit technologies. Many proposed spin-based devices, such as the well-known spin-transistor, require injection of spin polarized currents from ferromagnetic layers into semiconductor channels, where the degree of injected spin polarization is crucial to the overall device performance. Several ferromagnetic Heusler alloys are predicted to be half-metallic, meaning 100% spin-polarized at the Fermi level, and hence considered to be excellent candidates for electrical spin injection. Furthermore, they exhibit high Curie temperatures and close lattice matching to III-V semiconductors. Despite their promise, Heusler alloy/semiconductor heterostructures investigated in the past decade have failed to fulfill the expectation of near perfect spin injection and in certain cases have even demonstrated inferior behavior compared to their elemental ferromagnetic counterparts. To address this problem, a slew of theoretical and experimental work has emerged studying Heusler alloy/semiconductor interface properties. Here, we review the dominant prohibitive materials challenges that have been identified, namely atomic disorder in the Heusler alloy and in-diffusion of magnetic impurities into the semiconductor, and their ensuing detrimental effects on spin injection. To mitigate these effects, we propose the incorporation of half-metallic Heusler alloys grown at high temperatures (>200 °C) along with insertion of a MgO tunnel barrier at the ferromagnet/semiconductor interface to minimize magnetic impurity in-diffusion and potentially act as a spin-filter. By considering evidence from a variety of structural, optical, and electrical

  1. Epitaxial growth of the Heusler alloy Co2Cr1-xFexAl

    NASA Astrophysics Data System (ADS)

    Kelekar, R.; Clemens, B. M.

    2004-07-01

    We report a method for the growth of single-phase epitaxial thin films of compounds from the family of Heusler alloys Co2Cr1-xFexAl. Elemental targets were dc magnetron sputtered in 1.5 mtorr Ar gas onto MgO substrates held at 500 °C at a total growth rate of ≈0.8 Å/s. As the Fe content increases, the structural quality improves, the level of chemical ordering increases, and the slope of the resistivity versus temperature, dρ/dt, above 50 K changes from negative to positive. An extraordinary Hall resistivity exceeding 1×10-8 Ω m is observed in the Cr-containing alloys at low temperature and room temperature. Preliminary work on the incorporation of a single quaternary alloy into spin valves shows maximum giant magnetoresistances ranging from 4% at 15 K to 2% at room temperature.

  2. Topological RPdBi half-Heusler semimetals: A new family of noncentrosymmetric magnetic superconductors.

    PubMed

    Nakajima, Yasuyuki; Hu, Rongwei; Kirshenbaum, Kevin; Hughes, Alex; Syers, Paul; Wang, Xiangfeng; Wang, Kefeng; Wang, Renxiong; Saha, Shanta R; Pratt, Daniel; Lynn, Jeffrey W; Paglione, Johnpierre

    2015-06-01

    We report superconductivity and magnetism in a new family of topological semimetals, the ternary half-Heusler compound RPdBi (R: rare earth). In this series, tuning of the rare earth f-electron component allows for simultaneous control of both lattice density via lanthanide contraction and the strength of magnetic interaction via de Gennes scaling, allowing for a unique tuning of the normal-state band inversion strength, superconducting pairing, and magnetically ordered ground states. Antiferromagnetism with ordering vector (½,½,½) occurs below a Néel temperature that scales with de Gennes factor dG, whereas a superconducting transition is simultaneously supressed with increasing dG. With superconductivity appearing in a system with noncentrosymmetric crystallographic symmetry, the possibility of spin-triplet Cooper pairing with nontrivial topology analogous to that predicted for the normal-state electronic structure provides a unique and rich opportunity to realize both predicted and new exotic excitations in topological materials.

  3. Electron correlations and the minority-spin band gap in half-metallic Heusler alloys.

    PubMed

    Chioncel, L; Arrigoni, E; Katsnelson, M I; Lichtenstein, A I

    2006-04-07

    Electron-electron correlations affect the band gap of half-metallic ferromagnets by introducing nonquasiparticle states just above the Fermi level. In contrast with the spin-orbit coupling, a large asymmetric nonquasiparticle spectral weight is present in the minority-spin channel, leading to a peculiar finite-temperature spin depolarization effects. Using recently developed first-principle dynamical mean-field theory, we investigate these effects for the half-metallic ferrimagnetic Heusler compound FeMnSb. We discuss depolarization effects in terms of strength of local Coulomb interaction U and temperature in FeMnSb. We propose Ni(1-x)Fe(x)MnSb alloys as a perspective materials to be used in spin-valve structures and for experimental search of nonquasiparticle states in half-metallic materials.

  4. Understanding the transport properties of YNiBi half- Heusler alloy: An Ab-initio study

    NASA Astrophysics Data System (ADS)

    Sharma, Sonu; Kumar, Pradeep

    2017-05-01

    In the present work, we have studied the electronic and transport properties of YNiBi half-Heusler alloy by combining the first principles methods with the Boltzmann transport theory. The electronic band structure and total density of states plot suggest the presence of semiconducting ground state in the compound. The value of indirect band gap is found to be ˜0.21 eV. The origin of the band gap is associated primarily with the interaction between the Ni 3d and the Y 4d states. The room temperature value of Seebeck coefficient is ˜230 µVK-1. A moderate power factor of about 12×1014 μ Wcm-1 K-2 s-1 is obtained at 980 k.

  5. Structural transformation studies on the rare earth containing Heusler alloys Pd 2RESn

    NASA Astrophysics Data System (ADS)

    Umarji, A. M.; Malik, S. K.; Shenoy, G. K.

    1985-03-01

    The Heusler alloys Pd 2RESn form for rare-earths (RE) from Tb to Lu and for Sc and Y. Of these, the alloys containing Yb, Tm, Lu, Sc and Y are superconducting. We have carried out structural studies on all these alloys by studying the temperature dependence of the X-ray patterns in the temperature range 5 to 300 K. Some nonstoichiometric compositions were also investigated. Structural transformation is observed only in Tb and Dy containing alloys while none of the superconducting alloys show a transformation. The transformation temperature is lowered by about 50 K in going from stoichiometric Pd 2TbSn to nonstoichiometric Pd 2Tb 0.95Sn while it is completely suppressed in Pd 2Dy 0.95Sn. Magnetic and Mössbauer studies on Dy compound are also reported.

  6. Compound

    NASA Astrophysics Data System (ADS)

    Suzumura, Akitoshi; Watanabe, Masaki; Nagasako, Naoyuki; Asahi, Ryoji

    2014-06-01

    Recently, Cu-based chalcogenides such as Cu3SbSe4, Cu2Se, and Cu2SnSe3 have attracted much attention because of their high thermoelectric performance and their common feature of very low thermal conductivity. However, for practical use, materials without toxic elements such as selenium are preferable. In this paper, we report Se-free Cu3SbS4 thermoelectric material and improvement of its figure of merit ( ZT) by chemical substitutions. Substitutions of 3 at.% Ag for Cu and 2 at.% Ge for Sb lead to significant reductions in the thermal conductivity by 37% and 22%, respectively. These substitutions do not sacrifice the power factor, thus resulting in enhancement of the ZT value. The sensitivity of the thermal conductivity to chemical substitutions in these compounds is discussed in terms of the calculated phonon dispersion and previously proposed models for Cu-based chalcogenides. To improve the power factor, we optimize the hole carrier concentration by substitution of Ge for Sb, achieving a power factor of 16 μW/cm K2 at 573 K, which is better than the best reported for Se-based Cu3SbSe4 compounds.

  7. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials

    NASA Astrophysics Data System (ADS)

    Nayak, Ajaya K.; Kumar, Vivek; Ma, Tianping; Werner, Peter; Pippel, Eckhard; Sahoo, Roshnee; Damay, Franoise; Rößler, Ulrich K.; Felser, Claudia; Parkin, Stuart S. P.

    2017-08-01

    Magnetic skyrmions are topologically stable, vortex-like objects surrounded by chiral boundaries that separate a region of reversed magnetization from the surrounding magnetized material. They are closely related to nanoscopic chiral magnetic domain walls, which could be used as memory and logic elements for conventional and neuromorphic computing applications that go beyond Moore’s law. Of particular interest is ‘racetrack memory’, which is composed of vertical magnetic nanowires, each accommodating of the order of 100 domain walls, and that shows promise as a solid state, non-volatile memory with exceptional capacity and performance. Its performance is derived from the very high speeds (up to one kilometre per second) at which chiral domain walls can be moved with nanosecond current pulses in synthetic antiferromagnet racetracks. Because skyrmions are essentially composed of a pair of chiral domain walls closed in on themselves, but are, in principle, more stable to perturbations than the component domain walls themselves, they are attractive for use in spintronic applications, notably racetrack memory. Stabilization of skyrmions has generally been achieved in systems with broken inversion symmetry, in which the asymmetric Dzyaloshinskii-Moriya interaction modifies the uniform magnetic state to a swirling state. Depending on the crystal symmetry, two distinct types of skyrmions have been observed experimentally, namely, Bloch and Néel skyrmions. Here we present the experimental manifestation of another type of skyrmion—the magnetic antiskyrmion—in acentric tetragonal Heusler compounds with D2d crystal symmetry. Antiskyrmions are characterized by boundary walls that have alternating Bloch and Néel type as one traces around the boundary. A spiral magnetic ground-state, which propagates in the tetragonal basal plane, is transformed into an antiskyrmion lattice state under magnetic fields applied along the tetragonal axis over a wide range of temperatures

  8. Large low-field positive magnetoresistance in nonmagnetic half-Heusler ScPtBi single crystal

    NASA Astrophysics Data System (ADS)

    Hou, Zhipeng; Wang, Yue; Liu, Enke; Zhang, Hongwei; Wang, Wenhong; Wu, Guangheng

    2015-11-01

    High-quality nonmagnetic half-Heusler ScPtBi single crystals were synthesized by a Bi self-flux method. This compound was revealed to be a hole-dominated semimetal with a large low-field magnetoresistance up to 240% at 2 K in a magnetic field of 1 T. Magneto-transport measurements demonstrated that the large low-field magnetoresistance effect resulted from the coexistence of field-induced metal-semiconductor transition and weak-antilocalization effect. Moreover, Hall measurements indicated that ScPtBi single crystal showed a high mobility over a wide temperature region even up to room temperature (4050 cm2V-1s-1 at 2 K-2016 cm2V-1s-1 at 300 K). These findings not only suggest the nonmagnetic ScPtBi semimetal a potential material candidate for applications in high-sensitivity magnetic sensors but also are of great significance to comprehensively understand the rare-earth based half-Heusler compounds.

  9. Half-metallicity in the inverse Heusler Ti2RuSn alloy: A first-principles prediction

    NASA Astrophysics Data System (ADS)

    Taşkın, Ferhat; Atiş, Murat; Canko, Osman; Kervan, Selçuk; Kervan, Nazmiye

    2017-03-01

    The electronic and magnetic properties of the Ti2RuSn Heusler compound are investigated by means of ab initio calculations with the full-potential linearized augmented plane wave (FLAPW) method. The generalized gradient approximation (GGA) method is used for the calculations. The CuHg2Ti-type structure is energetically more stable than the AlCu2Mn-type structure. The inverse-Heusler Ti2RuSn represents half-metallic behavior. The total spin moment of the compound is 2 μB which coincides with the Slater-Pauling rule of Mt =Zt - 18 with the equilibrium lattice constant a0 = 6.44 Å and the strained lattice constants as well. The majority bands have metallic properties, but the minority bands have semiconductor properties with a gap of 0.35 eV, and the spin-flip gap is 0.23 eV. Moreover, the sensitivity of half-metallicity is investigated under two types of structural distortion, namely uniform strain and tetragonal distortion.

  10. Electronic structure and half-metallicity in new Heusler alloys CoYO2 (Y = Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, and Zn)

    NASA Astrophysics Data System (ADS)

    Esteki, S.; Ahmadian, F.

    2017-09-01

    First-principles calculations based on density functional theory (DFT) using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method were applied to study the electronic structures and magnetic properties of new Heusler alloys CoYO2 (Y = Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, and Zn). The calculated formation energies of these compounds were negative, therefore, they can be synthesized experimentally. All compounds were stable in ferromagnetic AlCu2Mn-type structure. In AlCu2Mn-type structure, CoScO2, CoFeO2, and CoNiO2 compounds were HM ferromagnets, CoCuO2 was a nearly half-metal, CoZnO2 was a spin gapless semiconductor, and other compounds were conventional ferromagnets. In CuHg2Ti-type structure, CoTiO2 compound had a nearly HM characteristic, CoVO2 was a spin gapless semiconductor, and other compounds were conventional ferromagnets. The origin of the half-metallic band gap for CoScO2 alloy Heusler alloy was well understood. The total magnetic moments of the three HM compounds obeyed Slater-Pauling rules (Mtot = 22-Ztot and Mtot = 32-Ztot). CoScO2 had the widest region of half-metallicity between the three half-metals indicating its high robustness of half-metallicity with respect to the variation of lattice constants.

  11. Half-metallic Co-based quaternary Heusler alloys for spintronics: Defect- and pressure-induced transitions and properties

    SciTech Connect

    Enamullah, .; Johnson, D. D.; Suresh, K. G.; Alam, Aftab

    2016-11-07

    Heusler compounds offer potential as spintronic devices due to their spin polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L21) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite disorder between Cr-Al pair in CoMnCrAl alloy is energetically the most favorable, and retains half-metallic character in Cr-excess regime. However, Co-deficient samples in both alloys undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half-metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground state, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. Furthermore, this information should help in controlling these potential spintronic materials.

  12. Half-metallic Co-based quaternary Heusler alloys for spintronics: Defect- and pressure-induced transitions and properties

    NASA Astrophysics Data System (ADS)

    Enamullah, Johnson, D. D.; Suresh, K. G.; Alam, Aftab

    2016-11-01

    Heusler compounds offer potential as spintronic devices due to their spin polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L 21 ) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite disorder between Cr-Al pair in CoMnCrAl alloy is energetically the most favorable, and retains half-metallic character in Cr-excess regime. However, Co-deficient samples in both alloys undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half-metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground state, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. This information should help in controlling these potential spintronic materials.

  13. Half-metallic Co-based quaternary Heusler alloys for spintronics: Defect- and pressure-induced transitions and properties

    DOE PAGES

    Enamullah, .; Johnson, D. D.; Suresh, K. G.; ...

    2016-11-07

    Heusler compounds offer potential as spintronic devices due to their spin polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L21) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisitemore » disorder between Cr-Al pair in CoMnCrAl alloy is energetically the most favorable, and retains half-metallic character in Cr-excess regime. However, Co-deficient samples in both alloys undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half-metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground state, the minority-spin band gap and magnetic moments vary depending on the atoms swapped. Furthermore, this information should help in controlling these potential spintronic materials.« less

  14. Structural, electronic and magnetic properties of Fe2-based full Heusler alloys: A first principle study

    NASA Astrophysics Data System (ADS)

    Dahmane, F.; Mogulkoc, Y.; Doumi, B.; Tadjer, A.; Khenata, R.; Bin Omran, S.; Rai, D. P.; Murtaza, G.; Varshney, Dinesh

    2016-06-01

    Using the first-principles density functional calculations, the structural, electronic and magnetic properties of the Fe2XAl (X=Cr, Mn, Ni) compounds in both the Hg2CuTi and Cu2MnAl-type structures were studied by the full-potential linearized augmented plane waves (FP-LAPW) method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA) where the results show that the Cu2MnAl-type structure is energetically more stable than the Hg2CuTi-type structure for the Fe2CrAl and Fe2MnAl compounds at the equilibrium volume. The full Heusler compounds Fe2XAl (X=Cr, Mn) are half-metallic in the Cu2MnAl-type structure. Fe2NiAl has a metallic character in both CuHg2Ti and AlCu2Mn-type structures. The total magnetic moments of the Fe2CrAl and Fe2MnAl compounds are 1.0 and 2.0 μB, respectively, which are in agreement with the Slater-Pauling rule Mtot=Ztot- 24.

  15. Fermi surface studies of Co-based Heusler alloys: Ab-initio study

    NASA Astrophysics Data System (ADS)

    Ram, Swetarekha; Kanchana, V.

    2013-02-01

    The electronic, Fermi surface (FS) and magnetic properties of ferromagnetic Heusler alloys Co2XY (X = Cr, Mn, Fe; Y=Al, Ga) have been investigated by means of first principles calculation. Out of these compounds, Co2CrAl is found to be perfectly half-metallic (HM) at ambient. Under pressure HM to nearly HM (NHM) transition is observed around 75 GPa for Co2CrAl and NHM to HM transition is observed around 40 GPa and 18 GPa for Co2CrGa and Co2MnAl, respectively, while no transition is observed for other compounds under study and is also analyzed from the FS studies. The states at the Fermi level in the majority spin are strongly hybridized Co-d and X-d like states. The majority band FS topology change is observed under pressure for the compounds where we observe a transition, while the minority band FS remain unaltered under pressure for all compounds except in Co2FeGa, where we observed an electron sheet at X point instead of hole pocket at Γ point.

  16. Ternary Phases (Heusler) in the System Ti-Co-Sn

    NASA Astrophysics Data System (ADS)

    Kosinskiy, Andrey; Karlsen, Ole Bjørn; Sørby, Magnus H.; Prytz, Øystein

    2016-12-01

    Some of the Heusler-phases ( XY 2 Z and XYZ) are known to have large homogeneity ranges which can be useful for tuning material properties. In this work, we have revised the isothermal section of the Ti-Co-Sn system at 973 K (700 °C). A total of 29 ternary compositions, mostly in the regions TiCo2- x Sn for 0 ≤ x ≤ 1 and Ti1+ y Co2Sn1- y for 0 ≤ y ≤ 1, were prepared by arc-melting, then ball-milled and annealed. The resulting annealed powder samples were studied by applying the Rietveld method to X-ray and neutron powder diffraction data. Half-Heusler TiCoSn was not observed. The Heusler phase observed in TiCo2- x Sn has compositions ranging from TiCo1.52Sn to TiCo2Sn and has the half-Heusler structure where the excess of Co is located on the semi-filled tetrahedral site 4 d (¾, ¾, ¾) in the space group F-43 m. At 1273 K (1000 °C), this solid solubility is expanded from TiCo2Sn to TiCo with full solid solubility where Ti is gradually replacing Sn (Ti1+ y Co2Sn1- y for 0 ≤ y ≤ 1), while at 973 K (700 °C) there is a small solubility gap for 0.0 ≤ y ≤ 0.2.

  17. Role of structural defects on the half-metallic character of Heusler alloys and their junctions with Ge and GaAs

    NASA Astrophysics Data System (ADS)

    Picozzi, Silvia; Continenza, Alessandra; Freeman, Arthur J.

    2003-03-01

    Heusler-alloys, such as Co_2MnGe and Co_2MnSi, have been predicted from first-principles calculations to be half-metallic and have recently attracted great attention for spin-injection purposes. However, spin polarizations of only 50%-60% were experimentally obtained for Heusler thin films(M.P.Raphael et al.), Phys. Rev. B 66, 104429 (2002); B.Ravel et al., Appl. Phys. Lett. 81, 2812 (2002) - a decrease attributed to defects in the Mn and Co sublattices. We performed ab-initio FLAPW(E.Wimmer, H.Krakauer, M.Weinert, A.J.Freeman, Phys. Rev. B 24, 864 (1981)) calculations in order to determine the effects of several types of defects (Co and Mn antisites, vacancies, etc.) on the electronic and magnetic properties of the bulk Heusler compounds and their interfaces with Ge and GaAs. We analyze the results in terms of spatial and energy behavior of the defect- and interface-induced states. Our findings show that Co-antisites have low formation energies and locally destroy half-metallicity; the defect states are spatially localized and are screened out in a couple of atomic shells.

  18. Properties of the quaternary half-metal-type Heusler alloy Co{sub 2}Mn{sub 1-x}Fe{sub x}Si

    SciTech Connect

    Balke, Benjamin; Fecher, Gerhard H.; Kandpal, Hem C.; Felser, Claudia; Kobayashi, Keisuke; Ikenaga, Eiji; Kim, Jung-Jin; Ueda, Shigenori

    2006-09-01

    This paper reports on the bulk properties of the quaternary Heusler alloy Co{sub 2}Mn{sub 1-x}Fe{sub x}Si with the Fe concentration x=0,1/2,1. All samples, which were prepared by arc melting, exhibit L2{sub 1} long-range order over the complete range of Fe concentration. The structural and magnetic properties of the Co{sub 2}Mn{sub 1-x}Fe{sub x}Si Heusler alloys were investigated by means of x-ray diffraction, high- and low-temperature magnetometry, Moessbauer spectroscopy, and differential scanning calorimetry. The electronic structure was explored by means of high-energy photoemission spectroscopy at about 8 keV photon energy. This ensures true bulk sensitivity of the measurements. The magnetization of the Fe-doped Heusler alloys is in agreement with the values of the magnetic moments expected for a Slater-Pauling-like behavior of half-metallic ferromagnets. The experimental findings are discussed on the basis of self-consistent calculations of the electronic and magnetic structure. To achieve good agreement with experiment, the calculations indicate that on-site electron-electron correlation must be taken into account, even at low Fe concentration. The present investigation focuses on searching for the quaternary compound where the half-metallic behavior is stable against outside influences. Overall, the results suggest that the best candidate may be found at an iron concentration of about 50%.

  19. Electronic, structural, and magnetic properties of the quaternary Heusler alloy NiCoMnZ (Z=Al, Ge, and Sn)

    NASA Astrophysics Data System (ADS)

    Halder, Madhumita; Mukadam, M. D.; Suresh, K. G.; Yusuf, S. M.

    2015-03-01

    The electronic, magnetic, and structural properties of the Heusler alloys NiCoMnZ (Z=Al, Ge, and Sn) have been investigated both theoretically and experimentally. NiCoMnGe and NiCoMnSn have ordered cubic Heusler structure (with a possible disorder between Ni and Co), while NiCoMnAl has a B2 type disordered Heusler structure with random occupancy between Mn and Al atom at their crystallographic sites. Electronic structure calculation shows that NiCoMnGe and NiCoMnSn are normal ferromagnets, whereas NiCoMnAl is nearly half metallic (~100% spin polarization) in nature with its magnetic moment close to an integer value following the Slater-Pauling rule. Ab-initio calculations show ~56% and ~60% spin polarization for NiCoMnGe and NiCoMnSn, respectively. Magnetization measurements show all the three compounds have a high Curie temperature (>583 K).

  20. On the origin of a band gap in compounds of diamond-like structures.

    PubMed

    Köhler, Jürgen; Deng, Shuiquan; Lee, Changhoon; Whangbo, Myung-Hwan

    2007-03-19

    Electronic structure calculations were performed to examine the origin of a band gap present in most 18-electron half-Heusler compounds and its absence in NaTl. In these compounds of diamond-like structures, the presence or absence of a band gap is controlled by the sigma antibonding between the valence s orbitals, and the bonding characteristics of the late-main-group elements depend on the extent of their ns/np hybridization. Implications of these observations on the formal oxidation state and the covalent bonding of the transition-metal atoms in 18-electron half-Heusler and related compounds were discussed.

  1. Uniaxial-stress tuned large magnetic-shape-memory effect in Ni-Co-Mn-Sb Heusler alloys

    NASA Astrophysics Data System (ADS)

    Salazar Mejía, C.; Küchler, R.; Nayak, A. K.; Felser, C.; Nicklas, M.

    2017-02-01

    Combined strain and magnetization measurements on the Heusler shape-memory alloys Ni45Co5Mn38Sb12 and Ni44Co6Mn38Sb12 give evidence for strong magneto-structural coupling. The sample length changes up to 1% at the martensitic transformation, between a ferromagnetic, austenitic phase at high temperatures and a weakly magnetic, low-symmetry martensitic phase at lower temperatures. Under moderate uniaxial stress, the change in the sample length increases to and saturates at about 3%, pointing to stabilization of a single martensitic variant. A reverse martensitic transformation can also be induced by applying magnetic field: we find that within the temperature range of thermal hysteresis of the martensitic transformation, applying a field can induce a metastable expansion of the sample, while at slightly lower temperatures, the field response is reversible. These findings provide key information for future use of Ni(Co)-Mn-Sb-based Heusler compounds in, e.g., actuators and mechanical switches.

  2. Magnetic properties of low-moment ferrimagnetic Heusler Cr 2 CoGa thin films grown by molecular beam epitaxy

    DOE PAGES

    Jamer, Michelle E.; Sterbinsky, George E.; Stephen, Gregory M.; ...

    2016-10-31

    Recently, theorists have predicted many materials with a low magnetic moment and large spin-polarization for spintronic applications. These compounds are predicted to form in the inverse Heusler structure; however, many of these compounds have been found to phase segregate. In this study, ordered Cr2CoGa thin films were synthesized without phase segregation using molecular beam epitaxy. The present as-grown films exhibit a low magnetic moment from antiferromagnetically coupled Cr and Co atoms as measured with superconducting quantum interface device magnetometry and soft X-ray magnetic circular dichroism. Electrical measurements demonstrated a thermally-activated semiconductor-like resistivity component with an activation energy of 87 meV.more » These results confirm spin gapless semiconducting behavior, which makes these thin films well positioned for future devices.« less

  3. First principle study of a new half-metallic ferrimagnetic full-Heusler alloy Ti2VPb

    NASA Astrophysics Data System (ADS)

    Hamri, Bouabdallah; Hamri, Abdelkrim; Hamri, Aida; Hallouche, Abbes; Abbar, Boucif

    2016-04-01

    In this paper, we performed an ab initio calculation of the different properties of the full-Heusler alloy Ti2VPb which is reported for the first time using the WIEN2k implementation of the full potential linearized augmented plane wave (FPLAPW) scheme. In order to study this compound for magneto-electronic applications we used the generalized gradient approximation (GGA) approximation for its mechanical properties but the MBJ-GGA to better investigate the electronic properties. Our results show that the Ti2VPb compound exhibits half-metallicity at its equilibrium lattice 6.548 Å but a metallic character would occur with an expansion of its lattice parameters to 6.85 Å.

  4. Substitution effect on magnetic and electrical properties of half-Heusler alloy Ni{sub 1−x}Co{sub x}Mn{sub 1−y}Fe{sub y}Sb

    SciTech Connect

    Kushwaha, Varun Sharma, Himanshu Dixit, Dinesh Tomy, C. V.; Tulapurkar, Ashwin

    2014-04-24

    We have studied the effects of Co and Fe doping on the magnetic and electrical properties of half-Heusler compound NiMnSb. The alloys were prepared by arc-melting method in the presence of Argon gas. The powder X-ray diffraction of the each alloy was performed in air at room temperature. The magnetic and electrical properties were performed in the temperature range 2–400 K and in magnetic field up to 1 T.

  5. Equiatomic quaternary Heusler alloys: A material perspective for spintronic applications

    NASA Astrophysics Data System (ADS)

    Bainsla, Lakhan; Suresh, K. G.

    2016-09-01

    Half-metallic ferromagnetic (HMF) materials show high spin polarization and are therefore interesting to researchers due to their possible applications in spintronic devices. In these materials, while one spin sub band has a finite density of states at the Fermi level, the other sub band has a gap. Because of their high Curie temperature (TC) and tunable electronic structure, HMF Heusler alloys have a special importance among the HMF materials. Full Heusler alloys with the stoichiometric composition X2YZ (where X and Y are the transition metals and Z is a sp element) have the cubic L21 structure with four interpenetrating fcc sublattices. When each of these four fcc sublattices is occupied by different atoms (XX'YZ), a quaternary Heusler structure with different structural symmetries (space group F-43m, #216) is obtained. Recently, these equiatomic quaternary Heusler alloys (EQHAs) with 1:1:1:1 stoichiometry have attracted a lot of attention due to their superior magnetic and transport properties. A special class of HMF materials identified recently is known as spin gapless semiconductors (SGS). The difference in this case, compared with HMFs, is that the density of states for one spin band is just zero at the Fermi level, while the other has a gap as in the case of HMFs. Some of the reported SGS materials belong to EQHAs family. This review is dedicated to almost all reported materials belonging to EQHAs family. The electronic structure and hence the physical properties of Heusler alloys strongly depend on the degree of structural order and distribution of the atoms in the crystal lattice. A variety of experimental techniques has been used to probe the structural parameters and degree of order in these alloys. Their magnetic properties have been investigated using the conventional methods, while the spin polarization has been probed by point contact Andreev reflection technique. The experimentally obtained values of saturation magnetization are found to be in

  6. Equiatomic quaternary Heusler alloys: A material perspective for spintronic applications

    SciTech Connect

    Bainsla, Lakhan E-mail: suresh@phy.iitb.ac.in; Suresh, K. G. E-mail: suresh@phy.iitb.ac.in

    2016-09-15

    Half-metallic ferromagnetic (HMF) materials show high spin polarization and are therefore interesting to researchers due to their possible applications in spintronic devices. In these materials, while one spin sub band has a finite density of states at the Fermi level, the other sub band has a gap. Because of their high Curie temperature (T{sub C}) and tunable electronic structure, HMF Heusler alloys have a special importance among the HMF materials. Full Heusler alloys with the stoichiometric composition X{sub 2}YZ (where X and Y are the transition metals and Z is a sp element) have the cubic L2{sub 1} structure with four interpenetrating fcc sublattices. When each of these four fcc sublattices is occupied by different atoms (XX′YZ), a quaternary Heusler structure with different structural symmetries (space group F-43m, #216) is obtained. Recently, these equiatomic quaternary Heusler alloys (EQHAs) with 1:1:1:1 stoichiometry have attracted a lot of attention due to their superior magnetic and transport properties. A special class of HMF materials identified recently is known as spin gapless semiconductors (SGS). The difference in this case, compared with HMFs, is that the density of states for one spin band is just zero at the Fermi level, while the other has a gap as in the case of HMFs. Some of the reported SGS materials belong to EQHAs family. This review is dedicated to almost all reported materials belonging to EQHAs family. The electronic structure and hence the physical properties of Heusler alloys strongly depend on the degree of structural order and distribution of the atoms in the crystal lattice. A variety of experimental techniques has been used to probe the structural parameters and degree of order in these alloys. Their magnetic properties have been investigated using the conventional methods, while the spin polarization has been probed by point contact Andreev reflection technique. The experimentally obtained values of saturation magnetization are

  7. Enhanced thermoelectric performance in the p-type half-Heusler (Ti/Zr/Hf)CoSb0.8Sn0.2 system via phase separation.

    PubMed

    Rausch, Elisabeth; Balke, Benjamin; Ouardi, Siham; Felser, Claudia

    2014-12-14

    A novel approach for optimization of the thermoelectric properties of p-type Heusler compounds with a C1b structure was investigated. A successful recipe for achieving intrinsic phase separation in the n-type material based on the TiNiSn system is isoelectronic partial substitution of Ti with its heavier homologues Zr and Hf. We applied this concept to the p-type system MCoSb0.8Sn0.2 by a systematic investigation of samples with different compositions at the Ti position (M = Ti, Zr, Hf, Ti0.5Zr0.5, Zr0.5Hf0.5, and Ti0.5Hf0.5). We thus achieved an approximately 40% reduction of the thermal conductivity and a maximum figure of merit ZT of 0.9 at 700 °C. This is a 80% improvement in peak ZT from 0.5 to 0.9 at 700 °C compared to the best published value of an ingot p-type half-Heusler compound. Thus far, comparable good thermoelectric p-type materials of this structure type have only been realized by a nanostructuring process via ball milling of premelted ingot samples followed by a rapid consolidation method, like hot pressing. The herein-presented simple arc-melting fabrication method reduces the fabrication time as compared to this multi-step nanostructuring process. The high mechanical stability of the Heusler compounds is favorable for the construction of thermoelectric modules. The Vickers hardness values are close to those of the n-type material, leading to good co-processability of both materials.

  8. First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors

    NASA Astrophysics Data System (ADS)

    Jakobsson, A.; Mavropoulos, P.; Şaşıoǧlu, E.; Blügel, S.; Ležaić, M.; Sanyal, B.; Galanakis, I.

    2015-05-01

    Employing first-principles electronic-structure calculations in conjunction with the frozen-magnon method, we calculate exchange interactions, spin-wave dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin gapless semiconductor (SGS) compounds Mn2CoAl , Ti2MnAl , Cr2ZnSi , Ti2CoSi , and Ti2VAs . We find that their magnetic behavior is similar to the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice exchange interactions play an essential role in the formation of the magnetic ground state and in determining the Curie temperature Tc. All compounds, except Ti2CoSi , possess a ferrimagnetic ground state. Due to the finite energy gap in one spin channel, the exchange interactions decay sharply with the distance, and hence magnetism of these SGSs can be described considering only nearest- and next-nearest-neighbor exchange interactions. The calculated spin-wave dispersion curves are typical for ferrimagnets and ferromagnets. The spin-wave stiffness constants turn out to be larger than those of the elementary 3 d ferromagnets. Calculated exchange parameters are used as input to determine the temperature dependence of the magnetization and Tc of the SGSs. We find that the Tc of all compounds is much above the room temperature. The calculated magnetization curve for Mn2CoAl as well as the Curie temperature are in very good agreement with available experimental data. This study is expected to pave the way for a deeper understanding of the magnetic properties of the inverse-Heusler-based SGSs and enhance the interest in these materials for application in spintronic and magnetoelectronic devices.

  9. Compensated Ferrimagnetic Tetragonal Heusler Thin Films for Antiferromagnetic Spintronics.

    PubMed

    Sahoo, Roshnee; Wollmann, Lukas; Selle, Susanne; Höche, Thomas; Ernst, Benedikt; Kalache, Adel; Shekhar, Chandra; Kumar, Nitesh; Chadov, Stanislav; Felser, Claudia; Parkin, Stuart S P; Nayak, Ajaya K

    2016-10-01

    Fully compensated ferrimagnets with tetragonal crystal structure have the potential for large spin-polarization and strong out-of-plane magnetic anisotropy; hence, they are ideal candidates for high-density-memory applications. Tetragonal Heusler thin films with compensated magnetic state are realized by substitution of Pt in Mn3-x Ptx Ga. Furthermore, the bilayer formed from compensated/uncompensated Mn-Pt-Ga layers is utilized to accomplish exchange bias up to room temperature. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  10. Lattice dynamics of Ni-Mn-Al Heusler alloys

    SciTech Connect

    Moya, Xavier; Manosa, Lluis; Planes, A.; Krenke, T.; Acet, Mehmet; Wassermann, E. F.; Morin, M.; Garlea, Vasile O; Lograsso, Tom; Zarestky, Jerel L.

    2008-01-01

    We have studied the lattice dynamics of a Ni54Mn23Al23 (at.%) Heusler single-crystalline alloy by means of neutron scattering and ultrasonic techniques. Results show the existence of a number of precursor phenomena. We have found an anomaly (dip) in the low TA2 phonon branch at the wave number 0.33 (in reciprocal lattice units) that becomes more pronounced (phonon softening) with decreasing temperature. We have also observed softening of the associated shear elastic constant (C ) with decreasing temperature. Ultrasonic measurements under applied magnetic field, both isothermally and varying the temperature show that the values of elastic constants depend on magnetic order thus evidencing magnetoelastic coupling.

  11. Optimized thermoelectric performance of the n-type half-Heusler material TiNiSn by substitution and addition of Mn

    NASA Astrophysics Data System (ADS)

    Lkhagvasuren, Enkhtaivan; Ouardi, Siham; Fecher, Gerhard H.; Auffermann, Gudrun; Kreiner, Guido; Schnelle, Walter; Felser, Claudia

    2017-04-01

    Alloys based on the half-Heusler compound TiNiSn with the addition of Mn or with a substitution of Ti by Mn are investigated as high-temperature thermoelectric materials. In both materials an intrinsic phase separation is observed, similar to TiNiSn where Ti has been partially substituted by Hf, with increasing Mn concentration the phase separation drastically reduces the lattice thermal conductivity while the power factor is increased. The thermoelectric performance of the n-type conducting alloy can be optimized both by substitution of Ti by Mn as well as the addition of Mn.

  12. Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases

    SciTech Connect

    Wei, Z. Y.; Liu, E. K. Chen, J. H.; Xi, X. K.; Zhang, H. W.; Wang, W. H.; Wu, G. H.; Li, Y.; Liu, G. D.; Luo, H. Z.

    2015-07-13

    Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.

  13. Tunnel magnetoresistance effect and magnetic damping in half-metallic Heusler alloys.

    PubMed

    Oogane, M; Mizukami, S

    2011-08-13

    Some full-Heusler alloys, such as Co(2)MnSi and Co(2)MnGe, are expected to be half-metallic ferromagnetic material, which has complete spin polarization. They are the most promising materials for realizing half-metallicity at room temperature owing to their high Curie temperature. We demonstrate a huge tunnel magnetoresistance effect in a magnetic tunnel junction using a Co(2)MnSi Heusler alloy electrode. This result proves high spin polarization of the Heusler alloy. We also demonstrate a small magnetic damping constant in Co(2)FeAl epitaxial film. The very high spin polarization and small magnetic constant of Heusler alloys will be a great advantage for future spintronic device applications.

  14. The Pressure Dependence of Structural, Electronic, Mechanical, Vibrational, and Thermodynamic Properties of Palladium-Based Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Çoban, Cansu

    2017-08-01

    The pressure dependent behaviour of the structural, electronic, mechanical, vibrational, and thermodynamic properties of Pd2TiX (X=Ga, In) Heusler alloys was investigated by ab initio calculations. The lattice constant, the bulk modulus and its first pressure derivative, the electronic band structure and the density of states (DOS), mechanical properties such as elastic constants, anisotropy factor, Young's modulus, etc., the phonon dispersion curves and phonon DOS, entropy, heat capacity, and free energy were obtained under pressure. It was determined that the calculated lattice parameters are in good agreement with the literature, the elastic constants obey the stability criterion, and the phonon dispersion curves have no negative frequency which shows that the compounds are stable. The band structures at 0, 50, and 70 GPa showed valence instability at the L point which explains the superconductivity in Pd2TiX (X=Ga, In).

  15. Stabilizing and increasing the magnetic moment of half-metals: The role of Li in half-Heusler LiMn Z (Z =N,P,Si)

    NASA Astrophysics Data System (ADS)

    Damewood, L.; Busemeyer, B.; Shaughnessy, M.; Fong, C. Y.; Yang, L. H.; Felser, C.

    2015-02-01

    Due to their similarities to metastable zinc-blende half-metals, we systematically examined the half-Heusler compounds β -LiMnZ (Z =N,P and Si) for their electronic, magnetic, and stability properties at optimized lattice constants and strained lattice constants that exhibit half-metallic properties. We also report the other phases of the half-Heusler structure (α and γ phases), but they are unlikely to be grown. The magnetic moments of these stable Li-based compounds are expected to reach as high as 4 μB per unit cell when Z =Si and 5 μB per unit cell when Z =N and P; however, the antiferromagnetic spin configuration is energetically favored when Z is a pnictogen. β -LiMnSi at a lattice constant 14% larger than its equilibrium lattice constant is a promising half-metal due to its large magnetic moment, large gap, and vibrational stability. The modified Slater-Pauling rule for these compounds is determined. Finally, we investigated a plausible method for developing half-metallic Li xMn Z at equilibrium by tuning x , but this type of alloying introduces local structural changes that preclude half-metallicity.

  16. Enhancing thermopower and hole mobility in bulk p-type half-Heuslers using full-Heusler nanostructures.

    PubMed

    Sahoo, Pranati; Liu, Yuanfeng; Makongo, Julien P A; Su, Xian-Li; Kim, Sung Joo; Takas, Nathan; Chi, Hang; Uher, Ctirad; Pan, Xiaoqing; Poudeu, Pierre F P

    2013-10-07

    The concept of band structure engineering near the Fermi level through atomic-scale alteration of a bulk semiconductor crystal structure using coherently embedded intrinsic semiconducting quantum dots provides a unique opportunity to manipulate the transport behavior of the existing ensembles of carriers within the semiconducting matrix. Here we show that in situ growth of coherent nanometer-scale full-Heusler quantum dots (fH-QDs) within the p-type Ti(0.5)Hf(0.5)CoSb(0.9)Sn(0.1) half-Heusler (hH) matrix induces a drastic decrease of the effective hole density within the hH/fH-QD nanocomposites at 300 K followed by a sharp increase with rising temperature. This behavior is associated with the formation of staggered heterojunctions with a valence band (VB) offset energy, ΔE at the hH/fH phase boundaries. The energy barrier (ΔE) discriminates existing holes with respect to their energy by trapping low energy (LE) holes, while promoting the transport of high energy (HE) holes through the VB of the fH-QDs. This "hole culling" results in surprisingly large increases in the mobility and the effective mass of HE holes contributing to electronic conduction. The simultaneous reduction in the density and the increase in the effective mass of holes resulted in large enhancements of the thermopower whereas; the increase in the mobility minimizes the drop in the electrical conductivity.

  17. Localized magnetic moments in the Heusler alloy Rh2MnGe

    NASA Astrophysics Data System (ADS)

    Klaer, P.; Kallmayer, M.; Elmers, H. J.; Basit, L.; Thöne, J.; Chadov, S.; Felser, C.

    2009-04-01

    X-ray magnetic circular dichroism (XMCD) of core-level absorption (x-ray absorption spectroscopy, XAS) spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Rh2MnGe at the Rh M3,2 and Mn L3,2 edges. The ratio of Rh and Mn spin moments amounts to 0.05 which is smaller than the ratio of 0.1 determined by a local density approximation electronic band structure calculation. We have found that the orbital moments of the Rh 4d and Mn 3d states are very small. The observed Rh 2p XAS spectrum can be understood on the basis of the Rh 3d partial density of unoccupied states as is typical for metals. The observed features of the Mn 2p XAS and XMCD spectra are dominated by final state multiplets as is typical for oxides. The comparison of experimental and ab initio calculated XAS/XMCD spectra reveals a strong narrowing of the Mn 3d bands, indicating strongly localized Mn moments. The magnetic moments are considerably more localized for Rh2MnGe in comparison with the isoelectronic compound Co2MnGe. In spite of the strong localization of the Mn moment, the temperature dependences of sublattice magnetization are equal for the Mn and Rh sublattices in contrast to the prediction by a Heisenberg model. This might be attributed to the remaining itinerant character of the Rh moment.

  18. Effect of disorder on electronic and magnetic properties of Co{sub 2}VGa Heusler alloy

    SciTech Connect

    Seema, K.; Kumar, Ranjan

    2015-08-28

    This paper presents the effect of disorder on electronic, magnetic and half-metallic properties of Co{sub 2}VGa Heusler alloy using density functional theory. Binary mixing is the most common form of atomic disorder in these compounds. We have considered three types of disorders: DO{sub 3}, A2 and B2 disorder which corresponds to X-Y, X-Z and Y-Z mixing respectively. After structural optimization, we found that A2 disorder has high formation energy and is most unlikely to occur. The half-metallic nature of the alloy is destroyed in presence of DO{sub 3} and A2 disorder. The destruction of half-metallicity is due to reconstruction of energy states. Also the loss of half-metallicity is accompanied by reversal of spin-polarization at the Fermi level. B2 disorder retains the half-metallic nature of the alloy but spin-polarization value is reduced as compared to the ordered alloy.

  19. Half-metallic properties of the new Ti2YPb(Y = Co, Fe) Heusler alloys

    NASA Astrophysics Data System (ADS)

    Hussain, Moaid K.; Gao, G. Y.; Yao, Kai-Lun

    2015-09-01

    The half-metallic properties of Ti2YPb(Y = Co, Fe) Heusler alloys with a CuHg2Ti-type structure were examined within the frame of the density functional theory and the Perdew-Burke-Ernzerh of generalized gradient approximation (GGA). Analysis of the electronic band structures and density of states for Ti2YPb(Y = Co, Fe) revealed that the spin-up bands are metallic, whereas the spin-down bands exhibit gaps of 0.73 and 0.70 eV, respectively. The magnetic moments calculated for the Ti2YPb(Y = Co, Fe) alloys were found to be equal to 3 μB/f.u. and 2 μB/f.u., values which both follows the Slater-Pauling rule of Mt = Zt - 18. The compounds’ negative enthalpy values should encourage their experimental realization in the future. The bandgap was elucidated to be mainly determined by the bonding and antibonding states created from the hybridizations of the d states between the Ti(1)-Ti(2) coupling and the Y = Co, Fe atom. The half-metallic properties of the Ti2YPb(Y = Co, Fe) compounds were found to be insensitive to lattice distortion, with full spin polarization achievable within a large range of lattice parameter values, making the alloys suitable for use in practical applications.

  20. Topological RPdBi half-Heusler semimetals: A new family of noncentrosymmetric magnetic superconductors

    PubMed Central

    Nakajima, Yasuyuki; Hu, Rongwei; Kirshenbaum, Kevin; Hughes, Alex; Syers, Paul; Wang, Xiangfeng; Wang, Kefeng; Wang, Renxiong; Saha, Shanta R.; Pratt, Daniel; Lynn, Jeffrey W.; Paglione, Johnpierre

    2015-01-01

    We report superconductivity and magnetism in a new family of topological semimetals, the ternary half-Heusler compound RPdBi (R: rare earth). In this series, tuning of the rare earth f-electron component allows for simultaneous control of both lattice density via lanthanide contraction and the strength of magnetic interaction via de Gennes scaling, allowing for a unique tuning of the normal-state band inversion strength, superconducting pairing, and magnetically ordered ground states. Antiferromagnetism with ordering vector (½,½,½) occurs below a Néel temperature that scales with de Gennes factor dG, whereas a superconducting transition is simultaneously supressed with increasing dG. With superconductivity appearing in a system with noncentrosymmetric crystallographic symmetry, the possibility of spin-triplet Cooper pairing with nontrivial topology analogous to that predicted for the normal-state electronic structure provides a unique and rich opportunity to realize both predicted and new exotic excitations in topological materials. PMID:26601201

  1. Composition induced metal-insulator quantum phase transition in the Heusler type Fe2VAl

    NASA Astrophysics Data System (ADS)

    Naka, Takashi; Nikitin, Artem M.; Pan, Yu; de Visser, Anne; Nakane, Takayuki; Ishikawa, Fumihiro; Yamada, Yuh; Imai, Motoharu; Matsushita, Akiyuki

    2016-07-01

    We report the magnetism and transport properties of the Heusler compound Fe2+x V1-x Al at  -0.10  ⩽  x  ⩽  0.20 under pressure and a magnetic field. A metal-insulator quantum phase transition occurred at x  ≈  -0.05. Application of pressure or a magnetic field facilitated the emergence of finite zero-temperature conductivity σ 0 around the critical point, which scaled approximately according to the power law (P  -  P c ) γ . At x  ⩽  -0.05, a localized paramagnetic spin appeared, whereas above the ferromagnetic quantum critical point at x  ≈  0.05, itinerant ferromagnetism was established. At the quantum critical points at x  =  -0.05 and 0.05, the resistivity and specific heat exhibited singularities characteristic of a Griffiths phase appearing as an inhomogeneous electronic state.

  2. Thermoelectric performance and the role of anti-site disorder in the 24-electron Heusler TiFe2Sn.

    PubMed

    Buffon, Malinda L C; Laurita, Geneva; Lamontagne, Leo; Levin, Emily E; Mooraj, Shahryar; Lloyd, Demetrious L; White, Natalie; Pollock, Tresa M; Seshadri, Ram

    2017-10-11

    Heusler compounds XY 2 Z with 24 valence electrons per formula unit are potential thermoelectric materials, given their thermal and chemical stability and their relatively earth-abundant constituent elements. We present results on the 24-electron compound TiFe2Sn here. First principles calculations on this compound suggest semiconducting behavior. A relatively flat conduction band that could be associated with a high Seebeck coefficient upon electron doping is found. A series of compounds have been prepared and characterized using a combination of synchrotron x-ray and neutron diffraction studies to understand the effects of site order/disorder phenomena and n-type doping. Samples fabricated by a three step processing approach were subjected to high temperature Seebeck and electrical resistivity measurements. Ti:Fe anti-site disorder is present in the stoichiometric compound and these defects are reduced when starting Ti-rich compositions are employed. Additionally, we investigate control of the Seebeck coefficient through the introduction of carriers through the substitution of Sb on the Sn site in these intrinsically p-type materials.

  3. Thermoelectric performance and the role of anti-site disorder in the 24-electron Heusler TiFe2Sn

    NASA Astrophysics Data System (ADS)

    Buffon, Malinda L. C.; Laurita, Geneva; Lamontagne, Leo; Levin, Emily E.; Mooraj, Shahryar; Lloyd, Demetrious L.; White, Natalie; Pollock, Tresa M.; Seshadri, Ram

    2017-10-01

    Heusler compounds XY 2 Z with 24 valence electrons per formula unit are potential thermoelectric materials, given their thermal and chemical stability and their relatively earth-abundant constituent elements. We present results on the 24-electron compound TiFe2Sn here. First principles calculations on this compound suggest semiconducting behavior. A relatively flat conduction band that could be associated with a high Seebeck coefficient upon electron doping is found. A series of compounds have been prepared and characterized using a combination of synchrotron x-ray and neutron diffraction studies to understand the effects of site order/disorder phenomena and n-type doping. Samples fabricated by a three step processing approach were subjected to high temperature Seebeck and electrical resistivity measurements. Ti:Fe anti-site disorder is present in the stoichiometric compound and these defects are reduced when starting Ti–rich compositions are employed. Additionally, we investigate control of the Seebeck coefficient through the introduction of carriers through the substitution of Sb on the Sn site in these intrinsically p-type materials.

  4. Magnetism and electronic structure of CoFeCrX (X = Si, Ge) Heusler alloys

    NASA Astrophysics Data System (ADS)

    Jin, Y.; Kharel, P.; Lukashev, P.; Valloppilly, S.; Staten, B.; Herran, J.; Tutic, I.; Mitrakumar, M.; Bhusal, B.; O'Connell, A.; Yang, K.; Huh, Y.; Skomski, R.; Sellmyer, D. J.

    2016-08-01

    The structural, electronic, and magnetic properties of CoFeCrX (X = Si, Ge) Heusler alloys have been investigated. Experimentally, the alloys were synthesized in the cubic L21 structure with small disorder. The cubic phase of CoFeCrSi was found to be highly stable against heat treatment, but CoFeCrGe disintegrated into other new compounds when the temperature reached 402 °C (675 K). Although the first-principle calculation predicted the possibility of tetragonal phase in CoFeCrGe, the tetragonal phase could not be stabilized experimentally. Both CoFeCrSi and CoFeCrGe compounds showed ferrimagnetic spin order at room temperature and have Curie temperatures (TC) significantly above room temperature. The measured TC for CoFeCrSi is 790 K but that of CoFeCrGe could not be measured due to its dissociation into new compounds at 675 K. The saturation magnetizations of CoFeCrSi and CoFeCrGe are 2.82 μB/f.u. and 2.78 μB/f.u., respectively, which are close to the theoretically predicted value of 3 μB/f.u. for their half-metallic phases. The calculated band gaps for CoFeCrSi and CoFeCrGe are, respectively, 1 eV and 0.5 eV. These materials have potential for spintronic device applications, as they exhibit half-metallic electronic structures with large band gaps, and Curie temperatures significantly above room temperature.

  5. Growth and transport properties of epitaxial lattice matched half Heusler CoTiSb/InAlAs/InP(001) heterostructures

    SciTech Connect

    Kawasaki, Jason K.; Johansson, Linda I. M.; Schultz, Brian D.; Palmstrøm, Chris J.

    2014-01-13

    We demonstrate the integration of the lattice matched single crystal epitaxial Half Heusler compound CoTiSb with In{sub 0.52}Al{sub 0.48}As/InP(001) heterostructures using molecular beam epitaxy. CoTiSb belongs to the subset of Half Heusler compounds that is expected to be semiconducting, despite being composed entirely of metallic constituents. The lattice matching and epitaxial alignment of the CoTiSb films were confirmed by reflection high energy electron diffraction and X-ray diffraction. Temperature dependent transport measurements indicate semiconducting-like behavior, with a room temperature Hall mobility of 530 cm{sup 2}/Vs and background Hall carrier density of 9.0 × 10{sup 17} cm{sup −3}, which is comparable to n-Si with similar carrier density. Below 100 K, the films show a large negative magnetoresistance, and possible origins of this negative magnetoresistance are discussed.

  6. Ru9Zn7Sb8: a structure with a 2 × 2 × 2 supercell of the half-Heusler phase.

    PubMed

    Xiong, Ding-Bang; Zhao, Yufeng; Okamoto, Norihiko L; Pietzonka, Clemens; Waki, Takeshi; Inui, Haruyuki

    2010-11-15

    The title compound Ru(9)Zn(7)Sb(8) was synthesized via a high-temperature reaction from the elements in a stoichiometric ratio, and its structure was solved by a single-crystal X-ray diffraction method. The structure [cubic, space group Fm3m, Pearson symbol cF96, a = 11.9062(14) Å (293 K), and Z = 4] adopts a unique 2a(hh) × 2a(hh) × 2a(hh) supercell of a normal half-Heusler phase and shows abnormal features of atomic coordination against the Pauling rule. The formation of this superstructure was discussed in light of the valence electron concentration per unit cell. It is a metallic conductor [ρ(300 K) = 16 μΩ·m], and differential scanning calorimetry revealed that Ru(9)Zn(7)Sb(8) undergoes a transformation at 1356(1) K and melts, by all indications, congruently at 1386 K. At room temperature, its thermal conductivity is about 3 W/m·K, which is only one-quarter of that of most normal half-Heusler phases. Ru(9)Zn(7)Sb(8) as well as its analogues of iron-, cobalt-, rhodium-, and iridium-containing compounds are expected to serve as a new structure type for exploring new thermoelectric materials.

  7. Electronic, magnetic and Fermi properties investigates on quaternary Heusler NiCoCrAl, NiCoCrGa and NiFeCrGa

    NASA Astrophysics Data System (ADS)

    Wei, Xiao-Ping; Zhang, Ya-Ling; Chu, Yan-Dong; Sun, Xiao-Wei; Sun, Ting; Guo, Peng; Deng, Jian-Bo

    2015-07-01

    Using the full-potential local-orbital minimum-basis method within the framework of density functional theory, we study the electronic, magnetic and Fermi properties of three quaternary Heusler compounds: NiCoCrAl, NiCoCrGa and NiFeCrGa. Results identify that these compounds are half-metallic ferromagnets with integer spin magnetic moment, and their spin moments follow the Slater-Pauling rule. Accordingly, the origin of gap and magnetic moment are also discussed. In addition, the Fermi surface is further plotted to explore the behavior of electronic states in the vicinity of Fermi level for these compounds. Finally, we argue the influence of tetragonal deformation on electronic and magnetic properties. Meanwhile, the possible L21 disorder is also discussed for NiCoCrAl and NiCoCrGa.

  8. Figure of Merit ZT achieved in Nanostructured Half Heusler alloys

    NASA Astrophysics Data System (ADS)

    Wu, Di; Zhu, Song; Xie, Wenjie; Poon, Joseph; Tritt, Terry; Thomas, Peter; Venkatasubramanian, Rama

    2012-02-01

    Half-Heusler (HH) phases have recently gained attention due to their high temperature thermoelectric performance, especially in the environment of growing concern over the dependence on the limited fossil fuels for energy production. These materials are investigated from the perspective of thermal and electronic transport properties for enhancing the dimensionless figure of merit (ZT) at 800-1000 K. Refinement on grain sizes and embedment of nanoparticles in HH alloy hosts were employed to produce fine-grained as well as nanocomposites and monolithic nanostructured materials. Present experiments indicated that HH alloys and their nanoparticles-embedded phases can attain ZT˜1, or slightly higher near 900-1000K. The observed ZT enhancements could be attributed to multiple origins. HH alloy hosts of nano-sized grains with/without embedment of different nanoparticles are also being investigated to further improve their thermoelectric performance.

  9. Exploring Half Metals in Li-based Half Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Busemeyer, B.; Shaughnessy, M.; Fong, C. Y.

    2011-11-01

    We examine the electronic and magnetic properties of three Li-related half Heusler alloys, namely LiMnN, LiMnP, and LiMnSi in a structure close to the well-known zinc-blende structure in the attempt to search for new half metallic materials. If they do demonstrate half metallic properties, this will open new grounds for finding half metallic spintronic materials. Our results will furnish guidelines for future exploration of alkali-related half metals. Using the primitive cell LiMnSi is a half metal, while the pnictides are not. However when the conventional cell is used, we find that Li3Mn4P4 and Li3Mn4N4 are half metals. The physical reason for these two pnictides to be half metallic and for their magnetic moment per unit cell will be presented.

  10. Lattice thermal conductivity of nanograined half-Heusler solid solutions

    SciTech Connect

    Geng, Huiyuan Meng, Xianfu; Zhang, Hao; Zhang, Jian

    2014-05-19

    We report a phenomenological model of atomic weight, lattice constant, temperature, and grain size to calculate the high-temperature lattice thermal conductivity of nanograined solid solutions. The theoretical treatment developed here is reasonably consistent with the experimental results of n-type MNiSn and p-type MCoSb alloys, where M is the combination of Hf, Zr, and Ti. For disordered half-Heusler alloys with moderated grain sizes, we predict that the reduction in lattice thermal conductivity due to grain boundary scattering is independent of the scattering parameter, which characterizes the phonon scattering cross section of point defects. In addition, the lattice thermal conductivity falls off with temperature as T{sup –1∕2} around the Debye temperature.

  11. Ab initio study of electronic, magnetic, elastic and optical properties of full Heusler Co2MnSb

    NASA Astrophysics Data System (ADS)

    Lashgari, H.; Abolhassani, M. R.; Boochani, A.; Sartipi, E.; Taghavi-Mendi, R.; Ghaderi, A.

    2016-08-01

    Ab-initio study of electronic, magnetic, elastic and optical properties of full Heusler Co2MnSb is performed in the framework of the Density Functional Theory to obtain the associated parameters. Equilibrium lattice parameter is calculated 6.03 Å. Studying the electronic properties of compound confirms its half-metallic property, whereas spin polarization at Fermi level is 100 %. This compound in the minority spin channel is a semiconductor with a calculated gap of 0.623 eV. Also, elastic properties of the compound including bulk modulus B, shear modulus G, Young's modulus E and Poisson's modulus υ are calculated. Investigation of the elastic properties of Co2MnSb indicates the elastic stability is greatly anisotropic. Besides, it is shown that the Co2MnSb is mechanically a ductile compound. Spin magnetic moment is obtained 6.0001 that are in good agreement with the previous experimental studies. In addition, in order to optical studies, dielectric function, reflectivity, energy loss function, absorption coefficient and optical conductivity are calculated.

  12. Natural off-stoichiometry causes carrier doping in half-Heusler filled tetrahedral structures

    NASA Astrophysics Data System (ADS)

    Yu, Yonggang G.; Zhang, Xiuwen; Zunger, Alex

    2017-02-01

    The half-Heusler filled tetrahedral structures (FTSs) are zinc-blende-like compounds, where an additional atom is filling its previously empty interstitial site. The FTSs having 18 valence electrons per formula unit are an emerging family of functional materials, whose intrinsic doping trends underlying a wide range of electronic functionalities are yet to be understood. Interestingly, even pristine compounds without any attempt at impurity/chemical doping exhibit intriguing trends in the free carriers they exhibit. Applying the first principles theory of doping to a few prototype compounds in the AIVBXCIV and AIVBIXCV groups, we describe the key ingredients controlling the materials' propensity for both intrinsic and extrinsic doping: (a) The spontaneous deviations from 1:1:1 stoichiometry reflect predictable thermodynamic stability of specific competing phases. (b) Bulk ABC compounds containing 3 d elements in the B position (ZrNiSn and ZrCoSb) are predicted to be naturally 3 d rich. The B =3 d interstitials are the prevailing shallow donors, whereas the potential acceptors (e.g., Zr vacancy and Sn-on-Zr antisite) are ineffective electron killers, resulting in an overall uncompensated n -type character, even without any chemical doping. In these materials, the band edges are "natural impurity bands" due to non-Daltonian off-stoichiometry, such as B interstitials, not intrinsic bulk controlled states as in a perfect crystal. (c) Bulk ABC compounds containing 5 d elements in the B position (ZrPtSn, ZrIrSb, and TaIrGe) are predicted to be naturally C rich and A poor. This promotes the hole-producing C -on-A antisite defects rather than B -interstitial donors. The resultant p -type character (without chemical doping) therein is "latent" for C =Sn and Sb; however, as the C -on-A hole-producing acceptors are rather deep and p typeness is manifest only at high temperature or via impurity doping. In contrast, in TaIrGe (B =Ir , 5 d ) , the prevailing hole-producing Ge

  13. Fully epitaxial C1b-type NiMnSb half-Heusler alloy films for current-perpendicular-to-plane giant magnetoresistance devices with a Ag spacer

    PubMed Central

    Wen, Zhenchao; Kubota, Takahide; Yamamoto, Tatsuya; Takanashi, Koki

    2015-01-01

    Remarkable magnetic and spin-dependent transport properties arise from well-designed spintronic materials and heterostructures. Half-metallic Heusler alloys with high spin polarization exhibit properties that are particularly advantageous for the development of high-performance spintronic devices. Here, we report fully (001)-epitaxial growth of a high-quality half-metallic NiMnSb half-Heusler alloy films, and their application to current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with Ag spacer layers. Fully (001)-oriented NiMnSb epitaxial films with very flat surface and high magnetization were prepared on Cr/Ag-buffered MgO(001) single crystalline substrates by changing the substrate temperature. Epitaxial CPP-GMR devices using the NiMnSb films and a Ag spacer were fabricated, and room-temperature (RT) CPP-GMR ratios for the C1b-type half-Heusler alloy were determined for the first time. A CPP-GMR ratio of 8% (21%) at RT (4.2 K) was achieved in the fully epitaxial NiMnSb/Ag/NiMnSb structures. Furthermore, negative anisotropic magnetoresistance (AMR) ratio and small discrepancy of the AMR amplitudes between RT and 10 K were observed in a single epitaxial NiMnSb film, indicating robust bulk half metallicity against thermal fluctuation in the half-Heusler compound. The modest CPP-GMR ratios could be attributed to interface effects between NiMnSb and Ag. This work provides a pathway for engineering a new class of ordered alloy materials with particular emphasis on spintronics. PMID:26672482

  14. Fully epitaxial C1b-type NiMnSb half-Heusler alloy films for current-perpendicular-to-plane giant magnetoresistance devices with a Ag spacer.

    PubMed

    Wen, Zhenchao; Kubota, Takahide; Yamamoto, Tatsuya; Takanashi, Koki

    2015-12-17

    Remarkable magnetic and spin-dependent transport properties arise from well-designed spintronic materials and heterostructures. Half-metallic Heusler alloys with high spin polarization exhibit properties that are particularly advantageous for the development of high-performance spintronic devices. Here, we report fully (001)-epitaxial growth of a high-quality half-metallic NiMnSb half-Heusler alloy films, and their application to current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with Ag spacer layers. Fully (001)-oriented NiMnSb epitaxial films with very flat surface and high magnetization were prepared on Cr/Ag-buffered MgO(001) single crystalline substrates by changing the substrate temperature. Epitaxial CPP-GMR devices using the NiMnSb films and a Ag spacer were fabricated, and room-temperature (RT) CPP-GMR ratios for the C1b-type half-Heusler alloy were determined for the first time. A CPP-GMR ratio of 8% (21%) at RT (4.2 K) was achieved in the fully epitaxial NiMnSb/Ag/NiMnSb structures. Furthermore, negative anisotropic magnetoresistance (AMR) ratio and small discrepancy of the AMR amplitudes between RT and 10 K were observed in a single epitaxial NiMnSb film, indicating robust bulk half metallicity against thermal fluctuation in the half-Heusler compound. The modest CPP-GMR ratios could be attributed to interface effects between NiMnSb and Ag. This work provides a pathway for engineering a new class of ordered alloy materials with particular emphasis on spintronics.

  15. Nuclear magnetic resonance study of thin Co2FeAl0.5Si0.5 Heusler films with varying thickness

    NASA Astrophysics Data System (ADS)

    Alfonsov, A.; Peters, B.; Yang, F. Y.; Büchner, B.; Wurmehl, S.

    2015-02-01

    Type, degree, and evolution of structural order are important aspects for understanding and controlling the properties of highly spin-polarized Heusler compounds, in particular, with respect to the optimal film growth procedure. In this work, we compare the structural order and the local magnetic properties revealed by nuclear magnetic resonance (NMR) spectroscopy with the macroscopic properties of thin Co2FeAl 0.5Si 0.5 Heusler films with varying thickness. A detailed analysis of the measured NMR spectra presented in this paper enables us to find a very high degree of L 21 -type ordering up to 81% concomitantly with excess Fe of 8%-13% at the expense of Al and Si. We show that the formation of certain types of order depends not only on the thermodynamic phase diagrams as in bulk samples, but also that the kinetic control may contribute to the phase formation in thin films. It is an exciting finding that Co2FeAl 0.5Si 0.5 can form an almost ideal L 21 structure in films, though with a considerable amount of Fe-Al/Si off stoichiometry. Moreover, the very good quality of the films as demonstrated by our NMR study suggests that the technique of off-axis sputtering used to grow the films sets the stage for the optimized performance of Co2FeAl 0.5Si 0.5 in spintronic devices.

  16. Magnetic properties of low-moment ferrimagnetic Heusler Cr 2 CoGa thin films grown by molecular beam epitaxy

    SciTech Connect

    Jamer, Michelle E.; Sterbinsky, George E.; Stephen, Gregory M.; DeCapua, Matthew C.; Player, Gabriel; Heiman, Don

    2016-10-31

    Recently, theorists have predicted many materials with a low magnetic moment and large spin-polarization for spintronic applications. These compounds are predicted to form in the inverse Heusler structure; however, many of these compounds have been found to phase segregate. In this study, ordered Cr2CoGa thin films were synthesized without phase segregation using molecular beam epitaxy. The present as-grown films exhibit a low magnetic moment from antiferromagnetically coupled Cr and Co atoms as measured with superconducting quantum interface device magnetometry and soft X-ray magnetic circular dichroism. Electrical measurements demonstrated a thermally-activated semiconductor-like resistivity component with an activation energy of 87 meV. These results confirm spin gapless semiconducting behavior, which makes these thin films well positioned for future devices.

  17. Magnetic and structural characterizations of Heusler Ni2FeGa nanoparticles

    NASA Astrophysics Data System (ADS)

    Xu, Yunli; Liu, Min; Huang, Xiufeng; Dai, Zhiwen; Qiu, Hongmei; Yu, Guanghua; Pan, Liqing

    2016-11-01

    This work describes the chemical preparation, structural characterization and magnetic properties of Heusler Ni2FeGa nanoparticles. The nanoparticles were synthesized by reducing a methanol solution mixture of NiCl2 · 6H2O, Fe(NO3)3 · 9H2O and Ga(NO3) · xH2O, and using SBA-15 as template. The obtained nanoparticles were investigated by means of x-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer. The structure of the prepared nanoparticles is Heusler L21 phase with a second γ phase, and the ratio of the two components could be adjusted by the concentration of SBA in the precursor solution. The prepared nanoscale Ni2FeGa heusler alloys with a tensile strain of 0.56% present ferromagnetism at room-temperature and its Curie temperature exceeds 340 K.

  18. Contradictory role of the magnetic contribution in inverse magnetocaloric Heusler materials

    NASA Astrophysics Data System (ADS)

    Gottschall, Tino; Skokov, Konstantin P.; Benke, Dimitri; Gruner, Markus E.; Gutfleisch, Oliver

    2016-05-01

    In this paper, we illustrate the dilemma of inverse magnetocaloric materials using the example of Heusler alloys. For such materials, the magnetic and lattice contribution to the total entropy change are competing with each other. For the two paradigmatic Heusler systems of Ni-Mn-In and Ni-Mn-In-Co, we provide a systematic comparison of experimental data under different magnetic fields and hydrostatic pressures with magnetic and the magnetocaloric properties obtained from the Heisenberg model. This allows us to separate the lattice and the magnetic contribution to the total entropy of the martensitic transition. Our analysis reveals that a large magnetization change is parasitic, but at the same time it is necessary to drive the magnetocaloric effect. This contradicting role of the magnetic contribution—the dilemma—is a general characteristic of inverse magnetocaloric Heusler materials.

  19. Unique topological surface states of full-Heusler topological crystalline insulators

    NASA Astrophysics Data System (ADS)

    Pham, Anh; Li, Sean

    2017-03-01

    Our theoretical analysis reveals that a family of full-Heusler materials exhibit unique topological surface states with type-I and type-II Dirac quasiparticles. The type-I Dirac surface state is characterized by an enclosed Fermi surface, while the type-II Dirac surfaces occur at the touching of the electron and hole pockets. In addition, due to the layered nature of the full-Heusler crystals structured with a wide range of various elements, such structures induce multiple Dirac surface states with different Lifshitz transitions protected by more than one mirror plane.

  20. Martensitic transformation and phase diagram in ternary Co-V-Ga Heusler alloys

    NASA Astrophysics Data System (ADS)

    Xu, Xiao; Nagashima, Akihide; Nagasako, Makoto; Omori, Toshihiro; Kanomata, Takeshi; Kainuma, Ryosuke

    2017-03-01

    We report the martensitic transformation behavior in Co-V-Ga Heusler alloys. Thermoanalysis and thermomagnetization measurements were conducted to observe the martensitic transformation. By using a transmission electron microscope and an in situ X-ray diffractometer, martensitic transformation was found to occur from the L21 Heusler parent phase to the D022 martensite phase. Phase diagrams were determined for two pseudo-binary sections where martensitic transformation was detected. Magnetic properties, including the Curie temperatures and spontaneous magnetization of the parent phase, were also investigated. The magnetic properties showing behaviors different from those of NiMn-based alloys were found.

  1. Structural and magnetic properties of Fe2CoGa Heusler nanoparticles

    NASA Astrophysics Data System (ADS)

    Wang, Changhai; Casper, Frederick; Gasi, Teuta; Ksenofontov, Vadim; Balke, Benjamin; Fecher, Gerhard H.; Felser, Claudia; Hwu, Yeu-Kuang; Lee, Jeu-Jau

    2012-07-01

    Fe2CoGa Heusler nanoparticles are synthesized by a chemical method. The structure and magnetic properties of Fe2CoGa Heusler nanoparticles are investigated by x-ray diffraction, extended x-ray absorption fine structure and Mössbauer spectroscopy. The crystal structure of Fe2CoGa nanoparticles is described by the X-type structure (prototype: Li2AgSb). Magnetic measurements reveal the presence of small Fe2CoGa nanoparticles and lower magnetic moments compared with the theoretically predicted values.

  2. Enhancement of ferromagnetism by Cr doping in Ni-Mn-Cr-Sb Heusler alloys

    NASA Astrophysics Data System (ADS)

    Khan, Mahmud; Dubenko, Igor; Stadler, Shane; Jung, J.; Stoyko, S. S.; Mar, Arthur; Quetz, Abdiel; Samanta, Tapas; Ali, Naushad; Chow, K. H.

    2013-03-01

    A series of Mn rich Ni50Mn37-xCrxSb13 Heusler alloys have been investigated by dc magnetization and electrical resistivity measurements. Due to the weakening of the Ni-Mn hybridization, the martensitic transition shifts to lower temperatures with increasing Cr concentration, while the saturation magnetization at 5 K increases. The magnetoresistance and exchange bias properties are dramatically suppressed with increasing Cr concentration. The observed behaviors suggest that substitution of Cr for Mn in Ni50Mn37-xCrxSb13 Heusler alloys not only destabilizes the martensitic phase but also enhances ferromagnetism in the system. The possible mechanisms responsible for the observed behavior are discussed.

  3. Size correlated long and short range order of ternary Co2FeGa Heusler nanoparticles

    NASA Astrophysics Data System (ADS)

    Wang, C. H.; Guo, Y. Z.; Casper, F.; Balke, B.; Fecher, G. H.; Felser, C.; Hwu, Y.

    2010-09-01

    The long and short range order of chemically prepared Co2FeGa Heusler nanoparticles with various sizes are determined by x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) spectroscopy. Specifically, EXAFS fittings reveal the size dependent crystal structure and short range order of the Heusler type Co2FeGa nanoparticles. With decreasing particle size, the degree of L21 order in the nanoparticles decreases and the probability of B2 disorder increases simultaneously. The consequences of antisite disorder on the size correlated structure of Co2FeGa nanoparticles are also discussed.

  4. Investigation of spin-gapless semiconductivity and half-metallicity in Ti{sub 2}MnAl-based compounds

    SciTech Connect

    Lukashev, P. Staten, B.; Hurley, N.; Kharel, P.; Gilbert, S.; Fuglsby, R.; Huh, Y.; Valloppilly, S.; Zhang, W.; Skomski, R.; Sellmyer, D. J.; Yang, K.

    2016-04-04

    The increasing interest in spin-based electronics has led to a vigorous search for new materials that can provide a high degree of spin polarization in electron transport. An ideal candidate would act as an insulator for one spin channel and a conductor or semiconductor for the opposite spin channel, corresponding to the respective cases of half-metallicity and spin-gapless semiconductivity. Our first-principle electronic-structure calculations indicate that the metallic Heusler compound Ti{sub 2}MnAl becomes half-metallic and spin-gapless semiconducting if half of the Al atoms are replaced by Sn and In, respectively. These electronic structures are associated with structural transitions from the regular cubic Heusler structure to the inverted cubic Heusler structure.

  5. Investigation of spin-gapless semiconductivity and half-metallicity in Ti2MnAl-based compounds

    NASA Astrophysics Data System (ADS)

    Lukashev, P.; Kharel, P.; Gilbert, S.; Staten, B.; Hurley, N.; Fuglsby, R.; Huh, Y.; Valloppilly, S.; Zhang, W.; Yang, K.; Skomski, R.; Sellmyer, D. J.

    2016-04-01

    The increasing interest in spin-based electronics has led to a vigorous search for new materials that can provide a high degree of spin polarization in electron transport. An ideal candidate would act as an insulator for one spin channel and a conductor or semiconductor for the opposite spin channel, corresponding to the respective cases of half-metallicity and spin-gapless semiconductivity. Our first-principle electronic-structure calculations indicate that the metallic Heusler compound Ti2MnAl becomes half-metallic and spin-gapless semiconducting if half of the Al atoms are replaced by Sn and In, respectively. These electronic structures are associated with structural transitions from the regular cubic Heusler structure to the inverted cubic Heusler structure.

  6. Half-Heusler Alloys for Efficient Thermoelectric Power Conversion

    NASA Astrophysics Data System (ADS)

    Chen, Long; Zeng, Xiaoyu; Tritt, Terry M.; Poon, S. Joseph

    2016-11-01

    Half-Heusler (HH) phases (space group F43m, Clb) are increasingly gaining attention as promising thermoelectric materials in view of their thermal stability and environmental benignity as well as efficient power output. Until recently, the verifiable dimensionless figure of merit ( ZT) of HH phases has remained moderate near 1, which limits the power conversion efficiency of these materials. We report herein ZT ˜ 1.3 in n-type (Hf,Zr)NiSn alloys near 850 K developed through elemental substitution and simultaneous embedment of nanoparticles in the HH matrix, obtained by annealing the samples close to their melting temperatures. Introduction of mass fluctuation and scattering centers play a key role in the high ZT measured, as shown by the reduction of thermal conductivity and increase of thermopower. Based on computation, the power conversion efficiency of a n-p couple module based on the new n-type (Hf,Zr,Ti)NiSn particles-in-matrix composite and recently reported high- ZT p-type HH phases is expected to reach 13%, comparable to that of state-of-the-art materials, but with the mentioned additional materials and environmental attributes. Since the high efficiency is obtained without tuning the microstructure of the HH phases, it leaves room for further optimization.

  7. Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Wang, Zhijun; Vergniory, M. G.; Kushwaha, S.; Hirschberger, Max; Chulkov, E. V.; Ernst, A.; Ong, N. P.; Cava, Robert J.; Bernevig, B. Andrei

    2016-12-01

    Weyl fermions have recently been observed in several time-reversal-invariant semimetals and photonics materials with broken inversion symmetry. These systems are expected to have exotic transport properties such as the chiral anomaly. However, most discovered Weyl materials possess a substantial number of Weyl nodes close to the Fermi level that give rise to complicated transport properties. Here we predict, for the first time, a new family of Weyl systems defined by broken time-reversal symmetry, namely, Co-based magnetic Heusler materials X Co2Z (X =IVB or VB; Z =IVA or IIIA). To search for Weyl fermions in the centrosymmetric magnetic systems, we recall an easy and practical inversion invariant, which has been calculated to be -1 , guaranteeing the existence of an odd number of pairs of Weyl fermions. These materials exhibit, when alloyed, only two Weyl nodes at the Fermi level—the minimum number possible in a condensed matter system. The Weyl nodes are protected by the rotational symmetry along the magnetic axis and separated by a large distance (of order 2 π ) in the Brillouin zone. The corresponding Fermi arcs have been calculated as well. This discovery provides a realistic and promising platform for manipulating and studying the magnetic Weyl physics in experiments.

  8. Characterization of half-Heusler unicouple for thermoelectric conversion

    NASA Astrophysics Data System (ADS)

    Hu, Xiaokai; Yamamoto, Atsushi; Nagase, Kazuo

    2015-06-01

    A thermoelectric unicouple is constructed with half-Heusler materials for characterizing power generation. The electric power and heat flow are measured simultaneously under various temperature differences, and the efficiencies are calculated. Phase structures and thermoelectric properties are characterized for the p- and n-type legs; the latter are used for finite-element simulations to calculate numerically the ideal electric power, heat flow and efficiency under the same temperature conditions. Seebeck voltage, internal resistance, electric power, heat flow, and efficiency are compared between the measurement and simulation. Thereby, the contact electrical resistance is estimated, and the extra thermal resistance is analyzed in terms of the compared open-circuit voltage and heat flow. The maximum efficiency of 4.0% is attained from the measurement with the hot side at 470 °C, in contrast to 5.7% from the simulation. The simulation permits an investigation on the one-dimensional heat flow through the p- and n-type legs that is coupled with Peltier, Thomson, and Joule heat.

  9. Artificially engineered Heusler ferrimagnetic superlattice exhibiting perpendicular magnetic anisotropy

    NASA Astrophysics Data System (ADS)

    Ma, Q. L.; Zhang, X. M.; Miyazaki, T.; Mizukami, S.

    2015-01-01

    To extend density limits in magnetic recording industry, two separate strategies were developed to build the storage bit in last decade, introduction of perpendicular magnetic anisotropy (PMA) and adoption of ferrimagnetism/antiferromagnetism. Meanwhile, these properties significantly improve device performance, such as reducing spin-transfer torque energy consumption and decreasing signal-amplitude-loss. However, materials combining PMA and antiferromagnetism rather than transition-metal/rare-earth system were rarely developed. Here, we develop a new type of ferrimagnetic superlattice exhibiting PMA based on abundant Heusler alloy families. The superlattice is formed by [MnGa/Co2FeAl] unit with their magnetizations antiparallel aligned. The effective anisotropy (Kueff) over 6 Merg/cm3 is obtained, and the SL can be easily built on various substrates with flexible lattice constants. The coercive force, saturation magnetization and Kueff of SLs are highly controllable by varying the thickness of MnGa and Co2FeAl layers. The SLs will supply a new choice for magnetic recording and spintronics memory application such as magnetic random access memory.

  10. Artificially engineered Heusler ferrimagnetic superlattice exhibiting perpendicular magnetic anisotropy

    PubMed Central

    Ma, Q. L.; Zhang, X. M.; Miyazaki, T.; Mizukami, S.

    2015-01-01

    To extend density limits in magnetic recording industry, two separate strategies were developed to build the storage bit in last decade, introduction of perpendicular magnetic anisotropy (PMA) and adoption of ferrimagnetism/antiferromagnetism. Meanwhile, these properties significantly improve device performance, such as reducing spin-transfer torque energy consumption and decreasing signal-amplitude-loss. However, materials combining PMA and antiferromagnetism rather than transition-metal/rare-earth system were rarely developed. Here, we develop a new type of ferrimagnetic superlattice exhibiting PMA based on abundant Heusler alloy families. The superlattice is formed by [MnGa/Co2FeAl] unit with their magnetizations antiparallel aligned. The effective anisotropy (Kueff) over 6 Merg/cm3 is obtained, and the SL can be easily built on various substrates with flexible lattice constants. The coercive force, saturation magnetization and Kueff of SLs are highly controllable by varying the thickness of MnGa and Co2FeAl layers. The SLs will supply a new choice for magnetic recording and spintronics memory application such as magnetic random access memory. PMID:25597496

  11. Artificially engineered Heusler ferrimagnetic superlattice exhibiting perpendicular magnetic anisotropy.

    PubMed

    Ma, Q L; Zhang, X M; Miyazaki, T; Mizukami, S

    2015-01-19

    To extend density limits in magnetic recording industry, two separate strategies were developed to build the storage bit in last decade, introduction of perpendicular magnetic anisotropy (PMA) and adoption of ferrimagnetism/antiferromagnetism. Meanwhile, these properties significantly improve device performance, such as reducing spin-transfer torque energy consumption and decreasing signal-amplitude-loss. However, materials combining PMA and antiferromagnetism rather than transition-metal/rare-earth system were rarely developed. Here, we develop a new type of ferrimagnetic superlattice exhibiting PMA based on abundant Heusler alloy families. The superlattice is formed by [MnGa/Co2FeAl] unit with their magnetizations antiparallel aligned. The effective anisotropy (K(u)(eff)) over 6 Merg/cm(3) is obtained, and the SL can be easily built on various substrates with flexible lattice constants. The coercive force, saturation magnetization and K(u)(eff) of SLs are highly controllable by varying the thickness of MnGa and Co2FeAl layers. The SLs will supply a new choice for magnetic recording and spintronics memory application such as magnetic random access memory.

  12. Reversibility of minor hysteresis loops in magnetocaloric Heusler alloys

    NASA Astrophysics Data System (ADS)

    Gottschall, Tino; Stern-Taulats, Enric; Mañosa, Lluís; Planes, Antoni; Skokov, Konstantin P.; Gutfleisch, Oliver

    2017-05-01

    The unavoidable existence of thermal hysteresis in magnetocaloric materials with a first-order phase transition is one of the central problems limiting their implementation in cooling devices. Using minor loops, however, allows achieving significant cyclic effects even in materials with relatively large hysteresis. Here, we compare thermometric measurements of the adiabatic temperature change Δ T ad and calorimetric measurements of the isothermal entropy change Δ S T when moving in minor hysteresis loops driven by magnetic fields. Under cycling in 2 T, the Ni-Mn-In-Co Heusler material provides a reversible magnetocaloric effect of Δ ST rev = 10.5 J kg-1 K-1 and Δ Tad rev = 3.0 K. Even though the thermodynamic conditions and time scales are very different in adiabatic and isothermal minor loops, it turns out that after a suitable scaling, a self-consistent reversibility region in the entropy diagram is found. This region is larger than expected from basic thermodynamic considerations based on isofield measurements alone, which opens new opportunities in application.

  13. Half-metallic interface between a Heusler alloy and Si.

    PubMed

    Abe, Kazutaka; Miura, Yoshio; Shiozawa, Yasunori; Shirai, Masafumi

    2009-02-11

    The interface between the half-Heusler alloy CoFeSi and Si is investigated by using first-principles density-functional calculations. Although CoFeSi has not been fabricated yet, its formation energy turns out to be negative. Within the generalized gradient approximation, CoFeSi shows nearly half-metallic properties, and its lattice constant is about 5.38 Å; this value is relatively close to the lattice constant of Si. We here chiefly investigate the CoFeSi/Si (110) interface, and find that the half-metallic properties are almost preserved at a specific (110) interface. Furthermore, the interfacial structure which leads to the high spin polarization has the lowest energy of the (110) interfacial patterns examined in this work. The half-metallicity at the interfaces is similarly observed in the densities of states projected onto delocalized sp states, and this suggests the relevance of the high spin polarization to transport properties.

  14. Large anomalous Hall effect in a half-Heusler antiferromagnet

    NASA Astrophysics Data System (ADS)

    Suzuki, T.; Chisnell, R.; Devarakonda, A.; Liu, Y.-T.; Feng, W.; Xiao, D.; Lynn, J. W.; Checkelsky, J. G.

    2016-12-01

    The quantum mechanical (Berry) phase of the electronic wavefunction plays a critical role in the anomalous and spin Hall effects, including their quantized limits. While progress has been made in understanding these effects in ferromagnets, less is known in antiferromagnetic systems. Here we present a study of antiferromagnet GdPtBi, whose electronic structure is similar to that of the topologically non-trivial HgTe (refs ,,), and where the Gd ions offer the possibility to tune the Berry phase via control of the spin texture. We show that this system supports an anomalous Hall angle ΘAH > 0.1, comparable to the largest observed in bulk ferromagnets and significantly larger than in other antiferromagnets. Neutron scattering measurements and electronic structure calculations suggest that this effect originates from avoided crossing or Weyl points that develop near the Fermi level due to a breaking of combined time-reversal and lattice symmetries. Berry phase effects associated with such symmetry breaking have recently been explored in kagome networks; our results extend this to half-Heusler systems with non-trivial band topology. The magnetic textures indicated here may also provide pathways towards realizing the topological insulating and semimetallic states predicted in this material class.

  15. Engineered Heusler Ferrimagnets with a Large Perpendicular Magnetic Anisotropy

    PubMed Central

    Ranjbar, Reza; Suzuki, Kazuya; Sugihara, Atsushi; Miyazaki, Terunobu; Ando, Yasuo; Mizukami, Shigemi

    2015-01-01

    Synthetic perpendicular magnetic anisotropy (PMA) ferrimagnets consisting of 30-nm-thick D022-MnGa and Co2MnSi (CMS) cubic Heusler alloys with different thicknesses of 1, 3, 5, 10 and 20 nm, buffered and capped with a Cr film, are successfully grown epitaxially on MgO substrate. Two series samples with and without post annealing at 400 ∘C are fabricated. The (002) peak of the cubic L21 structure of CMS films on the MnGa layer is observed, even for the 3-nm-thick CMS film for both un-annealed and annealed samples. The smaller remnant magnetization and larger switching field values of CMS (1–20 nm)/MnGa (30 nm) bilayers compared with 30-nm-thick MnGa indicates antiferromagnetic (AFM) interfacial exchange coupling (Jex) between MnGa and CMS films for both un-annealed and annealed samples. The critical thickness of the CMS film for observing PMA with AFM coupling in the CMS/MnGa bilayer is less than 10 nm, which is relatively large compared to previous studies. PMID:28793580

  16. Influence of chemical pressure in Sn-substituted Ni2MnGa Heusler alloy: Experimental and theoretical studies

    NASA Astrophysics Data System (ADS)

    Passamani, E. C.; Lozano, E.; Larica, C.; Marcelo, C. A. C.; Scopel, W. L.; Merino, I.; Alves, A. L.; Takeuchi, A. Y.; Nascimento, V. P.

    2012-10-01

    The influence of the chemical pressure on the magneto-structural properties of the Ni2MnGa Heusler alloy after a gradual substitution of Ga by Sn atoms was theoretical and experimentally studied in this work. Our data clearly show that an expansion of the L21-cell volume due Sn-substitution causes a diminution of the internal structural stress and favors the austenitic state in low temperatures where martensitic phase should prevail. It is also shown that the total magnetization reduces with increasing Sn-content, a behavior explained by a reduction of the Ni-magnetic moment, since an increase of Mn-magnetic moment was theoretically calculated. The Sn-substitution effect in the Ni2MnGa compound is similar to that found in experiments performed under high applied magnetic fields, which means that in both cases there is an increase of the L21 cell-volume favoring the austenitic state in low temperatures. Magnetization values in martensitic state of the pure Ga-compound systematically reduce after consecutive M(T) thermal cycle recorded at 5 mT; an effect not yet reported within our knowledge and attributed here to modifications in local magnetic anisotropies during the field cycles.

  17. Peculiarities of thermoelectric half-Heusler phase formation in Gd-Ni-Sb and Lu-Ni-Sb ternary systems

    NASA Astrophysics Data System (ADS)

    Romaka, V. V.; Romaka, L.; Horyn, A.; Rogl, P.; Stadnyk, Yu; Melnychenko, N.; Orlovskyy, M.; Krayovskyy, V.

    2016-07-01

    The phase equilibria in the Gd-Ni-Sb and Lu-Ni-Sb ternary systems were studied at 873 K by X-ray and metallographic analyses in the whole concentration range. The interaction of the elements in the Gd-Ni-Sb system results the formation of five ternary compounds at investigated temperature: Gd5Ni2Sb (Mo5SiB2-type), Gd5NiSb2 (Yb5Sb3-type), GdNiSb (MgAgAs-type), Gd3Ni6Sb5 (Y3Ni6Sb5-type), and GdNi0.72Sb2 (HfCuSi2-type). At investigated temperature the Lu-Ni-Sb system is characterized by formation of the LuNiSb (MgAgAs-type), Lu5Ni2Sb (Mo5SiB2-type), and Lu5Ni0.56Sb2.44 (Yb5Sb3-type) compounds. The disordering in the crystal structure of half-Heusler GdNiSb and LuNiSb was revealed by EPMA and studied by means of Rietveld refinement and DFT modeling. The performed electronic structure calculations are in good agreement with electrical transport property studies.

  18. Structure and magnetic properties of Heusler alloy Co2RuSi melt-spun ribbons

    NASA Astrophysics Data System (ADS)

    Xin, Yuepeng; Ma, Yuexing; Hao, Hongyue; Luo, Hongzhi; Meng, Fanbin; Liu, Heyan; Liu, Enke; Wu, Guangheng

    2017-08-01

    Heusler alloy Co2RuSi has been synthesized by melt-spinning technology successfully. Co2RuSi bulk sample after annealing is composed of an HCP Co-rich phase and a BCC Ru-Si phase, but melt-spinning can suppress the precipitation of the HCP phase and produce a single Co2RuSi Heusler phase. In the XRD pattern, it is found that Ru has a strong preference for the (A, C) sites, though it has fewer valence electrons compared with Co. This site preference is different from the case in Heusler alloys containing only 3d elements and is supported further by first-principles calculations. Melt-spun Co2RuSi has a Ms of 2.67 μB/f.u. at 5 K and a Tc of 491 K. An exothermic peak is observed at 871 K in the DTA curve, corresponding to the decomposition of the Heusler phase. Finally, the site preference and magnetic properties of Co2RuSi were discussed based on electronic structure calculation and charge density difference.

  19. Enhanced thermoelectric performance in TiNiSn-based half-Heuslers.

    PubMed

    Downie, R A; MacLaren, D A; Smith, R I; Bos, J W G

    2013-05-14

    Thermoelectric figures of merit, ZT > 0.5, have been obtained in arc-melted TiNiSn-based ingots. This promising conversion efficiency is due to a low lattice thermal conductivity, which is attributed to excess nickel in the half-Heusler structure.

  20. Half-Heusler phase related structural perturbations near stoichiometric composition FeZnSb

    SciTech Connect

    Xiong, Ding-Bang; Zhao, Yufeng

    2011-05-15

    Half-Heusler phases XYZ (Pearson symbol cF12) are chemically versatile and rich in physical properties. The half-Heusler phase in the Fe-Zn-Sb ternary system was reported in the year 2000. In this work, two new ternary phases are identified in the vicinity of the equiatomic composition FeZnSb in the same system: Fe{sub 1-x}ZnSb (tetragonal, space group P4/nmm, Pearson symbol tP6-{delta}, Z=2: a=4.1113(6) A, c=6.0127(12) A for x=0.08 (1), and a=4.1274(6) A, c=6.0068(12) A for x=0.12 (2)); and Fe{sub 7.87}Zn{sub 6.72}Sb{sub 8} (Fe{sub 0.98}Zn{sub 0.84}Sb) (3) (cubic, space group Fm-3m, Pearson symbol cF96-{delta}, Z=4, a=11.690(13) A). 1 and 2 crystallize in the PbFCl-type structure, and 3 adopts a unique 2x2x2 supercell of a normal half-Heusler structure. The structures of both the tetragonal and cubic phases can be described as assemblies of half-Heusler structure related subunits. Electrical resistivity measurement on the pure sample of 2 shows it has metallic-like behavior, and its thermal and magnetic properties are also characterized. -- Graphical Abstract: Three types of half-Heusler structure related subunit are identified in the vicinity of the equiatomic composition FeZnSb in the Fe-Zn-Sb system. Display Omitted Highlights: {yields} Two new related phases around equiatomic composition FeZnSb. {yields} Structural perturbation with the small variation of composition. {yields} Magnetic properties of the structure with defects.

  1. The electronic structure and spin polarization of Co2Mn0.75(Gd, Eu)0.25Z (Z=Si, Ge, Ga, Al) quaternary Heusler alloys

    NASA Astrophysics Data System (ADS)

    Berri, Saadi

    2016-03-01

    A first-principles approach is used to study the electronic and magnetic properties of Co2Mn0.75(Gd, Eu)0.25Z(Z=Si, Ge, Ga, Al) quaternary Heusler alloys. The investigation was done using the (FP-LAPW) method where the exchange-correlation potential was calculated with the frame of GGA-WC. At ambient conditions our calculated results of band structures reveal that for Co2Mn0.75(Gd, Eu)0.25Z(Z=Si, Ge) has a half-metallic (HM) band structure profile showing 100% spin polarization at the Fermi level. In contrast, Co2Mn0.75(Gd, Eu)0.25Z(Z=Ga, Al) alloys are found to be metallic. Finally, the half metallic compounds found in some structures of this series might be useful in spintronic devices.

  2. Chemical disorder determines the deviation of the Slater-Pauling rule for Fe2MnSi-based Heusler alloys: evidences from neutron diffraction and density functional theory.

    PubMed

    Tedesco, J C G; Pedro, S S; Caraballo Vivas, R J; Cruz, C; Andrade, V M; Dos Santos, A M; Carvalho, A M G; Costa, M; Venezuela, P; Rocco, D L; Reis, M S

    2016-11-30

    Fe2MnSi fails to follow the Slater-Pauling rule. This phenomenon is thought to originate from either: (i) an antiferromagnetic arrangement of Mn ions at low temperature and/or (ii) chemical disorder. An important insight on this issue could be achieved by considering Fe2MnSi1-x Ga x compounds, thoroughly studied here by means of magnetization, neutron diffraction and density functional calculations (DFT). Our results indicate that chemical disorder (and not the antiferromagnetic arrangement) is responsible for the deviation of the Slater-Pauling rule on Fe2MnSi-based Heusler alloys. Furthermore, evidences suggest that Ga substitution into Si site favors the Fe/Mn disorder, further enhancing the observed deviation.

  3. Chemical disorder determines the deviation of the Slater-Pauling rule for Fe2MnSi-based Heusler alloys: evidences from neutron diffraction and density functional theory

    NASA Astrophysics Data System (ADS)

    Tedesco, J. C. G.; Pedro, S. S.; Caraballo Vivas, R. J.; Cruz, C.; Andrade, V. M.; dos Santos, A. M.; Carvalho, A. M. G.; Costa, M.; Venezuela, P.; Rocco, D. L.; Reis, M. S.

    2016-11-01

    Fe2MnSi fails to follow the Slater-Pauling rule. This phenomenon is thought to originate from either: (i) an antiferromagnetic arrangement of Mn ions at low temperature and/or (ii) chemical disorder. An important insight on this issue could be achieved by considering Fe2MnSi1-x Ga x compounds, thoroughly studied here by means of magnetization, neutron diffraction and density functional calculations (DFT). Our results indicate that chemical disorder (and not the antiferromagnetic arrangement) is responsible for the deviation of the Slater-Pauling rule on Fe2MnSi-based Heusler alloys. Furthermore, evidences suggest that Ga substitution into Si site favors the Fe/Mn disorder, further enhancing the observed deviation.

  4. Magnetic Properties of MnFe2Ga Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Elgendy, Ahmed A.; Salehi-Fashami, Mohammad; Sellmyer, David; Hadjipanayis, George

    2015-03-01

    Recently, MnFe2Ga Heusler alloys have attracted significant attention due to their interesting physical properties such as large magnetic-field-induced strain, giant magnetocaloric effects,large magnetoresistance, and exchange bias behavior. These properties make them promising candidates for various practical applications in the field of smart materials, magnetic refrigeration and spintronics. In this work, we prepared MnFe2Ga alloys by melt-spinning and sputtering and studied the structural and magnetic properties. The melt-spun ribbons were prepared with a wheel speed of 30 m/s. The ribbons were annealed at different temperatures for 1 hour and grinded to make fine powders. The grinded powders were also used to make the target that is used in the cluster gun for the fabrication of MnFe2Ga nanoparticles. The structure of the as made, annealed ribbons, and powders displayed a face-centered-cubic structure. The microstructure of the as-made ribbons showed equiaxed grains with an average size of 3-5 μm while the annealed ribbons showed bigger grains with small particles covering homogeneously their surface. The magnetic properties show an enhancement of magnetization while coercivity remains the same with values M(3T) and HC of 85 emu/g and 150 Oe, respectively Transmission electron microscopy with elemental mapping is currently underway to determine the structure and composition of the surface nanoparticles. The work was supported by DOE-BES-DMSE (Grant No. DE-FG02-04ER4612).

  5. Magnetic properties and martensitic transformation of Ni-Mn-Ge Heusler alloys from first-principles and Monte Carlo studies

    NASA Astrophysics Data System (ADS)

    Sokolovskiy, V. V.; Zagrebin, M. A.; Buchelnikov, V. D.

    2017-05-01

    In the present study, the magnetic properties and possibility of martensitic transformation in a series of off-stoichiometric Ni2+x Mn1-x Ge and Ni2Mn1+x Ge1-x Heusler alloys have been studied by using both first-principles and Monte Carlo methods. It is shown that in both cases an increase in chemical disorder stimulates the austenite-martensite transformation and leads to an increase in transition temperature. Moreover, the calculated formation energies confirm that these compounds are stable chemically. By using the exchange coupling constants obtained from ab initio calculations in combination with the Heisenberg model and Monte Carlo methods, the temperature-dependent magnetizations as well as Curie temperatures of the cubic and tetragonal Ni2+x Mn1-x Ge and Ni2Mn1+x Ge1-x have been determined. The phase diagrams of alloys studied showing the compositions with magnetostructural transformation are obtained. Calculated results demonstrate a similar trend to the previous experimental and theoretic results for Ni-Mn-(Ga, In, Sn, Sb) alloys that makes them possible promising magnetic materials in technological applications.

  6. Effect of the Chalcogenide Element Doping on the Electronic Properties of Co2FeAl Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Huang, Ting; Cheng, Xiao-min; Guan, Xia-wei; Miao, Xiang-shui

    2016-02-01

    The electronic properties of the typical Heusler compound Co2FeAl with chalcogenide element doping were investigated by means of first principles calculations within the local spin-density approximation (LSDA) + Hubbard U parameter (U). The calculations indicate that, only when 25% of the number of Al atoms is substituted by the chalcogenide element, the chalcogenide element-doped Co2FeAl shows the half metallic properties. The Fermi energy ( E F) of the 25% chalcogenide element-doped Co2FeAl is located in the middle of the gap of the minority states instead of around the top of the valence band as in Co2FeAl. Moreover, the band gap of 25% Te-doped Co2FeAl (0.80 eV) is wider than that of Co2FeAl (0.74 eV). These improved electronic structures will make 25% chalcogenide element-doped Co2FeAl more stable against temperature variation. Therefore, the expected excellent stability of the 25% chalcogenide element-doped Co2FeAl make it more suitable for spintronic applications than Co2FeAl.

  7. Effect of structural disorder on the ground state properties of Co2CrAl Heusler alloy

    NASA Astrophysics Data System (ADS)

    Zagrebin, Mikhail A.; Sokolovskiy, Vladimir V.; Buchelnikov, Vasiliy D.; Pavlukhina, Oksana O.

    2017-08-01

    In order to discuss the difference between the available theoretical and experimental values of the total magnetic moment of Co2CrAl Heusler alloy, in this paper we studied the effects of a structural disorder on the magnetic and electronic ground state properties of the alloy studied by means of ab initio and Monte Carlo methods. On the one hand, it is shown that a calculated magnetic ground state of the austenite L21 structure is ferromagnetic, and the alloy demonstrates half-metallic behavior. However, the equilibrium lattice parameter and magnetic moment calculated for ferrimagnetic state (where the Cr atoms are ordered antiferromagnetically) are in better agreement with the available experimental data than the ferromagnetic one. On the other hand, an account of a structural disorder results in a decrease in the magnetic moment to a value close to the experimental. However, systems with a structural disorder are energetically unfavorable in comparison with the ordered L21 structure at zero temperature. Using the calculated exchange coupling parameters in the Heisenberg Hamiltonian, the temperature dependences of magnetization, specific heat, magnetic part of internal energy as well as Helmholtz energy are simulated in the framework of Monte Carlo technique for both ordered and disordered cases. Eventually, it is shown that the disordered structure with smaller magnetization is more stable at higher temperatures. This indicates that the experimental compound might be disordered.

  8. Magnetism and electronic structure of CoFeCrX (X = Si, Ge) Heusler alloys

    SciTech Connect

    Jin, Y.; O'Connell, A.; Kharel, P.; Lukashev, P. Staten, B.; Tutic, I.; Valloppilly, S.; Herran, J.; Mitrakumar, M.; Bhusal, B.; Huh, Y.; Yang, K.; Skomski, R.; Sellmyer, D. J.

    2016-08-07

    The structural, electronic, and magnetic properties of CoFeCrX (X = Si, Ge) Heusler alloys have been investigated. Experimentally, the alloys were synthesized in the cubic L2{sub 1} structure with small disorder. The cubic phase of CoFeCrSi was found to be highly stable against heat treatment, but CoFeCrGe disintegrated into other new compounds when the temperature reached 402 °C (675 K). Although the first-principle calculation predicted the possibility of tetragonal phase in CoFeCrGe, the tetragonal phase could not be stabilized experimentally. Both CoFeCrSi and CoFeCrGe compounds showed ferrimagnetic spin order at room temperature and have Curie temperatures (T{sub C}) significantly above room temperature. The measured T{sub C} for CoFeCrSi is 790 K but that of CoFeCrGe could not be measured due to its dissociation into new compounds at 675 K. The saturation magnetizations of CoFeCrSi and CoFeCrGe are 2.82 μ{sub B}/f.u. and 2.78 μ{sub B}/f.u., respectively, which are close to the theoretically predicted value of 3 μ{sub B}/f.u. for their half-metallic phases. The calculated band gaps for CoFeCrSi and CoFeCrGe are, respectively, 1 eV and 0.5 eV. These materials have potential for spintronic device applications, as they exhibit half-metallic electronic structures with large band gaps, and Curie temperatures significantly above room temperature.

  9. Fabrication and thermoelectric properties of fine-grained TiNiSn compounds

    SciTech Connect

    Zou Minmin; Li Jingfeng; Du Bing; Liu Dawei; Kita, Takuji

    2009-11-15

    Nearly single-phased TiNiSn half-Heusler compound thermoelectric materials were synthesized by combining mechanical alloying (MA) and spark plasma sintering (SPS) in order to reduce its thermal conductivity by refining the grain sizes. Although TiNiSn compound powders were not synthesized directly via MA, dense bulk samples of TiNiSn compound were obtained by the subsequent SPS treatment. It was found that an excessive Ti addition relative to the TiNiSn stoichiometry is effective in increasing the phase purity of TiNiSn half-Heusler phase in the bulk samples, by compensating for the Ti loss caused by the oxidation of Ti powders and MA processing. The maximum power factor value obtained in the Ti-compensated sample is 1720 muW m{sup -1} K{sup -2} at 685 K. A relatively high ZT value of 0.32 is achieved at 785 K for the present undoped TiNiSn compound polycrystals. - Graphical abstract: Nearly single-phased TiNiSn-based half-Heusler compound polycrystalline materials with fine grains were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). A high ZT value for undoped TiNiSn was obtained because of the reduced thermal conductivity.

  10. Shell-ferromagnetism of nano-Heuslers generated by segregation under magnetic field

    PubMed Central

    Çakır, A.; Acet, M.; Farle, M.

    2016-01-01

    We report on a new functional property in an AF martensitic Heusler Ni50Mn45In5, which when annealed at high temperatures under a magnetic field, segregates and forms Ni50Mn25In25 Heusler precipitates embedded in a Ni50Mn50 matrix. The precipitates are paramagnetic whereas the matrix is antiferromagnetic. However, the spins at the interface with the Ni50Mn50 matrix align with the field during their nucleation and growth and become strongly pinned in the direction of the applied field during annealing, whereas the core spins become paramagnetic. This shell-ferromagnetism persists up to 600 K and is so strongly pinned that the remanent magnetization remains unchanged, even when the field is reversed or when the temperature is cycled between low temperatures and close to the annealing temperature. PMID:27412644

  11. High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloy

    SciTech Connect

    Bainsla, Lakhan; Suresh, K. G.; Nigam, A. K.; Manivel Raja, M.; Varaprasad, B. S. D. Ch. S.; Takahashi, Y. K.; Hono, K.

    2014-11-28

    We report the structure, magnetic property, and spin polarization of CoFeMnGe equiatomic quaternary Heusler alloy. The alloy was found to crystallize in the cubic Heusler structure (prototype LiMgPdSn) with considerable amount of DO{sub 3} disorder. Thermal analysis result indicated the Curie temperature is about 750 K without any other phase transformation up to melting temperature. The magnetization value was close to that predicted by the Slater-Pauling curve. Current spin polarization of P = 0.70 ± 0.01 was deduced using point contact andreev reflection measurements. The temperature dependence of electrical resistivity has been fitted in the temperature range of 5–300 K in order to check for the half metallic behavior. Considering the high spin polarization and Curie temperature, this material appears to be promising for spintronic applications.

  12. Premartensite transition in Ni{sub 2}FeGa Heusler alloy

    SciTech Connect

    Nath, Hrusikesh; Phanikumar, G.

    2015-04-15

    Martensitic phase transformation of Ni{sub 2}FeGa Heusler alloy was studied by differential scanning calorimetry. Atomic ordering induced in the austenite structure by quenching from high temperature plays a significant role on martensitic phase transformation. Higher magnetization and larger magneto-crystalline anisotropy of martensite phase than that of austenite phase are noticed. Tweed contrast regions observed in the transmission electron microscopy were correlated to premartensite phenomena. A shift in premartensitic transition temperature prior to martensitic transformation as measured by differential scanning calorimetry is being reported for the first time in this system. - Highlights: • Atomic ordering influences martensitic transformation in Ni{sub 2}FeGa Heusler alloy. • Observation of tweed contrast in TEM was correlated to premartensite phenomena. • For the first time the shift in premartensite peak was observed in DSC.

  13. Pairing of j=3/2 Fermions in Half-Heusler Superconductors.

    PubMed

    Brydon, P M R; Wang, Limin; Weinert, M; Agterberg, D F

    2016-04-29

    We theoretically consider the superconductivity of the topological half-Heusler semimetals YPtBi and LuPtBi. We show that pairing occurs between j=3/2 fermion states, which leads to qualitative differences from the conventional theory of pairing between j=1/2 states. In particular, this permits Cooper pairs with quintet or septet total angular momentum, in addition to the usual singlet and triplet states. Purely on-site interactions can generate s-wave quintet time-reversal symmetry-breaking states with topologically nontrivial point or line nodes. These local s-wave quintet pairs reveal themselves as d-wave states in momentum space. Furthermore, due to the broken inversion symmetry in these materials, the s-wave singlet state can mix with a p-wave septet state, again with topologically stable line nodes. Our analysis lays the foundation for understanding the unconventional superconductivity of the half-Heuslers.

  14. Improvement of thermoelectric properties for half-Heusler TiNiSn by interstitial Ni defects

    SciTech Connect

    Hazama, Hirofumi; Matsubara, Masato; Asahi, Ryoji; Takeuchi, Tsunehiro

    2011-09-15

    We have synthesized off-stoichiometric Ti-Ni-Sn half-Heusler thermoelectrics in order to investigate the relation between randomly distributed defects and thermoelectric properties. A small change in the composition of Ti-Ni-Sn causes a remarkable change in the thermal conductivity. An excess content of Ni realizes a low thermal conductivity of 2.93 W/mK at room temperature while keeping a high power factor. The low thermal conductivity originates in the defects generated by an excess content of Ni. To investigate the detailed defect structure, we have performed first-principles calculations and compared with x ray photoemission spectroscopy measurement. Based on these analyses, we conclude that the excess Ni atoms randomly occupy the vacant sites in the half-Heusler structure, which play as phonon scattering centers, resulting in significant improvement of the figure of merit without any substitutions of expensive heavy elements, such as Zr and Hf.

  15. Tetragonal Heusler-Like Mn-Ga Alloys Based Perpendicular Magnetic Tunnel Junctions

    NASA Astrophysics Data System (ADS)

    Ma, Qinli; Sugihara, Atsushi; Suzuki, Kazuya; Zhang, Xianmin; Miyazaki, Terunobu; Mizukami, Shigemi

    2014-10-01

    Films of the Mn-based tetragonal Heusler-like alloys, such as Mn-Ga, exhibit a large perpendicular magnetic anisotropy (PMA), small damping constant, small saturation magnetization and large spin polarizations. These properties are attractive for the application to the next generation high density spin-transfer-torque (STT) magnetic random access memory (STT-MRAM). We reviewed the structure, magnetic properties and Gilbert damping of the alloy films with large PMA, and the current status of research on tunnel magnetoresistance (TMR) in perpendicular magnetic tunnel junctions (p-MTJs) based on Mn-based tetragonal Heusler-like alloy electrode, and also discuss the issues for the application of those to STT-MRAM.

  16. Half-metallicity and magnetism of Ti2Ni1-x CoxAl1-y Siy inverse Heusler alloys

    NASA Astrophysics Data System (ADS)

    Zhou, Ting; Feng, Yu; Chen, Xiaorui; Yuan, Hongkuan; Chen, Hong

    2017-02-01

    Half-metallicity and magnetism of Ti2Ni1-x CoxAl1-y Siy, which are obtained by Co/Si substitutions for Ni/Al of inverse Heusler alloy Ti2NiAl, are investigated by first-principle calculations based on density functional theory (DFT). The optimized lattice constants of the doped systems all conform to the Vegard law as the increase of the impurity concentration, and the magnetic moments obey the Slater-Pauling rule when the half-metallicity is retained. The defect formation energies of the codoped systems are lower than those of the monodoped systems due to the charge compensation effects, thus the Co+Si codoping is more favorable in energy than the Co/Si monodoping. Furthermore, for the Co and Si monodoped systems, the Co monodoping retains the minority-spin bandgap unchanged although the Fermi level moves towards high energy region, and the Si monodoping leads to the minority-spin bandgap narrowing and even the loss of half-metallicity at the high concentration, while for the Co+Si codoped systems, the majority of the codoped compounds obviously show more stable half-metallicity and the minority-spin gap get widened. In particular, the minority-spin band gap of the codoped compounds Ti2Ni0.5Co0.5Al0.5Si0.5 , Ti2Ni0.25Co0.75Al0.5Si0.5 , and Ti2NiCo Al0.25Si0.75 are widened distinctly and their Fermi level are adjusted to the middle of the minority-spin gap, indicating that they possess robust half-metallicity and thus they are promising candidates for spintronics applications.

  17. Mechanical, electronic and thermodynamic properties of full Heusler compounds Fe2VX(X = Al, Ga)

    NASA Astrophysics Data System (ADS)

    Khalfa, M.; Khachai, H.; Chiker, F.; Baki, N.; Bougherara, K.; Yakoubi, A.; Murtaza, G.; Harmel, M.; Abu-Jafar, M. S.; Omran, S. Bin; Khenata, R.

    2015-11-01

    The electronic structure, mechanical and thermodynamic properties of Fe2VX, (with X = Al and Ga), have been studied self consistently by employing state-of-the-art full-potential linearized approach of augmented plane wave plus local orbitals (FP-LAPW + lo) method. The exchange-correlation potential is treated with the local density and generalized gradient approximations (LDA and GGA). Our predicted ground state properties such as lattice constants, bulk modulus and elastic constants appear more accurate when we employed the GGA rather than the LDA, and these results are in very good agreement with the available experimental and theoretical data. Further, thermodynamic properties of Fe2VAl and Fe2VGa are predicted with pressure and temperature in the ranges of 0-40 GPa and 0-1500 K using the quasi-harmonic Debye model. We have obtained successfully the variations of the heat capacities, primitive cell volume and volume expansion coefficient.

  18. Growth and Electronic Structure of Heusler Compounds for Use in Electron Spin Based Devices

    DTIC Science & Technology

    2015-06-01

    spectroscopy (ARPES) at the beamlines i3 and i4 of MAX-lab in Lund University, SE as well as x-ray diffraction and SQUID magnetometry using a Quantum ...SECURITY CLASSIFICATION OF: Spintronic devices, where information is carried by the quantum spin state of the electron instead of purely its charge...Title Spintronic devices, where information is carried by the quantum spin state of the electron instead of purely its charge, have gained considerable

  19. Ab-initio study of structural, elastic, thermal, electronic and magnetic properties of quaternary Heusler alloys CoMnCrZ (Z = Al, As, Si, Ge)

    NASA Astrophysics Data System (ADS)

    Mohamedi, Mohamed Walid; Chahed, Abbes; Amar, Amina; Rozale, Habib; Lakdja, Abdelaziz; Benhelal, Omar; Sayede, Adlane

    2016-12-01

    First-principles approach is used to study the structural, electronic and magnetic properties of CoMnCrZ (Z = Al, Si, Ge and As) quaternary Heusler compounds, using full-potential linearized augmented plane wave (FP-LAPW) scheme within the generalized gradient approximation (GGA). The computed equilibrium lattice parameters agree well with the available theoretical data. The obtained negative formation energy shows that CoMnCrZ (Z = Al, Si, Ge, As) compounds have strong structural stability. The elastic constants Cij are calculated using the total energy variation with strain technique. The polycrystalline elastic moduli (namely: the shear modulus, Young's modulus, Poisson's ratio, sound velocities, Debye temperature and melting temperature were derived from the obtained single-crystal elastic constants. The ductility mechanism for the studied compounds is discussed via the elastic constants Cij. Our calculations with the GGA approximation predict that CoMnCrGe, CoMnCrAl, CoMnCrSi and CoMnCrAs are half-metallic ferrimagnets (HMFs) with a half-metallic gap EHM of 0.03 eV, 0.19 eV, 0.34 eV and 0.50 eV for, respectively. We also find that the half-metallicity is maintained on a wide range of lattice constants.

  20. Investigation of the electronic and thermoelectric properties of Fe2ScX (X = P, As and Sb) full Heusler alloys by using first principles calculations

    NASA Astrophysics Data System (ADS)

    Sharma, Sonu; Pandey, Sudhir K.

    2014-11-01

    By using ab initio electronic structure calculations here we report the three new full Heusler alloys—Fe2ScP, Fe2ScAs and Fe2ScSb. These alloys possess a very good thermoelectric behavior and are also expected to be synthesized in laboratories. The first two compounds are indirect band gap semiconductors and the last one shows a semimetallic ground state. The value of the band gap of Fe2ScP and Fe2ScAs is 0.3 eV and 0.09 eV, respectively. The presence of flat conduction bands along the Γ—X-direction suggest the large electron like effective mass and also promises a very good thermoelectric behavior of these compounds. At 200 K, the Seebeck coefficients of Fe2ScP, Fe2ScAs and Fe2ScSb compounds are -770, -386 and -192 µV K-1, respectively. The maximum power factor (PF) is expected for the n-type doping in these materials. The heavily doped Fe2ScP and Fe2ScAs have PF > 60 for a wide temperature range, which is comparable to the PF of Bi2Te3—a well known and one of the most commercially used thermoelectric materials. The present work suggests the possible thermoelectric applicability of these materials in a wide temperature range.

  1. Perpendicular magnetization of Co2FeAl full-Heusler alloy films induced by MgO interface

    NASA Astrophysics Data System (ADS)

    Wen, Zhenchao; Sukegawa, Hiroaki; Mitani, Seiji; Inomata, Koichiro

    2011-06-01

    The perpendicular magnetization of Co2FeAl (CFA) full-Heusler alloy films was achieved in the structures of CFA/MgO and MgO/CFA with the perpendicular magnetic anisotropy energy density (KU) of 2-3×106 erg/cm3, which can be used as the perpendicular ferromagnetic electrodes of MgO-based magnetic tunnel junctions (MTJs) with high thermal stability at sub-50-nm dimension. The CFA thickness dependence of KU was investigated at different annealing temperatures, indicating that the perpendicular anisotropy of CFA is contributed by the interfacial anisotropy between CFA and MgO. This letter will open up a way for obtaining perpendicular magnetization of Co-based full-Heusler alloys, which is promising for further reduction in the critical current of current induced magnetization switching in MgO-based MTJ nanopillars with perpendicular full-Heusler alloy electrodes.

  2. A half-metallic half-Heusler alloy having the largest atomic-like magnetic moment at optimized lattice constant

    SciTech Connect

    Zhang, R. L.; Damewood, L.; Fong, C. Y.; Yang, L. H.; Peng, R. W.; Felser, C.

    2016-11-02

    For half-Heusler alloys, the general formula is XYZ, where X can be a transition or alkali metal element, Y is another transition metal element, typically Mn or Cr, and Z is a group IV element or a pnicitide. The atomic arrangements within a unit-cell show three configurations. Before this study, most of the predictions of half-metallic properties of half-Heusler alloys at the lattice constants differing from their optimized lattice constant. Based on the electropositivity of X and electronegativity of Z for half-Heusler alloys, we found that one of the configurations of LiCrS exhibits half-metallic properties at its optimized lattice constant of 5.803Å, and has the maximum atomic-like magnetic moment of 5μB. In conclusion, the challenges of its growth and the effects of the spin-orbit effect in this alloy will be discussed.

  3. A half-metallic half-Heusler alloy having the largest atomic-like magnetic moment at optimized lattice constant

    DOE PAGES

    Zhang, R. L.; Damewood, L.; Fong, C. Y.; ...

    2016-11-02

    For half-Heusler alloys, the general formula is XYZ, where X can be a transition or alkali metal element, Y is another transition metal element, typically Mn or Cr, and Z is a group IV element or a pnicitide. The atomic arrangements within a unit-cell show three configurations. Before this study, most of the predictions of half-metallic properties of half-Heusler alloys at the lattice constants differing from their optimized lattice constant. Based on the electropositivity of X and electronegativity of Z for half-Heusler alloys, we found that one of the configurations of LiCrS exhibits half-metallic properties at its optimized lattice constantmore » of 5.803Å, and has the maximum atomic-like magnetic moment of 5μB. In conclusion, the challenges of its growth and the effects of the spin-orbit effect in this alloy will be discussed.« less

  4. A half-metallic half-Heusler alloy having the largest atomic-like magnetic moment at optimized lattice constant

    NASA Astrophysics Data System (ADS)

    Zhang, R. L.; Damewood, L.; Fong, C. Y.; Yang, L. H.; Peng, R. W.; Felser, C.

    2016-11-01

    For half-Heusler alloys, the general formula is XYZ, where X can be a transition or alkali metal element, Y is another transition metal element, typically Mn or Cr, and Z is a group IV element or a pnicitide. The atomic arrangements within a unit-cell show three configurations. Before this study, most of the predictions of half-metallic properties of half-Heusler alloys at the lattice constants differing from their optimized lattice constant. Based on the electropositivity of X and electronegativity of Z for half-Heusler alloys, we found that one of the configurations of LiCrS exhibits half-metallic properties at its optimized lattice constant of 5.803Å, and has the maximum atomic-like magnetic moment of 5μB. The challenges of its growth and the effects of the spin-orbit effect in this alloy will be discussed.

  5. A Comparison of Surface Segregation for two semi-Heusler alloys: TiCoSb and NiMnSb

    NASA Astrophysics Data System (ADS)

    Caruso, A. N.; Borca, C. N.; Dowben, P. A.; Ristoiu, D.; Nozieres, J. P.

    2002-03-01

    Very different types of surface segregation are found for very similar Heusler alloy materials. We observed significant manganese and antimony segregation to the surfaces of the semi-Heusler alloys NiMnSb and TiCoSb respectively. The Mn and Sb surface enrichment was characterized by angle-resolved core level photoemission. Indications of surface disorder from low energy electron diffraction provide complimentary evidence of segregation. It has been established [1,2] that segregation will significantly affect polarization so surface/interface segregation of the types observed for the half Heusler alloys will have substantial implications for spin-electronic devices made from these nominally high polarization materials. [1] D. Ristoiu, et al., Europhysics Letters 49 (2000) 624-630 [2] C. N. Borca, et al., Europhysics Letters 56 (2001) 722-728

  6. Structural and magnetic properties of Ni 2MnIn Heusler thin films grown on modulation-doped InAs heterostructures with metamorphic buffer

    NASA Astrophysics Data System (ADS)

    Bohse, S.; Zolotaryov, A.; Volland, A.; Landgraf, B.; Albrecht, O.; Bastjan, M.; Vossmeyer, T.; Görlitz, D.; Heyn, Ch.; Hansen, W.

    2012-01-01

    This paper reports on the morphological, structural, magnetic, and magneto-optic properties of Ni 2MnIn Heusler films grown on InAs-high electron-mobility transistor structures (HEMT) with metamorphic buffers for spintronic applications. Similar to our previous results on the Ni 2MnIn/InAs (001) system, the Heusler layer is found to have a (110) orientation relative to the (001) InAs-HEMT surface. We observe almost equal spin-polarizations for Heusler films on (001) InAs-HEMT as well as on (001) InAs. In addition, we find further support for interfacial intermixing previously reported for the Ni 2MnIn/InAs (001) system. On the other hand, the Heusler/InAs-HEMT system shows distinct morphologic, structural, and magnetic properties as compared to the Ni 2MnIn/InAs (001) system. In particular, more rapid and complex plastic deformation effects resulting in a high surface density of pin-holes in the Heusler films are found. We report on complex mutual deformation effects between the Heusler films and the underlying InAs-HEMT structure. Furthermore, a hysteresis loop squareness close to 1 for a 50 nm Heusler film on InAs-HEMT is observed. We tentatively associate these phenomena with the higher mismatch strain of the Ni 2MnIn/InAs-HEMT system compared to Ni 2MnIn films grown on (001) InAs.

  7. Ab initio ground state and L{sub 2,3} x-ray magnetic circular dichroism of Mn-based Heusler alloys

    SciTech Connect

    Galanakis, I.; Ostanin, S.; Alouani, M.; Dreysse, H.; Wills, J. M.

    2000-02-01

    Relativistic full-potential calculations within the generalized gradient approximation (GGA) for a series of Mn-based Heusler alloys are presented. Calculated equilibrium lattice parameters deviate less than 1.2% from the experimental values. The main features of a half metallic system are present in the density of states for the PtMnSb and NiMnSb. We predict that PdMnSb shows half metallic character under hydrostatic pressure. The substitution of Sb in PtMnSb by Sn or Te destroys the minority spin band gap. Spin and orbital magnetic moments for all the systems are in good agreement with previous calculations and experimental data. L{sub 2,3} x-ray absorption and x-ray magnetic circular dichroism (XMCD) spectra are calculated for all the five compounds. Pt spectra present big deviations from system to system in the PtMnY (Y=Sn,Sb,Te) compounds while Mn spectra show only small deviations. For all these spectra GGA underestimates the L{sub 3}/L{sub 2} integrated branching ratio and produces a much smaller L{sub 2} peak intensity for the Ni site in NiMnSb. The XMCD sum rules are used to compute the spin and orbital magnetic moments and the results are compared to the direct calculations. (c) 2000 The American Physical Society.

  8. Ab initio phonon properties of half-Heusler NiTiSn, NiZrSn and NiHfSn.

    PubMed

    Andrea, Luc; Hug, Gilles; Chaput, Laurent

    2015-10-28

    A theoretical investigation of phonon properties from first-principles calculations is carried out for the half-Heusler compounds NiXSn, [Formula: see text], Zr and Hf. The crystal structures are optimised via ab initio calculations within the framework of density functional theory. The phonon properties are retrieved from harmonic and anharmonic interatomic force constants calculations using the finite size displacements method and many-body perturbation theory. A solution to the linearized phonon Boltzmann transport equation is then used to compute the ab initio thermal conductivities. For X   =   Ti, Zr and Hf, we found 15.4, 13.3 and 15.8 W m(-1) K(-1) at 300 K, respectively. Thanks to a spectral analysis of the velocities and lifetimes we were able appreciate the differences in the thermal conductivities between the three compounds under study. Our results provide insights to understand the behaviour of the thermal conductivity and therefore to improve the thermoelectric figure of merit for such materials.

  9. Structural, Electronic and Magnetic Properties of Ti1+xFeSb and TiFe0.75M0.25Sb (M = Ni, Mn) Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Al Azar, Said; Mousa, Ahmad

    Density functional theory calculations based on full potential linearized augmented plane-wave (FPLAPW) plus local orbital method in the framework of GGA-PBE, as embodied in the WIEN2k code, is used to investigate the structural, electronic and magnetic properties of intermetallic Ti1+xFeS Heusler compounds, where (x = i/4, i =-3,-2,-1,0,1,2,3,4) and the TiFe0.75M0.25Sb (M = Ni, Mn) quaternary semi-Heusler compounds. Moreover, the modified Becke-Johnson exchange potential, as a semi-local method, was employed to predict the band-gap more precisely. We examined the site preference of the parent compound TiFeSb and varying the electron concentration by doping or removing a Ti atom. It is found that they play a crucial role in physical properties of these material systems. The lattice parameters and spin magnetic moment calculated were consistent with the previous experimental and theoretical data available. Moreover, alloys with x<0 are found to exhibit a ferrimagnetic phase, and the alloy with x =0.25 exhibit a non-magnetic properties, whereas the rest have shown ferromagnetic phase. The band-structure analysis of Ti1.75FeSb, Ti2FeSb and TiFe0.75Ni0.25Sb alloys suggested that they could be a ferromagnetic half-metallic members with band-gaps 0.67, 0.41 and 0.54 eV, respectively.

  10. Suppression of Martensitic Transformation in Co2Cr(Ga,Si) Heusler Alloys by Thermal Cycling

    NASA Astrophysics Data System (ADS)

    Liang, Xiao; Xiao, Fei; Jin, Xuejun; Fukuda, Takashi; Kakeshita, Tomoyuki

    2017-05-01

    We have investigated the influence of thermal cycles on martensitic transformation of a Co2Cr(Ga,Si) ferromagnetic Heusler alloy. The as-quenched specimen exhibits successive L21(L)- D022- L21(H) martensitic transformation in the cooling process, which is known as reentrant martensitic transformation. However, heating to 800 K (527 °C) for reverse D022- L21 transformation with a rate of 10 K/min (10 °C/min) stabilizes the parent phase, meaning that the martensitic transformation is suppressed by the thermal cycles. We found precipitate after thermal cycles, and it will be the reason for the stabilization of parent phase.

  11. On the Broadening of the Martensitic Transition in Heusler Alloys: From Microscopic Features to Magnetocaloric Properties

    NASA Astrophysics Data System (ADS)

    Cugini, F.; Porcari, G.; Rimoldi, T.; Orsi, D.; Fabbrici, S.; Albertini, F.; Solzi, M.

    2017-08-01

    We report on the sharpness of the magnetostructural transition in a Ni-Co-Mn-Ga-In Heusler alloy, which has a key role in defining the magnetocaloric effect exploitable in efficient and environmentally friendly magnetic refrigerators. Magnetic and calorimetric analysis together with optical microscopy in phase-imaging mode are compared to correlate the transition broadening with microscopic features of the sample. The three techniques give the same temperature behavior of the martensitic transition. Microscopy analysis of different sample areas and during cycles in temperature reveals the spatial homogeneity of the transition width and its reproducibility in subsequent transformations.

  12. Chemical disorder as an engineering tool for spin polarization in Mn3Ga -based Heusler systems

    NASA Astrophysics Data System (ADS)

    Chadov, S.; D'Souza, S. W.; Wollmann, L.; Kiss, J.; Fecher, G. H.; Felser, C.

    2015-03-01

    Our study highlights spin-polarization mechanisms in metals by focusing on the mobilities of conducting electrons with different spins instead of their quantities. Here, we engineer electron mobility by applying chemical disorder induced by nonstoichiometric variations. As a practical example, we discuss the scheme that establishes such variations in tetragonal Mn3Ga Heusler material. We justify this approach using first-principles calculations of the spin-projected conductivity components based on the Kubo-Greenwood formalism. It follows that, in the majority of cases, even a small substitution of some other transition element instead of Mn may lead to a substantial increase in spin polarization along the tetragonal axis.

  13. Uniaxial pressure setup for piezoresistance and magnetoresistance measurements in Heusler materials.

    PubMed

    Bourgault, D; Porcar, L; Bruyère, C; Jacquet, P; Courtois, P

    2013-01-01

    We report on a new uniaxial pressure experimental setup for electrical resistivity measurements working in a 77 K-500 K temperature range and in a magnetic field up to 8 T. Such a continuous uniaxial pressure device enables the study of the piezoresistance and the pressure induced change in electrical properties of bulk samples. Strong influence of uniaxial pressure on transport properties is shown for Ni-Co-Mn-In Heusler single crystal material. A shift of the martensite-austenite first order transformation temperature is measured with an applied uniaxial pressure leading to an electrical resistance changed by up to 120%.

  14. Disorder dependent half-metallicity in Mn{sub 2}CoSi inverse Heusler alloy

    SciTech Connect

    Singh, Mukhtiyar; Saini, Hardev S.; Thakur, Jyoti; Reshak, Ali H.; Kashyap, Manish K.

    2013-12-15

    Heusler alloys based thin-films often exhibit a degree of atomic disorder which leads to the lowering of spin polarization in spintronic devices. We present ab-initio calculations of atomic disorder effects on spin polarization and half-metallicity of Mn{sub 2}CoSi inverse Heusler alloy. The five types of disorder in Mn{sub 2}CoSi have been proposed and investigated in detail. The A2{sub a}-type and B2-type disorders destroy the half-metallicity whereas it sustains for all disorders concentrations in DO{sub 3a}- and A2{sub b}-type disorder and for smallest disorder concentration studied in DO{sub 3b}-type disorder. Lower formation energy/atom for A2{sub b}-type disorder than other four disorders in Mn{sub 2}CoSi advocates the stability of this disorder. The total magnetic moment shows a strong dependence on the disorder and the change in chemical environment. The 100% spin polarization even in the presence of disorders explicitly supports that these disorders shall not hinder the use of Mn{sub 2}CoSi inverse Heusler alloy in device applications. - Graphical abstract: Minority-spin gap (E{sub g↓}) and HM gap (E{sub sf}) as a function of concentrations of various possible disorder in Mn{sub 2}CoSi inverse Heusler alloy. The squares with solid line (black color)/dotted line (blue color)/dashed line (red color) reperesents E{sub g↓} for DO{sub 3a}-/DO{sub 3b}-/A2{sub b}-type disorder in Mn{sub 2}CoSi and the spheres with solid line (black color)/dottedline (blue color)/dashed line (red color) represents E{sub sf} for DO{sub 3a}-/DO{sub 3b}-/A2{sub b}-type disorder in Mn{sub 2}CoSi. - Highlights: • The DO{sub 3}- and A2-type disorders do not affect the half-metallicity in Mn{sub 2}CoSi. • The B2-type disorder solely destroys half-metallicity in Mn{sub 2}CoSi. • The A2-type disorder most probable to occur out of all three types. • The total spin magnetic moment strongly depends on the disorder concentrations.

  15. Suppression of Martensitic Transformation in Co2Cr(Ga,Si) Heusler Alloys by Thermal Cycling

    NASA Astrophysics Data System (ADS)

    Liang, Xiao; Xiao, Fei; Jin, Xuejun; Fukuda, Takashi; Kakeshita, Tomoyuki

    2017-03-01

    We have investigated the influence of thermal cycles on martensitic transformation of a Co2Cr(Ga,Si) ferromagnetic Heusler alloy. The as-quenched specimen exhibits successive L21(L)-D022-L21(H) martensitic transformation in the cooling process, which is known as reentrant martensitic transformation. However, heating to 800 K (527 °C) for reverse D022-L21 transformation with a rate of 10 K/min (10 °C/min) stabilizes the parent phase, meaning that the martensitic transformation is suppressed by the thermal cycles. We found precipitate after thermal cycles, and it will be the reason for the stabilization of parent phase.

  16. Electronic properties of thin Ni 2MnIn Heusler films

    NASA Astrophysics Data System (ADS)

    von Oehsen, S.; Scholtyssek, J. M.; Pels, C.; Neuber, G.; Rauer, R.; Rübhausen, M.; Meier, G.

    2005-04-01

    The half-metallic Heusler alloy Ni 2MnIn is of high interest for use in spin electronics since at the Ni 2MnIn/InAs interface a spin polarization of 100% is predicted. We prepare high-quality thin films of 20-60 nm thickness by co-evaporation and DC magnetron sputtering. Point-contact Andreev reflection spectroscopy yields a spin polarization of up to 54%. By spectral generalized magneto-optical ellipsometry, the dielectric and magneto-optical properties are determined and ferromagnetic behavior below the Curie temperature TC=318 K is proved.

  17. ``All-Heusler alloy'' current-perpendicular-to-plane giant magnetoresistance

    NASA Astrophysics Data System (ADS)

    Nikolaev, Konstantin; Kolbo, Paul; Pokhil, Taras; Peng, Xilin; Chen, Yonghua; Ambrose, Thomas; Mryasov, Oleg

    2009-06-01

    A materials system of ternary full Heusler alloys exhibiting substantial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) has been theoretically proposed and experimentally realized. Observed trends in magnetoresistance are broadly consistent with the modeling results. A CPP-GMR of 6.7% and ΔRA of 4 mΩ μm2 have been demonstrated in the bottom spin-valve configuration. The spin-stand testing of narrow-track recording heads confirmed compatibility of these materials with hard disk drive reader technology.

  18. Elastic constants determined by nanoindentation for p-type thermoelectric half-Heusler

    SciTech Connect

    Gahlawat, S.; Wheeler, L.; White, K. W. E-mail: kwwhite@uh.edu; He, R.; Chen, S.; Ren, Z. F. E-mail: kwwhite@uh.edu

    2014-08-28

    This paper presents a study of the elastic properties of the p-type thermoelectric half-Heusler material, Hf{sub 0.44}Zr{sub 0.44}Ti{sub 0.12}CoSb{sub 0.8}Sn{sub 0.2}, using nanoindentation. Large grain-sized polycrystalline specimens were fabricated for these measurements, providing sufficient indentation targets within single grains. Electron Backscatter Diffraction methods indexed the target grains for the correlation needed for our elastic analysis of individual single crystals for this cubic thermoelectric material. Elastic properties, including the Zener ratio and the Poisson ratio, obtained from the elasticity tensor are also reported.

  19. Electron dominated thermoelectric response in MNiSn (M: Ti, Zr, Hf) half-Heusler alloys.

    PubMed

    Gandi, Appala Naidu; Schwingenschlögl, Udo

    2016-05-18

    We solve the transport equations of the electrons and phonons to understand the thermoelectric behaviour of the technologically important half-Heusler alloys MNiSn (M: Ti, Zr, Hf). Doping is simulated within the rigid band approximation. We clarify the origin of the electron dominated thermoelectric response and determine the carrier concentrations with maximal figures of merit. The phonon mean free path is studied to calculate the grain size below which grain refinement methods can enforce ballistic heat conduction to enhance the figure of merit.

  20. Energies and lifetimes of magnons in complex ferromagnets: a first-principle study of Heusler alloys.

    PubMed

    Buczek, Paweł; Ernst, Arthur; Bruno, Patrick; Sandratskii, Leonid M

    2009-06-19

    The energies and lifetimes of magnons in several Mn-based Heusler alloys are studied using linear response density functional theory. The number of the spin wave branches in Co(2)MnSi corresponds to the number of its magnetic sublattices in contrast with the NiMnSb case in which the induced Ni sublattice cannot support optical magnons. The half-metallicity of these systems results in long-living acoustic spin waves. The example of non-half-metallic Cu(2)MnAl shows that the hybridization with Stoner continuum leads not only to the damping of magnons but also to a renormalization of their energies.

  1. First principle prediction of half-metallic ferromagnetism above room temperature in half-heusler alloys

    SciTech Connect

    Van An Dinh; Sato, Kazunori; Katayama-Yoshida, Hiroshi

    2010-01-04

    A first principle study of half-metallicity and ferromagnetism in half-heusler alloys NiMnZ (Z = Si, P, Ge, As, and Sb) is given. The half-metallicity and ferromagnetism are predicted via the calculation of electronic structure, and Curie temperature. The stability of the orthorhombic and tetragonal structures and C1{sub b} at various values of lattice parameters is also studied by means of the pseudo-potential method. All alloys exhibit the half-metallicity and ferromagnetism above room temperature.

  2. The magnetic and transport properties of the Co2FeGa Heusler alloy

    NASA Astrophysics Data System (ADS)

    Zhang, Ming; Brück, Ekkes; de Boer, Frank R.; Li, Zhuangzhi; Wu, Guangheng

    2004-08-01

    The magnetic and transport properties of the Co2FeGa Heusler alloy have been investigated. The results show that the temperature dependence of the magnetization follows the spin-wave behaviour at low temperature. The electrical resistivity behaves according to a ~T2 power law, which may be mainly attributed to electron-electron scattering, and the contribution of electron-phonon scattering to the resistivity seems to be small. We have not observed remarkable magnetoresistance in our measurements. Point contact Andreev reflection measurements of the spin-polarization yield a polarization of 59%, which is consistent with the theoretical prediction by a first-principles calculation.

  3. Martensitic transformation and shape memory effect in ferromagnetic Heusler alloy Ni2FeGa

    NASA Astrophysics Data System (ADS)

    Liu, Z. H.; Zhang, M.; Cui, Y. T.; Zhou, Y. Q.; Wang, W. H.; Wu, G. H.; Zhang, X. X.; Xiao, Gang

    2003-01-01

    We have synthesized ferromagnetic Heusler alloy Ni2FeGa using the melt-spinning technique. The Ni2FeGa ribbon, having a high chemical ordering L21 structure, exhibits a thermoelastic martensitic transformation from cubic to orthorhombic structure at 142 K and a premartensitic transformation. The alloy has a relatively high Curie temperature of 430 K, a magnetization of 73 Am2/kg, and a low saturated field of 0.6 T. The textured samples with preferentially oriented grains show a completely recoverable two-way shape memory effect with a strain of 0.3% upon the thermoelastic martensitic transformation.

  4. Effect of doping and disorder on the half metallicity of full Heusler alloys

    NASA Astrophysics Data System (ADS)

    Galanakis, I.; Özdoǧan, K.; Aktaş, B.; Şaşıoǧlu, E.

    2006-07-01

    Heusler alloys containing Co and Mn are amongst the most heavily studied half metallic ferromagnets for future applications in spintronics. Using state-of-the-art electronic structure calculations, we investigate the effect of doping and disorder on their electronic and magnetic properties. Small degrees of doping by substituting Fe or Cr for Mn scarcely affect the half metallicity. A similar effect is also achieved by mixing the sublattices occupied by the Mn and sp atoms. Thus the half metallicity is a robust property of these alloys.

  5. Electronic structure and half-metallicity of new quaternary Heusler alloys NiFeTiZ (Z=Si, P, Ge, and As)

    NASA Astrophysics Data System (ADS)

    Karimian, N.; Ahmadian, F.

    2015-12-01

    The electronic structure and magnetic properties of NiFeTiZ (Z=Si, P, Ge, and As) quaternary Heusler compounds were investigated using the full-potential linearized augmented plane wave (FPLAPW) method in framework of the density functional theory (DFT). The results showed that NiFeTiZ (Z=Si, P, Ge, and As) compounds were stable in YI structure and the NiFeTiP and NiFeTiSi compounds were true half-metallic (HM) ferromagnets. The NiFeTiGe had a nearly HM characteristic, while NiFeTiAs was a conventional ferromagnet. The majority and half-metallic band gaps were respectively 0.44 and 0.3 eV for NiFeTiP and 0.18 and 0.08 eV for NiFeTiSi. The origin and mechanism of the formation of majority band gap in NiFeTiP were also verified. The total magnetic moments of NiFeTiP and NiFeTiSi compounds were respectively obtained 1μB and 2μB per formula unit, which were in agreement with Slater-Pauling rule (Mtot=28-Ztot). Half-metalliciy exists in relatively wide ranges of 5.43-5.80 Å and 5.60-5.87 Å for NiFeTiP and NiFeTiSi compounds, respectively, which makes them promising candidates in spintronics.

  6. Thermoelectric and mechanical properties of gapless Zr2MnAl compound

    NASA Astrophysics Data System (ADS)

    Yousuf, Saleem; Gupta, Dinesh C.

    2017-01-01

    We present the study of elastic and magnetic properties of Zr2MnAl full-Heusler alloys within the first-principles density functional theory. The lattice constant, magnetic moment, bulk modulus and density of states are calculated using the full potential linearized augmented plane wave method in the generalized gradient approximation scheme. The thermoelectric properties are studied between the temperature range of 50-800 K. Seebeck coefficient (S) measurements indicate the material as n-type with large S value of -83.06 μV/K at 400 K. The material shows higher efficiency for thermoelectric use with figure of merit equal to 0.92 at 400 K relatively higher in comparison for the other full Heusler compounds in these temperature ranges. The behaviour of gapless character is mainly responsible for the anomalous transport properties of the material required for the thermoelectric applications.

  7. The crystal structure of the LiAg 2In compound

    NASA Astrophysics Data System (ADS)

    Pavlyuk, V. V.; Dmytriv, G. S.; Chumak, I. V.; Ehrenberg, H.; Pauly, H.

    2005-11-01

    The newly established intermetallic compound LiAg 2In crystallizes in the MnCu 2Al-type structure ( Fm-3 m, Heusler phase) with a=6.5681(5) Å. The homogeneity range of this phase in the ternary Li-Ag-In phase diagram along the adjacent quasibinary cut Li 0.25(Ag 1-xIn x) 0.75 was determined by X-ray powder diffraction and extends from x˜0.33, Li 0.25Ag 0.50In 0.25, up to x˜0.44, Li 0.25Ag 0.42In 0.33. The homogeneity ranges of Heusler- and Zintl-type phases in the Li-Ag-In system are separated from each other by a broad heterogeneous region.

  8. Hyperfine magnetic field on Cd-111 in Heusler alloys Co2MnZ (Z = Si, Ga, Ge, Sn)

    NASA Technical Reports Server (NTRS)

    Jha, S.; Mitros, C.; Lahamer, Amer; Yehia, Sherif; Julian, Glenn M.

    1989-01-01

    The time differential perturbed angular correlation method has been used to measure, as a function of temperature, the hyperfine magnetic field at Cd sites in the Heusler alloys Co2MnZ (Z = Si, Ga, Ge, Sn). The hyperfine fields, normalized to the total magnetic moment per formula unit, show an approximately linear trend toward more positive values with increasing lattice parameter.

  9. Efficient and Robust Thermoelectric Power Generation Device Using Hot-Pressed Metal Contacts on Nanostructured Half-Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Joshi, Giri; Poudel, Bed

    2016-12-01

    We report an efficient thermoelectric device with power density of 8.9 W/cm2 and efficiency of 8.9% at 678°C temperature difference using hot-pressed titanium metal contact layers on nanostructured half-Heusler materials. The high power density and efficiency are due to the efficient nanostructured materials and very low contact resistance of 1 μΩ cm2 between the titanium layer and half-Heusler material. Moreover, the bonding strength between the titanium and half-Heusler is more than 50 MPa, significantly higher compared with conventional contact metallization methods. The low contact resistance and high bonding strength are due to thin-layer diffusion of titanium (<100 μm) into the half-Heusler at high temperature (>600°C). The low contact resistance and high bonding strength result in a stable and efficient power generation device with great potential for use in recovery of waste heat, e.g., in automotive and industrial applications.

  10. Evidence of half-metallic interface magnetism via local moment formation in Co based Heusler alloys

    SciTech Connect

    Telling, N. D.; Keatley, P.S.; van der Laan, G.; Hicken, R.J.; Arenholz, E.; Sakuraba, Y.; Oogane, M.; Ando, Y.; Miyazaki, T.

    2008-08-18

    In this work we use a combination of x-ray magnetic circular and linear dichroism (XMCD and XMLD) techniques to examine the formation of local moments in Heusler alloys of the composition Co{sub 2}MnX (where X=Si or Al). The existence of local moments in a half-metallic system is reliant upon the band gap in the minority-spin states. By utilizing the element-specific nature of x-ray techniques we are able to explore the origin of the minority-spin band gap in the partial density of states (PDOS), via the degree of localization of moments on Co and Mn atoms. We observe a crucial difference in the localization of the Co moment when comparing Co{sub 2}MnSi (CMS) and Co{sub 2}MnAl (CMA) films that is consistent with the predicted larger minority-spin gap in the Co PDOS for CMS. These results provide important evidence for the dominant role of the Co minority-spin states in realizing half-metallic ferromagnetism (HMF) in these Heusler alloys. They also demonstrate a direct method for measuring the degree of interfacial HMF in the raw materials without the need for fabricating spin-transport devices.

  11. Thermoelectric properties of ultra-low thermal conductivity half-Heusler alloy

    SciTech Connect

    Mallick, Md. Mofasser; Vitta, Satish

    2016-05-23

    The half-Heusler alloy HfNiGe has been synthesized by arc melting from high purity elements followed by annealing at 1000 K for 6 days to homogenize completely. The X-ray diffraction pattern indicates the presence of mainly an orthorhombic phase with small amount of other binary phases. The electrical resistivity is found to be low and increases slightly with temperature from 14 µΩ-m to 24 µΩ-m, indicating a semi metallic behavior. As a result the Seebeck coefficient is found to be low and also increases with temperature from −11 µV K{sup −1} to −19.5 µV K{sup −1}. The thermal conductivity has been determined using a combination of heat capacity and thermal diffusivity. It decreases from ~ 1.9 Wm{sup −1}K{sup −1} at room temperature to ~ 0.007 Wm{sup −1}K{sup −1} at 843 K, an extremely low value for a half-Heusler alloy. The thermal conductivity reduction is found to be mainly due to a sharp decrease in heat capacity for T> 650 K. This leads to a divergence of figure of merit at high temperatures, >800 K, from ~0.05 to 2 at 843 K.

  12. Phase transition of Ni43Mn41Co5Sn11 Heusler alloy

    NASA Astrophysics Data System (ADS)

    Elwindari, N.; Kurniawan, C.; Manaf, A.

    2017-07-01

    In the recent years, Heusler alloy has been extensively studied. Among various the Heusler alloys, Ni-Mn-Sn has gained considerable interest due to their multifunctional properties like magnetoresistance, shape memory effect and magnetocaloric effect associated with a first order phase transition martensite to austenite. In this paper, we report the magneto-structural phase transitions under varying temperature of Ni43Mn41Co5Sn11 synthesized through vacuum arc-melting process. The magnetization of the sample was obtained after annealing process at 1173 K for 12 hours. It was evaluated by magnetic measurement using vibrating sample magnetometer (VSM250) up to a field of 21 kOe. The magnetic isotherm (M-H curves) shows the vicinity of the structural phase transition point. Magnetic saturation (Ms) of the NMCS alloy decreased and ferromagnetic transition shift towards higher temperature from 297-372 K. It might occur due to the alignment of the atomic magnetic moments depends on temperature. We discussed also the context of structural disorder and the ferromagnetic correlations in this study. The structural disorder of these alloys will explain the magnetic transition and the entropy change related magnetic properties.

  13. Enhanced thermoelectric figure of merit of p-type half-Heuslers.

    PubMed

    Yan, Xiao; Joshi, Giri; Liu, Weishu; Lan, Yucheng; Wang, Hui; Lee, Sangyeop; Simonson, J W; Poon, S J; Tritt, T M; Chen, Gang; Ren, Z F

    2011-02-09

    Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.

  14. High thermoelectric figure of merit by resonant dopant in half-Heusler alloys

    NASA Astrophysics Data System (ADS)

    Chen, Long; Liu, Yamei; He, Jian; Tritt, Terry M.; Poon, S. Joseph

    2017-06-01

    Half-Heusler alloys have been one of the benchmark high temperature thermoelectric materials owing to their thermal stability and promising figure of merit ZT. Simonson et al. early showed that small amounts of vanadium doped in Hf0.75Zr0.25NiSn enhanced the Seebeck coefficient and correlated the change with the increased density of states near the Fermi level. We herein report a systematic study on the role of vanadium (V), niobium (Nb), and tantalum (Ta) as prospective resonant dopants in enhancing the ZT of n-type half-Heusler alloys based on Hf0.6Zr0.4NiSn0.995Sb0.005. The V doping was found to increase the Seebeck coefficient in the temperature range 300-1000 K, consistent with a resonant doping scheme. In contrast, Nb and Ta act as normal n-type dopants, as evident by the systematic decrease in electrical resistivity and Seebeck coefficient. The combination of enhanced Seebeck coefficient due to the presence of V resonant states and the reduced thermal conductivity has led to a state-of-the-art ZT of 1.3 near 850 K in n-type (Hf0.6Zr0.4)0.99V0.01NiSn0.995Sb0.005 alloys.

  15. Large Closed-Circuit Seebeck Current in Quaternary (Ti,Zr)NiSn Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Wunderlich, Wilfried; Motoyama, Yuichiro; Sugisawa, Yuta; Matsumura, Yoshihiro

    2011-05-01

    In the (Ti x ,Zr y )Ni w Sn z quaternary system with a composition near ( x + y): w: z = 1:1:1 the existence of the half-Heusler (HH) phase has been confirmed, where Ti and Zr occupy one of the three lattice positions substitutionally. The goal of this study is to characterize the thermoelectric (TE) properties of such materials. TE properties were measured at large temperature differences up to Δ T = 800 K, exhibiting Seebeck voltages of about ±50 mV corresponding to Seebeck coefficients above 0.07 mV/K, with the highest value measured for the (Ti0.4Zr0.6)Ni0.9Sn1.1 composition. Fe and Mn doping could not improve these values further. Measurements under closed-circuit conditions showed very high currents of 0.4 mA for specimens at this particular composition. According to the composition, interfaces between full-Heusler and HH phases are responsible for an electron pull-out phenomenon due to the electric field at their interfaces. First-principle calculations of the electronic band structure confirm this explanation for why (TiZr)NiSn and CrNiSn are p-type TEs whereas NbNiSn is an n-type TE. These considerations will be useful in the search for other such systems.

  16. Structural and magnetic properties of Co 2CrAl Heusler alloys prepared by mechanical alloying

    NASA Astrophysics Data System (ADS)

    Hakimi, M.; Kameli, P.; Salamati, H.

    2010-11-01

    Mechanical alloying has been used to produce nanocrystalline samples of Co 2CrAl Heusler alloys. The samples were characterized by using different methods. The results indicate that, it is possible to produce L2 1-Co 2CrAl powders after 15 h of ball-milling. The grain size of 15 h ball milled L2 1-Co 2CrAl Heusler phase, calculated by analyzing the XRD peak broadening using Williamson and Hall approach was 14 nm. The estimated magnetic moment per formula unit is ˜2 μ B. The obtained magnetic moment is significantly smaller than the theoretical value of 2.96 μ B for L2 1 structure. It seems that an atomic disorder from the crystalline L2 1-type ordered state and two-phase separation depresses the ferromagnetic ordering in alloy. Also, the effect of annealing on the structural and magnetic properties of ball milled powders was investigated. Two structures were identified for annealed sample, namely L2 1 and B2. The obtained value for magnetic moment of annealed sample is smaller than the as-milled sample due to the presence of disordered B2 phase and improvement of phase separation.

  17. Large Enhancements in Thermopower and Electrical Conductivity in Nano-structured Half-Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Page, Alexander; van der Ven, Anton; Poudeu, Pierre; Uher, Ctirad

    2014-03-01

    Recent improvements have often been made to thermoelectric materials by adding nano-structures in order to scatter heat carrying phonons, however, the reduction in thermal conductivity is accompanied by large drops in the electrical conductivity caused by mobility reductions. In this work we show that Half-Heusler (HH) alloys can be combined with nano-scale Full-Heusler (FH) inclusions to simultaneously improve the power factor and reduce thermal conductivity. HH structures are of the form MNiSn and MCoSb (M = Ti, Zr, or Hf) and the FH counterparts are created by filling the vacancies on the Ni or Co planes respectively, giving MNi2Sn and MCo2Sb. Experimental results show FH nano-inclusions were coherently integrated into the matrix HH material resulting in enhanced ZT which is attributed to energy filtering effects that occur at the HH-FH grain boundaries as well as moderate reductions in thermal conductivity by nano-inclusion phonon scattering. Ab Initio calculations, in combination with a cluster expansion, are used to test the stability of FH structures in HH matrix and create thermodynamic pseudo-binary phase diagram for MNiSn-MNi2Sn compositions, elucidating the possibilities for future approaches to enhance ZT.

  18. Model Hamiltonian and time reversal breaking topological phases of antiferromagnetic half-Heusler materials

    NASA Astrophysics Data System (ADS)

    Yu, Jiabin; Yan, Binghai; Liu, Chao-Xing

    2017-06-01

    In this work, we construct a generalized Kane model with a coupling term between itinerant electron spins and local magnetic moments of antiferromagnetic ordering in order to describe the low-energy effective physics in a large family of antiferromagnetic half-Heusler materials. The topological properties of this generalized Kane model are studied and a large variety of topological phases, including the Dirac semimetal phase, Weyl semimetal phase, nodal line semimetal phase, type-B triple point semimetal phase, topological mirror (or glide) insulating phase, and antiferromagnetic topological insulating phase, are identified in different parameter regions of our effective models. In particular, we find that the system is always driven into the antiferromagnetic topological insulator phase once a bulk band gap is open, irrespective of the magnetic moment direction, thus providing a robust realization of antiferromagentic topological insulators. Furthermore, we discuss the possible realization of these topological phases in realistic antiferromagnetic half-Heusler materials. Our effective model provides a basis for the future study of physical phenomena in this class of materials.

  19. Peculiarities of thermoelectric half-Heusler phase formation in Gd-Ni-Sb and Lu-Ni-Sb ternary systems

    SciTech Connect

    Romaka, V.V.; Romaka, L.; Horyn, A.; Rogl, P.; Stadnyk, Yu; Melnychenko, N.; Orlovskyy, M.; Krayovskyy, V.

    2016-07-15

    The phase equilibria in the Gd–Ni–Sb and Lu-Ni-Sb ternary systems were studied at 873 K by X-ray and metallographic analyses in the whole concentration range. The interaction of the elements in the Gd–Ni–Sb system results the formation of five ternary compounds at investigated temperature: Gd{sub 5}Ni{sub 2}Sb (Mo{sub 5}SiB{sub 2}-type), Gd{sub 5}NiSb{sub 2} (Yb{sub 5}Sb{sub 3}-type), GdNiSb (MgAgAs-type), Gd{sub 3}Ni{sub 6}Sb{sub 5} (Y{sub 3}Ni{sub 6}Sb{sub 5}-type), and GdNi{sub 0.72}Sb{sub 2} (HfCuSi{sub 2}-type). At investigated temperature the Lu-Ni-Sb system is characterized by formation of the LuNiSb (MgAgAs-type), Lu{sub 5}Ni{sub 2}Sb (Mo{sub 5}SiB{sub 2}-type), and Lu{sub 5}Ni{sub 0.56}Sb{sub 2.44} (Yb{sub 5}Sb{sub 3}-type) compounds. The disordering in the crystal structure of half-Heusler GdNiSb and LuNiSb was revealed by EPMA and studied by means of Rietveld refinement and DFT modeling. The performed electronic structure calculations are in good agreement with electrical transport property studies. - Graphical abstract: Crystal structure model and electron localization function of Lu{sub 5}Ni{sub 2}Sb. Display Omitted - Highlights: • Gd-Ni-Sb and Lu-Ni-Sb phase diagrams were constructed at 873 K. • GdNiSb and LuNiSb are characterized by disordered crystal structure. • Crystal structure optimization with DFT calculations confirmed crystal structure disorder in GdNiSb and LuNiSb.

  20. Itinerant G -type antiferromagnetism in D 03 -type V3Z (Z =Al , Ga, In) compounds: A first-principles study

    NASA Astrophysics Data System (ADS)

    Galanakis, Iosif; Tırpancı, Şaban; Özdoǧan, Kemal; Şaşıoǧlu, Ersoy

    2016-08-01

    Heusler compounds are widely studied due to their variety of magnetic properties making them ideal candidates for spintronic and magnetoelectronic applications. V3Al in its metastable D0 3-type Heusler structure is a prototype for a rare antiferromagnetic gapless behavior. We provide an extensive study on the electronic and magnetic properties of V3Al , V3Ga , and V3In compounds based on electronic structure calculations. We show that the ground state for all three is a G -type itinerant antiferromagnetic gapless semiconductor. The large antiferromagnetic exchange interactions lead to very high Néel temperatures, which are predicted to be around 1000 K. The coexistence of the gapless and antiferromagnetic behaviors in these compounds can be explained considering the simultaneous presence of three V atoms at the unit cell using arguments which have been employed for usual inverse Heusler compounds. We expect our study on these compounds to enhance further the interest on them towards the optimization of their growth conditions and their eventual incorporation in devices.

  1. Electronic structure, magnetism, and antisite disorder in CoFeCrGe and CoMnCrAl quaternary Heusler alloys

    SciTech Connect

    Enamullah, .; Venkateswara, Y.; Gupta, Sachin; Varma, Manoj Raama; Singh, Prashant; Suresh, K. G.; Alam, Aftab

    2015-12-10

    In this study, we present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be 3 μB, 866 K and 0.9 μB, 358 K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y-type structure while CMCA has L21 disordered structure. The antisite disorder adds a somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. Ab initio simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess concentration of Al in the alloy.

  2. Electronic, magnetic and thermal properties of Co2CrxFe1-xX (X=Al, Si) Heusler alloys: First-principles calculations

    NASA Astrophysics Data System (ADS)

    Guezlane, M.; Baaziz, H.; El Haj Hassan, F.; Charifi, Z.; Djaballah, Y.

    2016-09-01

    Density functional theory (DFT) based on the full-potential linearized augmented plane wave (FP-LAPW) method is used to investigate the structural, electronic, magnetic and thermal properties of Co2CrxFe1-xX (X=Al, Si) full Heusler alloys, with L21 structure. The structural properties and spin magnetic moments are investigated by the generalized gradient approximations (GGA) minimizing the total energy. For band structure calculations, GGA, the Engel-Vosko generalized gradient approximation (EVGGA) and modified Becke-Johnson (mBJ) schemes are used. Results of density of states (DOS) and band structures show that these alloys are half-metallic ferromagnets (HMFS). A regular-solution model has been used to investigate the thermodynamic stability of the compounds Co2CrxFe1-xX that indicates a phase miscibility gap. The thermal effects using the quasi-harmonic Debye model are investigated within the lattice vibrations. The temperature and pressure effects on the heat capacities, Debye temperatures and entropy are determined from the non-equilibrium Gibbs functions.

  3. Electronic structure, magnetism, and antisite disorder in CoFeCrGe and CoMnCrAl quaternary Heusler alloys

    DOE PAGES

    Enamullah, .; Venkateswara, Y.; Gupta, Sachin; ...

    2015-12-10

    In this study, we present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be 3 μB, 866 K and 0.9 μB, 358 K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y-type structure while CMCA has L21 disordered structure. The antisite disorder adds amore » somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. Ab initio simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess concentration of Al in the alloy.« less

  4. Theoretical investigations of half-metallic ferromagnetism in new Half-Heusler YCrSb and YMnSb alloys using first-principle calculations

    NASA Astrophysics Data System (ADS)

    Atif Sattar, M.; Rashid, Muhammad; Hashmi, M. Raza; Ahmad, S. A.; Imran, Muhammad; Hussain, Fayyaz

    2016-10-01

    Structural, electronic, and magnetic properties of new predicted half-Heusler YCrSb and YMnSb compounds within the ordered MgAgAs C1b-type structure are investigated by employing first-principal calculations based on density functional theory. Through the calculated total energies of three possible atomic placements, we find the most stable structures regarding YCrSb and YMnSb materials, where Y, Cr(Mn), and Sb atoms occupy the (0.5, 0.5, 0.5), (0.25, 0.25, 0.25), and (0, 0, 0) positions, respectively. Furthermore, structural properties are explored for the non-magnetic and ferromagnetic and anti-ferromagnetic states and it is found that both materials prefer ferromagnetic states. The electronic band structure shows that YCrSb has a direct band gap of 0.78 eV while YMnSb has an indirect band gap of 0.40 eV in the majority spin channel. Our findings show that YCrSb and YMnSb materials exhibit half-metallic characteristics at their optimized lattice constants of 6.67 Å and 6.56 Å, respectively. The half-metallicities associated with YCrSb and YMnSb are found to be robust under large in-plane strains which make them potential contenders for spintronic applications.

  5. Native defects in the Co2Ti Z (Z =Si ,Ge,Sn) full Heusler alloys: Formation and influence on the thermoelectric properties

    NASA Astrophysics Data System (ADS)

    Popescu, Voicu; Kratzer, Peter; Wimmer, Sebastian; Ebert, Hubert

    2017-08-01

    We have performed first-principles investigations on the native defects in the half-metallic, ferromagnetic full Heusler alloys Co2Ti Z (Z one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that the Co vacancies (VcCo) and the TiSn as well as the CoZ or CoTi antisites exhibit the smallest formation energies. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination with the multiple-scattering theory Green function approach. The self-consistent potentials determined this way were used to calculate the residual resistivity via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck coefficient of the systems. The latter is shown to depend significantly on the type of defect, leading to variations that are related to subtle, spin-orbit coupling induced changes in the electronic structure above the half-metallic gap. Two of the systems, VcCo and CoZ, are found to exhibit a negative Seebeck coefficient. This observation, together with their low formation energy, offers an explanation for the experimentally observed negative Seebeck coefficient of the Co2Ti Z compounds as being due to unintentionally created native defects.

  6. Electronic structure, magnetism, and antisite disorder in CoFeCrGe and CoMnCrAl quaternary Heusler alloys

    NASA Astrophysics Data System (ADS)

    Enamullah; Venkateswara, Y.; Gupta, Sachin; Varma, Manoj Raama; Singh, Prashant; Suresh, K. G.; Alam, Aftab

    2015-12-01

    We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be 3 μB , 866 K and 0.9 μB , 358 K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y -type structure while CMCA has L 21 disordered structure. The antisite disorder adds a somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. Ab initio simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess concentration of Al in the alloy.

  7. Thermal and electronic charge transport in bulk nanostructured Zr{sub 0.25}Hf{sub 0.75}NiSn composites with full-Heusler inclusions

    SciTech Connect

    Makongo, Julien P.A.; Misra, Dinesh K.; Salvador, James R.; Takas, Nathan J.; Wang, Guoyu; Shabetai, Michael R.; Pant, Aditya; Paudel, Pravin; Uher, Ctirad; Stokes, Kevin L.; Poudeu, Pierre F.P.

    2011-11-15

    Bulk Zr{sub 0.25}Hf{sub 075}NiSn half-Heusler (HH) nanocomposites containing various mole fractions of full-Heusler (FH) inclusions were prepared by solid state reaction of pre-synthesized HH alloy with elemental Ni at 1073 K. The microstructures of spark plasma sintered specimens of the HH/FH nanocomposites were investigated using transmission electron microscopy and their thermoelectric properties were measured from 300 K to 775 K. The formation of coherent FH inclusions into the HH matrix arises from solid-state Ni diffusion into vacant sites of the HH structure. HH(1-y)/FH(y) composites with mole fraction of FH inclusions below the percolation threshold, y{approx}0.2, show increased electrical conductivity, reduced Seebeck coefficient and increased total thermal conductivity arising from gradual increase in the carrier concentration for composites. A drastic reduction ({approx}55%) in {kappa}{sub l} was observed for the composite with y=0.6 and is attributed to enhanced phonon scattering due to mass fluctuations between FH and HH, and high density of HH/FH interfaces. - Graphical abstract: Large reduction in the lattice thermal conductivity of bulk nanostructured half-Heusler/full-Heusler (Zr{sub 0.25}Hf{sub 075}NiSn/ Zr{sub 0.25}Hf{sub 075}Ni{sub 2}Sn) composites, obtained by solid-state diffusion at 1073 K of elemental Ni into vacant sites of the half-Heusler structure, arising from the formation of regions of spinodally decomposed HH and FH phases with a spatial composition modulation of {approx}2 nm. Highlights: > Bulk composites from solid state transformation of half-Heusler matrix through Ni diffusion. > Formation of coherent phase boundaries between half-Heusler matrix and full-Heusler inclusion. > Alteration of thermal and electronic transports with increasing full-Heusler inclusion. > Enhanced phonon scattering at half-Heusler/ full-Heusler phase boundaries.

  8. Covalence and ionicity in MgAgAs-type compounds.

    PubMed

    Bende, David; Grin, Yuri; Wagner, Frank R

    2014-07-28

    MgAgAs-type "half-Heusler" compounds are known to realize two out of three possible atomic arrangements of this structure type. The number of transition metal components typically determines which of the alternatives is favored. On the basis of DFT calculations for all three variants of 20 eight- and eighteen-valence-electron compounds, the experimentally observed structural variant was found to be determined by basically two different bonding patterns. They are quantified by employing two complementary position-space bonding measures. The Madelung energy E((M)(QTAIM)) calculated with the QTAIM effective charges reflects contributions of the ionic interactions to the total energy. The sum of nearest-neighbor delocalization indices ςnn characterizes the covalent interactions through electron sharing. With the aid of these quantities, the energetic sequence of the three atomic arrangements for each compound is rationalized. The resulting systematic is used to predict a scenario in which an untypical atomic arrangement becomes most favorable.

  9. Lattice Dynamics and Phonon Softening in NiMnAl Heusler Alloys

    SciTech Connect

    Moya, Xavier; Manosa, L.; Planes, A.; Krenke, T.; Acet, Mehmet; Garlea, Vasile O; Lograsso, Tom; Schlagel, D. L.; Zarestky, Jarel

    2006-01-01

    Inelastic and elastic neutron scattering have been used to study a single crystal of the Ni{sub 54}Mn{sub 23}Al{sub 23} Heusler alloy over a broad temperature range. The paper reports the experimental determination of the low-lying phonon dispersion curves for this alloy system. We find that the frequencies of the TA2 modes are relatively low. This branch exhibits an anomaly (dip) at a wave number {xi}{sub 0}=1/3{approx}0.33, which softens with decreasing temperature. Associated with this anomalous dip at {xi}{sub 0}, an elastic central peak scattering is also present. We have also observed satellites due to the magnetic ordering.

  10. Electron spin resonance probed competing states in NiMnInSi Heusler alloy

    NASA Astrophysics Data System (ADS)

    Chen, Y. S.; Lin, J. G.; Titov, I. S.; Granovsky, A. B.

    2016-06-01

    Shape memory Heusler alloy Ni50Mn35In12Si3 is investigated with electron spin resonance (ESR) technique in a temperature range of 200-300 K. ESR is a dynamic probe allowing us to separate the responses from various magnetic phases, thus to study the complex phase transitions. The sample shows three transition temperatures: TcA (271 K), TM (247 K) and TcM (212 K), where TcA is the Curie temperature of austenitic phase, TM and TcM are the temperatures of magnetostructural martensitic transition and the Curie temperature of martensitic phase, respectively. Furthermore, ESR data reveals the coexistence of two magnetic modes in whole temperature range of 200-300 K. Particularly in martensitic phase, two magnetic modes are attributed to two different kinds of lattice deformation, the slip and twinning deformations.

  11. The structural and magnetic properties of Fe2-xNiGa1+x Heusler alloys

    NASA Astrophysics Data System (ADS)

    Zhang (张玉洁), Y. J.; Xi (郗学奎), X. K.; Meng (孟凡斌), F. B.; Wang (王文洪), W. H.; Liu (刘恩克), E. K.; Chen (陈京兰), J. L.; Wu (吴光恒), G. H.

    2015-04-01

    The structural and magnetic properties of Fe2-xNiGa1+x (x=0~1) Heusler alloys have been investigated by experimental observation and calculation. In this system, a structural transition is found as a function of composition. A higher Ga content leads to an atomic-order transformation from Hg2CuTi to B2. The magnetization decreases due to the dilution effect and the competition between the magnetic interactions and enhanced covalent bonding. The calculation of electronic structure indicates that adding Ga enhances the p-d orbital hybridization between the transition-metal and main-group-element atoms at nearest-neighbor distance. A magnetic and a structural phase diagram have been obtained in which the composition dependences of the lattice constant, the ordering temperature and the Curie temperature show cusps at a critical composition of x=0.32.

  12. A unified approach to describe the thermal and magnetic hysteresis in Heusler alloys

    NASA Astrophysics Data System (ADS)

    Blázquez, J. S.; Franco, V.; Conde, A.; Gottschall, T.; Skokov, K. P.; Gutfleisch, O.

    2016-09-01

    Different excitations, like temperature, magnetic field, or pressure, can drive a martensitic transition in Heusler alloys. Coupled phenomena in these materials lead to interesting magnetocaloric and barocaloric effects ascribed to this transition. In this work, we demonstrate that isothermal transformations induced by a magnetic field and isofield transformations induced by the temperature can be described using the same framework. By defining an effective temperature that relates field and temperature through the properties of the system (magnetic moment and entropy of the transition), both kinds of loops can be transformed into the other kind, therefore providing a more effective way of characterizing hysteretic samples. The validity of this effective temperature approach to describe the transition holds for martensite to austenite transformations as well as reversal ones, and thus, the hysteresis phenomena can be described using this single general excitation.

  13. Fabrication and characterization of the gapless half-Heusler YPtSb thin films

    SciTech Connect

    Wang Wenhong; Du Yin; Liu Enke; Wu, G. H.; Liu Zhongyuan

    2012-11-15

    Half-Heusler YPtSb thin films were fabricated by magnetron co-sputtering method on MgO-buffered SiO{sub 2}/Si(001) substrates. X-ray diffraction pattern and energy dispersive X-ray spectroscopy confirmed the high-quality growth and stoichiometry. The temperature dependence of the resistivity shows a semiconducting-type behavior down to low temperature. The Hall mobility was determined to be 450 cm{sup 2}/V s at 300 K, which is much higher than the bulk value ({approx}300 cm{sup 2}/V s). In-plane magnetoresistance (MR) measurements with fields applied along and perpendicular to the current direction show opposite MR signs, which suggest the possible existence of the topological surface states.

  14. The Potential of FeVSb Half-Heusler Phase for Practical Thermoelectric Material

    NASA Astrophysics Data System (ADS)

    Yamamoto, Akio; Takeuchi, Tsunehiro

    2016-07-01

    The electronic structure and electron transport properties of FeVSb half-Heusler phase were calculated using first-principles calculations and Boltzmann transport equation. We found, as a result of theoretical calculations, that the magnitude of the dimensionless of figure of merit (ZT) could be increased up to ˜0.84 due to the drastic increase of the power factor when the carrier concentration of p-type FeVSb is optimized through element substitution to vary the valence electron concentration. We revealed that this large increase in ZT is closely related to the significant variations both in electronic density of states and spectral conductivity with decreasing energy at the top of the valence band.

  15. Magnetic properties and martensitic transformation in quaternary Heusler alloy of NiMnFeGa

    NASA Astrophysics Data System (ADS)

    Liu, Z. H.; Zhang, M.; Wang, W. Q.; Wang, W. H.; Chen, J. L.; Wu, G. H.; Meng, F. B.; Liu, H. Y.; Liu, B. D.; Qu, J. P.

    2002-11-01

    Quaternary Heusler alloy Ni2)(Mn,FeGa has been studied systematically for the structure, martensitic transformation, and magnetic properties in two systems of Ni50.5Mn25-xFexGa24.5 and Ni50.4Mn28-xFexGa21.6. Substituting Fe for Mn up to about 70%, the pure L21 phase and the thermoelastic martensitic transformation still can be observed in these quaternary systems. Iron doping dropped the martensitic transformation temperature from 220 to 140 K, increased the Curie temperature from 351 to 429 K, and broadened the thermal hysteresis from about 7 to 18 K. Magnetic analysis revealed that Fe atoms contribute to the net magnetization of the material with a moment lower than that of Mn. The temperature dependence of magnetic-field-induced strains has been improved by this doping method.

  16. Simulation of stress-modulated magnetization precession frequency in Heusler-based spin torque oscillator

    NASA Astrophysics Data System (ADS)

    Huang, Houbing; Zhao, Congpeng; Ma, Xingqiao

    2017-03-01

    We investigated stress-modulated magnetization precession frequency in Heusler-based spin transfer torque oscillator by combining micromagnetic simulations with phase field microelasticity theory, by encapsulating the magnetic tunnel junction into multilayers structures. We proposed a novel method of using an external stress to control the magnetization precession in spin torque oscillator instead of an external magnetic field. The stress-modulated magnetization precession frequency can be linearly modulated by externally applied uniaxial in-plane stress, with a tunable range 4.4-7.0 GHz under the stress of 10 MPa. By comparison, the out-of-plane stress imposes negligible influence on the precession frequency due to the large out-of-plane demagnetization field. The results offer new inspiration to the design of spin torque oscillator devices that simultaneously process high frequency, narrow output band, and tunable over a wide range of frequencies via external stress.

  17. Designing magnetic compensated states in tetragonal Mn3Ge-based Heusler alloys

    NASA Astrophysics Data System (ADS)

    You, Yurong; Xu, Guizhou; Hu, Fang; Gong, Yuanyuan; Liu, Er; Peng, Guo; Xu, Feng

    2017-05-01

    Magnetic compensated materials attracted much interests due to the observed large exchange bias and large coercivity, and also their potential applications in the antiferromagnetic spintronics with merit of no stray field. In this work, by using ab-initio studies, we designed several Ni (Pd, Pt) doped Mn3Ge-based D022-type tetragonal Heusler alloys with fully compensated states. Theoretically, we find the total moment change is asymmetric across the compensation point (at x=0.3) in Mn3-xYxGe (Y=Ni, Pd, Pt). In addition, an uncommon discontinuous jump is observed across the critical zero-moment point, indicating that some non-trivial properties may emerge at this point. Further electronic analyses of these compensated alloys reveal high spin polarizations at the Fermi level, which is advantageous for spin transfer torque applications.

  18. Structural, magnetic and transport properties of Co2FeAl Heusler films with varying thickness

    NASA Astrophysics Data System (ADS)

    Wang, Xiaotian; Li, Yueqing; Du, Yin; Dai, Xuefang; Liu, Guodong; Liu, Enke; Liu, Zhongyuan; Wang, Wenhong; Wu, Guangheng

    2014-08-01

    We report on a systematic study of the structural, magnetic properties and the anomalous Hall effect, in the Heusler alloy Co2FeAl (CFA) epitaxial films on MgO (001), as a function of film thickness. It was found that the epitaxial CFA films show a highly ordered B2 structure with an in-plane uniaxial magnetic anisotropy. The electrical transport properties reveal that the lattice and magnon scattering contributions to the longitudinal resistivity. Independent on the thickness of films, the anomalous Hall resistivity of CFA films is found to be dominated by skew scattering only. Moreover, the anomalous Hall resistivity shows weakly temperature dependent behavior, and its absolute value increases as the thickness decreases. We attribute this temperature insensitivity in the anomalous Hall resistivity to the weak temperature dependent of tunneling spin-polarization in the CFA films, while the thickness dependence behavior is likely due to the increasing significance of interface or free surface electronic states.

  19. High tunneling magnetoresistance ratio in perpendicular magnetic tunnel junctions using Fe-based Heusler alloys

    NASA Astrophysics Data System (ADS)

    Wang, Yu-Pu; Lim, Sze-Ter; Han, Gu-Chang; Teo, Kie-Leong

    2015-12-01

    Heulser alloys Fe2Cr1-xCoxSi (FCCS) with different Co compositions x have been predicted to have high spin polarization. High perpendicular magnetic anisotropy (PMA) has been observed in ultra-thin FCCS films with magnetic anisotropy energy density up to 2.3 × 106 erg/cm3. The perpendicular magnetic tunnel junctions (p-MTJs) using FCCS films with different Co compositions x as the bottom electrode have been fabricated and the post-annealing effects have been investigated in details. An attractive tunneling magnetoresistance ratio as high as 51.3% is achieved for p-MTJs using Fe2CrSi (FCS) as the bottom electrode. The thermal stability Δ can be as high as 70 for 40 nm dimension devices using FCS, which is high enough to endure a retention time of over 10 years. Therefore, Heusler alloy FCS is a promising PMA candidate for p-MTJ application.

  20. Galvanomagnetic properties of Heusler alloy Co2 YAl ( Y = Ti, V, Cr, Mn, Fe, and Ni)

    NASA Astrophysics Data System (ADS)

    Kourov, N. I.; Marchenkov, V. V.; Perevozchikova, Yu. A.; Weber, H. W.

    2017-01-01

    The Hall effect and the magnetoresistance of ferromagnetic Heusler alloys Co2 YAl, where Y = Ti, V, Cr, Mn, Fe, and Ni have been studied at T = 4.2 K in magnetic fields H ≤ 100 kOe. Normal R 0 and anomalous R S Hall coefficients are shown to be maximal in magnitudes in the middle of the 3 d period of the periodic table of elements. Coefficient R 0 changes the negative sign to positive sign in going from weak ( Y = Ti, V) to strong ( Y = Cr, Mn, Fe, and Ni) ferromagnetic alloys. Constant R S is positive and proportional to ρ2.9 in all the alloys. The magnetoresistance of the alloys is not higher than several percent and its magnitude is changed fairly significantly in the dependence on the number of valence electrons z; the magnetoresistance signs vary arbitrarily.

  1. Ab initio studies of Co2FeAl1-xSix Heusler alloys

    NASA Astrophysics Data System (ADS)

    Szwacki, N. Gonzalez; Majewski, Jacek A.

    2016-07-01

    We present results of extensive theoretical studies of Co2FeAl1-xSix Heusler alloys, which have been performed in the framework of density functional theory employing the all-electron full-potential linearized augmented plane-wave scheme. It is shown that the Si-rich alloys are more resistive to structural disorder and as a consequence Si stabilizes the L21 structure. Si alloying changes position of the Fermi level, pushing it into the gap of the minority spin-band. It is also shown that the hyperfine field on Co nuclei increases with the Si concentration, and this increase originates mostly from the changes in the electronic density of the valence electrons.

  2. Magnetic properties and atomic ordering of BCC Heusler alloy Fe2MnGa ribbons

    NASA Astrophysics Data System (ADS)

    Xin, Yuepeng; Ma, Yuexing; Luo, Hongzhi; Meng, Fanbin; Liu, Heyan

    2016-05-01

    The electronic structure, atomic disorder and magnetic properties of the Heusler alloy Fe2MnGa have been investigated experimentally and theoretically. BCC Fe2MnGa ribbon samples were prepared. Experimentally, a saturation magnetic moment (3.68 μB at 5 K) much larger than the theoretical value (2.04 μB) has been reported. First-principles calculations indicate that the difference is related to the Fe-Mn disorder between A, B sites, as can also be deduced from the XRD pattern. L21 type Fe2MnGa is a ferrimagnet with antiparallel Fe and Mn spin moments. However, when Fe-Mn disorder occurs, part of Mn moments will be parallel to Fe moments, and the Fe moments also clearly increase simultaneously. All this results in a total moment of 3.74 μB, close to the experimental value.

  3. Simultaneous large enhancements in thermopower and electrical conductivity of bulk nanostructured half-Heusler alloys.

    PubMed

    Makongo, Julien P A; Misra, Dinesh K; Zhou, Xiaoyuan; Pant, Aditya; Shabetai, Michael R; Su, Xianli; Uher, Ctirad; Stokes, Kevin L; Poudeu, Pierre F P

    2011-11-23

    Large reductions in the thermal conductivity of thermoelectrics using nanostructures have been widely demonstrated. Some enhancements in the thermopower through nanostructuring have also been reported. However, these improvements are generally offset by large drops in the electrical conductivity due to a drastic reduction in the mobility. Here, we show that large enhancements in the thermopower and electrical conductivity of half-Heusler (HH) phases can be achieved simultaneously at high temperatures through coherent insertion of nanometer scale full-Heusler (FH) inclusions within the matrix. The enhancements in the thermopower of the HH/FH nanocomposites arise from drastic reductions in the "effective" carrier concentration around 300 K. Surprisingly, the mobility increases drastically, which compensates for the decrease in the carrier concentration and minimizes the drop in the electrical conductivity. Interestingly, the carrier concentration in HH/FH nanocomposites increases rapidly with temperature, matching that of the HH matrix at high temperatures, whereas the temperature dependence of the mobility significantly deviates from the typical T(-α) law and slowly decreases (linearly) with rising temperature. This remarkable interplay between the temperature dependence of the carrier concentration and mobility in the nanocomposites results in large increases in the power factor at 775 K. In addition, the embedded FH nanostructures also induce moderate reductions in the thermal conductivity leading to drastic increases in the ZT of HH(1 - x)/FH(x) nanocomposites at 775 K. By combining transmission electron microscopy and charge transport data, we propose a possible charge carrier scattering mechanism at the HH/FH interfaces leading to the observed anomalous electronic transport in the synthesized HH(1 - x)/FH(x) nanocomposites.

  4. Spin-dependent electron momentum density in the Ni2MnSn Heusler alloy

    NASA Astrophysics Data System (ADS)

    Deb, Aniruddha; Hiraoka, N.; Itou, M.; Sakurai, Y.; Onodera, M.; Sakai, N.

    2001-05-01

    The spin-dependent electron momentum distribution in Ni2MnSn Heusler alloy single crystals was studied using 270 keV circularly polarized synchrotron radiation, through magnetic Compton profile measurements, on the high energy inelastic scattering beamline at SPring-8. The experiments were carried out for the three principal crystallographic directions [100], [110], and [111] at 10 K. The results show that the conduction electrons have a negative spin polarization of 0.34μB the 3d spin moment on the nickel site was found to be negligible. A band structure calculation was performed including a hyperfine field study using the full potential linearized augmented plane wave (FLAPW) method, with the generalized gradient approximation (GGA) for the electronic exchange and correlation. The spin moment on the Mn site at 10 K was observed as 4.39μB. The spin-dependent Compton profiles for the [100], [110], and [111] directions reported here show anisotropy in the momentum density, which is in good agreement with the FLAPW-GGA results, based on a ferromagnetic ground state. The hyperfine fields calculated were compared with previously calculated hyperfine field of Cu2MnAl and Co2FeGa Heusler alloys. From the comparison it is seen that the value of Hval (valence contribution to the hyperfine field) is roughly proportional to the spin polarization (ms) of the s electrons at the X (Ni,Cu,Co) and Y (Mn of Ni2MnSn and Cu2MnAl, Fe of Co2FeGa) atom positions.

  5. Spin gapless semiconducting behavior in equiatomic quaternary CoFeMnSi Heusler alloy

    NASA Astrophysics Data System (ADS)

    Bainsla, Lakhan; Mallick, A. I.; Raja, M. Manivel; Nigam, A. K.; Varaprasad, B. S. D. Ch. S.; Takahashi, Y. K.; Alam, Aftab; Suresh, K. G.; Hono, K.

    2015-03-01

    In this paper, we report the signature of spin gapless semiconductor (SGS) in CoFeMnSi that belongs to the Heusler family. SGS is a new class of magnetic semiconductors which have a band gap for one spin subband and zero band gap for the other, and thus are useful for tunable spin transport based applications. We show various experimental evidences for SGS behavior in CoFeMnSi by carefully carrying out the transport and spin-polarization measurements. SGS behavior is also confirmed by first-principles band-structure calculations. The most stable configuration obtained by the theoretical calculation is verified by experiment. The alloy is found to crystallize in the cubic Heusler structure (LiMgPdSn type) with some amount of disorder and has a saturation magnetization of 3.7 μB/f .u . and Curie temperature of ˜620 K. The saturation magnetization is found to follow the Slater-Pauling behavior, one of the prerequisites for SGS. Nearly-temperature-independent carrier concentration and electrical conductivity are observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data have yielded the current spin-polarization value of 0.64, which is found to be robust against the structural disorder. All these properties strongly suggest SGS nature of the alloy, which is quite promising for the spintronic applications such as spin injection as it can bridge the gap between the contrasting behaviors of half-metallic ferromagnets and semiconductors.

  6. Anti-site disorder and improved functionality of Mn₂NiX (X = Al, Ga, In, Sn) inverse Heusler alloys

    SciTech Connect

    Paul, Souvik; Kundu, Ashis; Ghosh, Subhradip; Sanyal, Biplab

    2014-10-07

    Recent first-principles calculations have predicted Mn₂NiX (X = Al, Ga, In, Sn) alloys to be magnetic shape memory alloys. Moreover, experiments on Mn₂NiGa and Mn₂NiSn suggest that the alloys deviate from the perfect inverse Heusler arrangement and that there is chemical disorder at the sublattices with tetrahedral symmetry. In this work, we investigate the effects of such chemical disorder on phase stabilities and magnetic properties using first-principles electronic structure methods. We find that except Mn₂NiAl, all other alloys show signatures of martensitic transformations in presence of anti-site disorder at the sublattices with tetrahedral symmetry. This improves the possibilities of realizing martensitic transformations at relatively low fields and the possibilities of obtaining significantly large inverse magneto-caloric effects, in comparison to perfect inverse Heusler arrangement of atoms. We analyze the origin of such improvements in functional properties by investigating electronic structures and magnetic exchange interactions.

  7. Antiferromagnetic half-metals, gapless half-metals, and spin gapless semiconductors: The D0{sub 3}-type Heusler alloys

    SciTech Connect

    Gao, G. Y. Yao, Kai-Lun

    2013-12-02

    High-spin-polarization materials are desired for the realization of high-performance spintronic devices. We combine recent experimental and theoretical findings to theoretically design several high-spin-polarization materials in binary D0{sub 3}-type Heusler alloys: gapless (zero-gap) half-metallic ferrimagnets of V{sub 3}Si and V{sub 3}Ge, half-metallic antiferromagnets of Mn{sub 3}Al and Mn{sub 3}Ga, half-metallic ferrimagnets of Mn{sub 3}Si and Mn{sub 3}Ge, and a spin gapless semiconductor of Cr{sub 3}Al. The high spin polarization, zero net magnetic moment, zero energy gap, and slight disorder compared to the ternary and quaternary Heusler alloys make these binary materials promising candidates for spintronic applications. All results are obtained by the electronic structure calculations from first-principles.

  8. The effect of defects on the magnetic properties and spin polarization of Ti2FeAl Heusler alloy

    NASA Astrophysics Data System (ADS)

    Fang, Qing-Long; Zhang, Jian-Min; Xu, Ke-Wei; Ji, Vincent

    2014-02-01

    The effect of antisite, swap and vacancy defects on the magnetic properties and spin-polarization of the full-Heusler Ti2FeAl alloy with the Hg2CuTi-type structure is studied by using the first-principles calculations within density functional theory. The perfect Ti2FeAl Heusler alloy exhibits a ferromagnetic half-metallic behavior with the total magnetic moment of 1µΒ and indirect band gap of 0.543 eV. Among swap defect, only the total magnetic moment of the Ti2-Al swap defected is close to the perfect alloy. All defected structures destroy the half-metallicity and only AlTi1 and AlTi2 antisite and Fe vacancy defects maintain relatively high spin polarization.

  9. Enhanced magnetocaloric effect in a Co-doped Heusler Mn50Ni37Co3In10 unidirectional crystal

    NASA Astrophysics Data System (ADS)

    Ren, Jian; Feng, Shutong; Fang, Yue; Zhai, Qijie; Luo, Zhiping; Zheng, Hongxing

    2016-11-01

    A high-pressure optical zone-melting technique was employed to grow a Mn-rich Heusler Mn50Ni37Co3In10 unidirectional crystal in the present study. It was found that the Co-doped Mn50Ni37Co3In10 unidirectional crystal showed a low magnetic hysteretic loss and a widened working temperature interval in the vicinity of the martensitic transformation. The inverse magnetic entropy change (∆SM) reached 7.84 Jkg-1K-1 around 237.5 K under a magnetic field change of 30 kOe, and the corresponding effective refrigeration capacity (RCeff) was about 127.2 Jkg-1. The experimental results demonstrated a high potential to develop high-performance Mn-rich Heusler Mn-Ni-In magnetocaloric materials by means of Co doping in combination with the high-pressure optical zone-melting fabrication technique.

  10. Metamagnetic shape memory effect in a Heusler-type Ni43Co7Mn39Sn11 polycrystalline alloy

    NASA Astrophysics Data System (ADS)

    Kainuma, R.; Imano, Y.; Ito, W.; Morito, H.; Sutou, Y.; Oikawa, K.; Fujita, A.; Ishida, K.; Okamoto, S.; Kitakami, O.; Kanomata, T.

    2006-05-01

    Shape memory and magnetic properties of a Ni43Co7Mn39Sn11 Heusler polycrystalline alloy were investigated by differential scanning calorimetry, the sample extraction method, and the three-terminal capacitance method. A unique martensitic transformation from the ferromagnetic parent phase to the antiferromagneticlike martensite phase was detected and magnetic-field-induced "reverse" transition was confirmed in a high magnetic field. In addition, a large magnetic-field-induced shape recovery strain of about 1.0% was observed to accompany reverse martensitic transformation, and the metamagnetic shape memory effect, which was firstly reported in a Ni45Co5Mn36.7In13.3 Heusler single crystal, was confirmed in a polycrystalline specimen.

  11. Large Linear Magnetoresistance and Shubnikov-de Hass Oscillations in Single Crystals of YPdBi Heusler Topological Insulators

    PubMed Central

    Wang, Wenhong; Du, Yin; Xu, Guizhou; Zhang, Xiaoming; Liu, Enke; Liu, Zhongyuan; Shi, Youguo; Chen, Jinglan; Wu, Guangheng; Zhang, Xi-xiang

    2013-01-01

    We report the observation of a large linear magnetoresistance (MR) and Shubnikov-de Hass (SdH) quantum oscillations in single crystals of YPdBi Heusler topological insulators. Owning to the successfully obtained the high-quality YPdBi single crystals, large non-saturating linear MR of as high as 350% at 5K and over 120% at 300 K under a moderate magnetic field of 7 T is observed. In addition to the large, field-linear MR, the samples exhibit pronounced SdH quantum oscillations at low temperature. Analysis of the SdH data manifests that the high-mobility bulk electron carriers dominate the magnetotransport and are responsible for the observed large linear MR in YPdBi crystals. These findings imply that the Heusler-based topological insulators have superiorities for investigating the novel quantum transport properties and developing the potential applications. PMID:23846531

  12. Technical Operations Support III (TOPS III). Task Order 0061: Fundamental Theory Based Assessment of Thermoelectric Merit Factor for Heusler Alloys

    DTIC Science & Technology

    2010-10-01

    is believed to posses the phonon behavior of a glass and electronic conduction of a crystal. The interest in Half-Heusler alloys is resulted from...concentration. Glasses are known for low kl a. But, electrons in glasses are scattered making them poor thermoelectrics. Low or High m*? From...result of large electronegativity differences, but both have a good zT. Final Verdict One needs a “phonon- glass and an electron crystal”1 to realize

  13. First-principles study of spin-dependent thermoelectric properties of half-metallic Heusler thin films between platinum leads

    NASA Astrophysics Data System (ADS)

    Comtesse, Denis; Geisler, Benjamin; Entel, Peter; Kratzer, Peter; Szunyogh, László

    2014-03-01

    The electronic and magnetic bulk properties of half-metallic Heusler alloys such as Co2FeSi,Co2FeAl, Co2MnSi, and Co2MnAl are investigated by means of ab initio calculations in combination with Monte Carlo simulations. The electronic structure is analyzed using the plane-wave code quantum espresso and the magnetic exchange interactions are determined using the Korringa-Kohn-Rostoker (KKR) method. From the magnetic exchange interactions, the Curie temperature is obtained via Monte Carlo simulations. In addition, electronic transport properties of trilayer systems consisting of two semi-infinite platinum leads and a Heusler layer in-between are obtained from the fully relativistic screened KKR method by employing the Kubo-Greenwood formalism. The focus is on thermoelectric properties, namely, the Seebeck effect and its spin dependence. It turns out that already thin Heusler layers provide highly spin-polarized currents. This is attributed to the recovery of half-metallicity with increasing layer thickness. The absence of electronic states of spin-down electrons around the Fermi level suppresses the contribution of this spin channel to the total conductance, which strongly influences the thermoelectric properties and results in a spin polarization of thermoelectric currents.

  14. First-principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys

    NASA Astrophysics Data System (ADS)

    Chico, Jonathan; Keshavarz, Samara; Kvashnin, Yaroslav; Pereiro, Manuel; Di Marco, Igor; Etz, Corina; Eriksson, Olle; Bergman, Anders; Bergqvist, Lars

    2016-06-01

    Heusler alloys have been intensively studied due to the wide variety of properties that they exhibit. One of these properties is of particular interest for technological applications, i.e., the fact that some Heusler alloys are half-metallic. In the following, a systematic study of the magnetic properties of three different Heusler families Co2Mn Z ,Co2FeZ , and Mn2V Z with Z=(Al,Si,Ga,Ge) is performed. A key aspect is the determination of the Gilbert damping from first-principles calculations, with special focus on the role played by different approximations, the effect that substitutional disorder and temperature effects. Heisenberg exchange interactions and critical temperature for the alloys are also calculated as well as magnon dispersion relations for representative systems, the ferromagnetic Co2Fe Si and the ferrimagnetic Mn2V Al . Correlation effects beyond standard density-functional theory are treated using both the local spin density approximation including the Hubbard U and the local spin density approximation plus dynamical mean field theory approximation, which allows one to determine if dynamical self-energy corrections can remedy some of the inconsistencies which were previously reported for these alloys.

  15. Magnetic and anomalous electronic transport properties of the quaternary Heusler alloys Co2Ti1-xFexGe

    NASA Astrophysics Data System (ADS)

    Venkateswarlu, B.; Midhunlal, P. V.; Babu, P. D.; Kumar, N. Harish

    2016-06-01

    The half-metallic Heusler alloy Co2TiGe has a ferromagnetic ground state with a low magnetic moment (2 μB). It is free of atomic antisite disorder but has low Curie temperature (~390 K). In contrast the other cobalt based Heusler alloy Co2FeGe has high Curie temperature (~980 K) and high magnetic moment (5.6 μB) while exhibiting antisite disorder and lack of half-metallicity. Hence it is of interest to investigate the magnetic and transport properties of solid solutions of these two materials with contrasting characteristics. We report the structural, magnetic and electronic transport properties of quaternary Co2Ti1-x FexGe (x=0.2, 0.4, 0.6, 0.8) Heusler alloys. The alloys crystallize in L21 structure but with antisite disorder. The magnetization measurements revealed that the alloys were of soft ferromagnetic type with high Curie temperatures. Deviation from Slater-Pauling behavior and drastic change in electronic transport properties with some anomalous features were observed.The complex electronic transport properties have been explained using different scattering mechanisms.

  16. High-performance giant-magnetoresistance junctions based on the all-Heusler architecture with matched energy bands and Fermi surfaces

    NASA Astrophysics Data System (ADS)

    Bai, Zhaoqiang; Cai, Yongqing; Shen, Lei; Han, Guchang; Feng, Yuanping

    2013-04-01

    We present an all-Heusler architecture which could be used as a rational design scheme for achieving high spin-filter efficiency in the current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices. A Co2MnSi/Ni2NiSi/Co2MnSi trilayer stack is chosen as the prototype of such an architecture, of which the electronic structure and magnetotransport properties are systematically investigated by first principles approaches. Well matched energy bands and Fermi surfaces between the all-Heusler electrode-spacer pair are found, which, in combination with the electrode half-metallicity, indicate large bulk and interfacial spin-asymmetry, high spin-filter efficiency, and consequently good magnetoresistance performance. Transport calculations further confirm the superiority of the all-Heusler architecture over the conventional Heusler/transition-metal structure by comparing their transmission coefficients and interfacial resistances of parallel conduction electrons, as well as the macroscopic current-voltage characteristics. We suggest future theoretical and experimental efforts in developing high-performance all-Heusler CPP-GMR junctions for the read heads of the next generation high-density hard disk drives.

  17. Current-perpendicular-to-plane giant magnetoresistive properties in Co2Mn(Ge0.75Ga0.25)/Cu2TiAl/Co2Mn(Ge0.75Ga0.25) all-Heusler alloy pseudo spin valve

    NASA Astrophysics Data System (ADS)

    Li, S.; Takahashi, Y. K.; Sakuraba, Y.; Chen, J.; Furubayashi, T.; Mryasov, O.; Faleev, S.; Hono, K.

    2016-03-01

    Nonmagnetic Cu2TiAl (CTA) Heusler compound is proposed as a new spacer material for all-Heusler alloy current-perpendicular-to-plane giant magnetoresistance devices based on first-principle calculations. The (001)-orientated Co2Mn(Ge0.75Ga0.25) (CMGG)/CTA/CMGG epitaxial pseudo spin valve (PSV) grown on MgO(100) single crystal substrate showed relatively large MR output, ΔR A ˜5.4 mΩ μm2, which is comparable to that of the CMGG/Ag/CMGG PSV prepared with the same condition. Considering the short spin diffusion length in CTA, this result indicates a better electronic band matching for the CTA/CMGG interface than the Ag/CMGG interface in agreement with the first-principle calculation results. (001)- and (011)-orientated CMGG/CTA/CMGG PSV show similar MR outputs, indicating that the CTA spacer suppresses the orientation dependence of MR output which is usually observed in Ag-spacer CPP-PSV.

  18. A (25)Mg, (89)Y and (115)In solid state MAS NMR study of YT2X and Y(T0.5T'0.5)2X (T/T' = Pd, Ag, Au; X = Mg, In) Heusler phases.

    PubMed

    Benndorf, Christopher; Stein, Sebastian; Heletta, Lukas; Kersting, Marcel; Eckert, Hellmut; Pöttgen, Rainer

    2016-12-20

    Yttrium-transition metal-magnesium (indium) Heusler phases YPd2Mg, YPd2In, YAg2Mg, YAg2In, YAu2Mg, and YAu2In and their quaternary compounds (solid solutions) Y(Pd0.5Ag0.5)2Mg, Y(Pd0.5Ag0.5)2In, Y(Pd0.5Au0.5)2Mg, Y(Pd0.5Au0.5)2In, Y(Ag0.5Au0.5)2Mg and Y(Ag0.5Au0.5)2In were synthesized from the elements in sealed niobium ampoules in a high-frequency furnace or by arc-melting, respectively. All compounds crystallize with the cubic MnCu2Al type structure (Heusler phase), space group Fm3[combining macron]m. The structure of Y(Ag0.39Au0.61)2Mg was refined from single crystal X-ray diffractometer data: a = 689.97(5) pm, wR2 = 0.0619, 52 F(2) values, 6 parameters. Magnetic susceptibility measurements show Pauli paramagnetic behavior for all samples. The compounds were investigated by (25)Mg, (89)Y and (115)In solid state MAS NMR spectroscopy. Large positive resonance shifts are observed for all nuclei. A review of the present data in the context of literature data on isotypic Heusler phases with Cd and Sn indicates that the (89)Y shifts show a correlation with the electronegativity of the main group atoms (Mg, Cd, In, Sn). The solid solutions Y(Ag1-xTx)2Mg (x = 0.1, 0.25, 0.33, 0.5; T = Pd, Au) clearly show Vegard-like behavior concerning their lattice parameters, and their main group element resonance shifts arising from spin and orbital contributions are close to the interpolated values of the corresponding end-member compounds.

  19. Magnetic entropy changes and exchange bias effects associated with phase transitions in ferromagnetic Heusler alloys

    NASA Astrophysics Data System (ADS)

    Khan, Mahmud

    Magnetic entropy changes and exchange bias effect associated with magnetostructural phase transitions of several Mn based ferromagnetic Heusler alloys have been investigated by x-ray diffraction, magnetization, thermal expansion, and electrical resistivity measurements. The alloys include Ni2+xMn1-xGa (0.16 ≤ x ≤ 0.20), Ni2Mn1-xCuxGa (0.245 ≤ x ≤ 0.26), Ni2Mn0.75Cu0.25-xCo xGa (0.245 ≤ x ≤ 0.26), Ni2Mn0.71Cu 0.27Fe0.02Ga, Ni2Mn0.70Cu0.30 Ga0.95Ge0.05, Ni50Mn25+xSb 25-x (0 ≤ x ≤ 15), and Ni50Mn50-xSn x (10 ≤ x ≤ 17). The study of the Ga based Heusler alloys listed above shows that the alloys possess the cubic Heusler L21 structure or the lower symmetry martensitic structure (tetragonal/orthorhombic) at room temperature. Each alloy undergoes a first order martensitic structural phase transition and a second order ferromagnetic phase transition at the same temperature. This coupled magnetostructural phase transition occurs at different temperatures ranging from 295 K to 375 K. In the vicinity of each of these transitions large magnetic entropy changes have been observed. The maximum peak magnetic entropy change value, DeltaSM, is found to be around -64 J/kg.K. Except the Ni2+xMn1-xGa alloys, all of the other alloys exhibit the large magnetic entropy changes at temperatures near room temperature. The study of the Ni50Mn25+xSb25-x and Ni50Mn50-xSnx reveals many interesting properties of the alloys. For some critical Sb and Sn concentrations, martensitic transitions are observed in the samples, in the vicinity of which large inverse magnetic entropy changes have been observed. X-ray diffraction patterns of the alloys suggest that the martensitic phases possess 10M modulated orthorhombic structures, while the high temperature phase is purely cubic with the L21 structure. The martensitic phases in these alloys are found to host both ferromagnetic and antiferromagnetic coupling. Due to this coexistence of both ferromagnetic and antiferromagnetic coupling

  20. Molecular beam epitaxy growth and magnetic properties of Cr-Co-Ga Heusler alloy films

    SciTech Connect

    Feng, Wuwei Wang, Weihua; Zhao, Chenglong; Van Quang, Nguyen; Cho, Sunglae; Dung, Dang Duc

    2015-11-15

    We have re-investigated growth and magnetic properties of Cr{sub 2}CoGa films using molecular beam epitaxy technique. Phase separation and precipitate formation were observed experimentally again in agreement with observation of multiple phases separation in sputtered Cr{sub 2}CoGa films by M. Meinert et al. However, significant phase separation could be suppressed by proper control of growth conditions. We showed that Cr{sub 2}CoGa Heusler phase, rather than Co{sub 2}CrGa phase, constitutes the majority of the sample grown on GaAs(001) at 450 {sup o}C. The measured small spin moment of Cr{sub 2}CoGa is in agreement with predicted HM-FCF nature; however, its Curie temperature is not as high as expected from the theoretical prediction probably due to the off-stoichiometry of Cr{sub 2}CoGa and the existence of the disorders and phase separation.

  1. On the Challenges of Reducing Contact Resistances in Thermoelectric Generators Based on Half-Heusler Alloys

    NASA Astrophysics Data System (ADS)

    Ngan, Pham Hoang; Van Nong, Ngo; Hung, Le Thanh; Balke, Benjamin; Han, Li; Hedegaard, Ellen Marie Jensen; Linderoth, Søren; Pryds, Nini

    2016-01-01

    A method using fast hot pressing to join half-Heusler (HH) thermoelectric materials directly to an electrical current collector (Ag electrode) without using a third filler material is introduced. The compositions of the HH alloys used are Hf0.5Zr0.5CoSn0.2Sb0.8 and Ti0.6Hf0.4NiSn for p- and n-type, respectively. Using this method, the quality of the HH-electrode contacts is improved due to their low electrical contact resistance and less reaction-diffusion layer. The microstructure and chemical composition of the joints were examined using a scanning electron microscope equipped with energy-dispersive x-ray analysis. The electrical characteristics of the interfaces at the contacts were studied based on electrical contact resistance and Seebeck scanning microprobe measurements. In this paper, we show that joining the HH to a Ag electrode directly using fast hot pressing resulted in lower contact resistance and better performance compared with the method of using active brazing filler alloy.

  2. Half-metallicity and tetragonal distortion in semi-Heusler alloy FeCrSe

    SciTech Connect

    Huang, H. M. Luo, S. J.; Yao, K. L.

    2014-01-28

    Full-potential linearized augmented plane wave methods are carried out to investigate the electronic structures and magnetic properties in semi-Heusler alloy FeCrSe. Results show that FeCrSe is half-metallic ferromagnet with the half-metallic gap 0.31 eV at equilibrium lattice constant. Calculated total magnetic moment of 2.00μ{sub B} per formula unit follows the Slater-Pauling rule quite well. Two kinds of structural changes are used to investigate the sensitivity of half-metallicity. It is found that the half-metallicity can be retained when lattice constant is changed by −4.56% to 3.52%, and the results of tetragonal distortion indicate the half-metallicity can be kept at the range of c/a ratio from 0.85 to 1.20. The Curie temperature, cohesive energy, and heat of formations of FeCrSe are also discussed.

  3. Magnetostructural transition behavior in Fe-doped Heusler Mn-Ni-In ribbon materials

    NASA Astrophysics Data System (ADS)

    Li, Hongwei; Fang, Yue; Feng, Shutong; Zhai, Qijie; Luo, Zhiping; Zheng, Hongxing

    2016-11-01

    In the present work, we investigated magnetostructural transition behavior in Mn-rich Heusler Mn50-xFexNi41In9 (x=0, 1, 2, 3 at%) ribbon materials. Microstructural observations showed that substituting Mn with Fe in Mn50Ni41In9 led to striking grain refinement from ∼50 μm to 5-10 μm, and formation of a secondary phase when Fe content was increased up to 2 at%. Differential scanning calorimetric and thermomagnetic measurements indicated that a paramagnetic→ferromagnetic transition in austenite occurred first, followed with a weak-magnetic martensitic transition upon cooling for the Mn50-xFexNi41In9 (x=0, 1, 2). In case of Mn47Fe3Ni41In9, the martensitic transformation happened between paramagnetic austenite and weak-magnetic martensite, without the presence of the magnetic transition in austenite. The effective refrigeration capacity of Mn49Fe1Ni41In9 reached 137.1 J kg-1 under a magnetic field change of 30 kOe.

  4. Optimizing Magnetocaloric Properties of Heusler-Type Magnetic Shape Memory Alloys by Tuning Magnetostructural Transformation Parameters

    NASA Astrophysics Data System (ADS)

    Huang, Lian; Qu, Yuhai; Cong, Daoyong; Sun, Xiaoming; Wang, Yandong

    2017-08-01

    Heusler-type magnetic shape memory alloys show a magnetostructural transformation from the low-magnetization phase to the high-magnetization phase upon the application of external magnetic fields. As a result, these alloys exhibit fascinating multifunctional properties, such as magnetic shape memory effect, magnetocaloric effect, magnetoresistance, and magnetic superelasticity. All these functional properties are intimately related to the coupling of the structural and magnetic transitions. Therefore, deliberate tuning of the magnetostructural transformation parameters is essential for obtaining optimal multifunctional properties. Here, we show that by tuning the magnetostructural transformation parameters, we are able to achieve a variety of novel magnetocaloric properties with different application potentials: (1) large magnetic entropy change of 31.9 J kg-1 K-1 under a magnetic field of 5 T; (2) giant effective magnetic refrigeration capacity (251 J kg-1) with a broad operating temperature window (33 K) under a magnetic field of 5 T; (3) large reversible field-induced entropy change (about 15 J kg-1 K-1) and large reversible effective magnetic refrigeration capacity (77 J kg-1) under a magnetic field of 5 T. The balanced tuning of magnetostructural transformation parameters of magnetic shape memory alloys may provide an instructive reference to the shape memory and magnetic refrigeration communities.

  5. First principles calculation of elastic and magnetic properties of Cr-based full-Heusler alloys

    NASA Astrophysics Data System (ADS)

    Aly, Samy H.; Shabara, Reham M.

    2014-06-01

    We present an ab-initio study of the elastic and magnetic properties of Cr-based full-Heusler alloys within the first-principles density functional theory. The lattice constant, magnetic moment, bulk modulus and density of states are calculated using the full-potential nonorthogonal local-orbital minimum basis (FPLO) code in the Generalized Gradient Approximation (GGA) scheme. Only the two alloys Co2CrSi and Fe2CrSi are half-metallic with energy gaps of 0.88 and 0.55 eV in the spin-down channel respectively. We have predicted the metallicity state for Fe2CrSb, Ni2CrIn, Cu2CrIn, and Cu2CrSi alloys. Fe2CrSb shows a strong pressure dependent, e.g. exhibits metallicity at zero pressure and turns into a half-metal at P≥10 GPa. The total and partial magnetic moments of these alloys were studied under higher pressure, e.g. in Co2CrIn, the total magnetic moment is almost unchanged under higher pressure up to 500 GPa.

  6. Resonant impurity states in chemically disordered half-Heusler Dirac semimetals

    NASA Astrophysics Data System (ADS)

    Chadova, K.; Ködderitzsch, D.; Minár, J.; Ebert, H.; Kiss, J.; D'Souza, S. W.; Wollmann, L.; Felser, C.; Chadov, S.

    2016-05-01

    We address the electron transport characteristics in bulk half-Heusler alloys with their compositions tuned to the borderline between topologically nontrivial semimetallic and trivial semiconducting phases. Accurate first-principles calculations based on the coherent potential approximation (CPA) reveal that all the studied systems exhibit sets of dispersionless impurity-like resonant levels, with one of them being located at the Dirac point. By means of the Kubo-Bastin formalism we reveal that the residual conductivity of these alloys is strongly suppressed by impurity scattering, whereas the spin Hall conductivity exhibits a rather complex behavior induced by the resonant states. In particular for LaPt0.5Pd0.5Bi we find that the total spin Hall conductivity is strongly suppressed by two large and opposite contributions: the negative Fermi-surface contribution produced by the resonant impurity and the positive Fermi-sea term stemming from the occupied states. At the same time, we identify no conductivity contributions from the conical states.

  7. Effect of Ti Substitution on Thermoelectric Properties of W-Doped Heusler Fe2VAl Alloy

    NASA Astrophysics Data System (ADS)

    Mikami, M.; Ozaki, K.; Takazawa, H.; Yamamoto, A.; Terazawa, Y.; Takeuchi, T.

    2013-07-01

    Effects of element substitutions on thermoelectric properties of Heusler Fe2VAl alloys were evaluated. By W substitution at the V site, the thermal conductivity is reduced effectively because of the enhancement of phonon scattering resulting from the introduction of W atoms, which have much greater atomic mass and volume than the constituent elements of Fe2VAl alloy. W substitution is also effective to obtain a large negative Seebeck coefficient and high electrical conductivity through an electron injection effect. To change the conduction type from n-type to p-type, additional Ti substitution at the V site, which reduces the valence electron density, was examined. A positive Seebeck coefficient as high as that of conventional p-type Fe2VAl alloy was obtained using a sufficient amount of Ti substitution. Electrical resistivity was reduced by the hole doping effect of the Ti substitution while maintaining low thermal conductivity. Compared with the conventional solo-Ti-substituted p-type Fe2VAl alloy, the ZT value was improved, reaching 0.13 at 450 K.

  8. Effect of rapid solidification on the site preference of Heusler alloy Mn2NiSb

    NASA Astrophysics Data System (ADS)

    Hongzhi, Luo; Wei, Zhu; Li, Ma; Guodong, Liu; Yangxian, Li; Xiaoxi, Zhu; Chengbao, Jiang; Huibin, Xu; Guangheng, Wu

    2009-05-01

    The site preference of Mn atoms in Heusler alloy Mn2NiSb can be influenced obviously by different preparing methods. Mn atoms enter the A and B sites after arc-melting and subsequent annealing, and form an Hg2CuTi-type of structure. However, after melt-spinning, the Mn atoms tend to occupy the (A, C) sites and form the Cu2MnAl-type of structure. The electronic structure calculations suggest that the Hg2CuTi-type of structure is lower in energy than the Cu2MnAl one and more stable. The lattice constant of the former is a bit larger than the latter one, agreeing with experimental results. Ferromagnetism is observed in Mn2NiSb with both structures. Calculations give a total moment of 4.21 μB for the Hg2CuTi-type of structure and 3.93 μB for the Cu2MnAl-type one. These results fit the saturation magnetization at 5 K quite well. The difference between the Curie temperatures of the bulk and ribbon samples is about 77 K.

  9. First principles investigation of magnetocrystalline anisotropy at Full Heusler / MgO interfaces

    NASA Astrophysics Data System (ADS)

    Vadapoo, Rajasekarakumar; Hallal, Ali; Chshiev, Mairbek

    2014-03-01

    Magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) have the potential for realizing next generation high density nonvolatile memories and logic devices. The origin of high PMA in these interfaces has been explained by orbital hybridizations at interface along with spin-orbit interactions. Here we present a systematic study of PMA in Heusler alloy [X2YZ]/ MgO interfaces using first principle methods with X =Co, YZ =FeAl, MnGe and MnSi. Among the interfaces studied, we found that Co terminated interface of Co2FeAl/MgO gives rise to PMA value of 1.2erg/cm2 in agreement with recent experimental observations. On the contrary, FeAl terminated interfaces of the same structure shows in-plane magnetic anisotropy (IMA). We also found that the most of PMA contribution originates from dyz and dz2 orbitals of Co atoms at the interface. Finally, Co2MnGe and Co2MnSi structures tend to favor IMA for any termination.

  10. Magnetic and superconducting phase diagram of the half-Heusler topological semimetal HoPdBi.

    PubMed

    Nikitin, A M; Pan, Y; Mao, X; Jehee, R; Araizi, G K; Huang, Y K; Paulsen, C; Wu, S C; Yan, B H; de Visser, A

    2015-07-15

    We report a study of the magnetic and electronic properties of the non-centrosymmetric half-Heusler antiferromagnet HoPdBi (TN = 2.0 K). Magnetotransport measurements show HoPdBi has a semimetallic behavior with a carrier concentration n = 3.7 × 10(18) cm(-3) extracted from the Shubnikov-de Haas effect. The magnetic phase diagram in the field-temperature plane has been determined by transport, magnetization, and thermal expansion measurements: magnetic order is suppressed at BM ~ 3.6 T for T --> 0. Superconductivity with Tc ~ 1.9 K is found in the antiferromagnetic phase. Ac-susceptibility measurements provide solid evidence for bulk superconductivity below Tc = 0.75 K with a screening signal close to a volume fraction of 100%. The upper critical field shows an unusual linear temperature variation with Bc2(T --> 0) = 1.1 T. We also report electronic structure calculations that classify HoPdBi as a new topological semimetal, with a non-trivial band inversion of 0.25 eV.

  11. Thickness dependencies of structural and magnetic properties of cubic and tetragonal Heusler alloy bilayer films

    NASA Astrophysics Data System (ADS)

    Ranjbar, R.; Suzuki, K. Z.; Sugihara, A.; Ando, Y.; Miyazaki, T.; Mizukami, S.

    2017-07-01

    The thickness dependencies of the structural and magnetic properties for bilayers of cubic Co-based Heusler alloys (CCHAs: Co2FeAl (CFA), Co2FeSi (CFS), Co2MnAl (CMA), and Co2MnSi (CMS)) and D022-MnGa were investigated. Epitaxy of the B2 structure of CCHAs on a MnGa film was achieved; the smallest thickness with the B2 structure was found for 3-nm-thick CMS and CFS. The interfacial exchange coupling (Jex) was antiferromagnetic (AFM) for all of the CCHAs/MnGa bilayers except for unannealed CFA/MnGa samples. A critical thickness (tcrit) at which perpendicular magnetization appears of approximately 4-10 nm for the CMA/MnGa and CMS/MnGa bilayers was observed, whereas this thickness was 1-3 nm for the CFA/MnGa and CFS/MnGa films. The critical thickness for different CCHAs materials is discussed in terms of saturation magnetization (Ms) and the Jex .

  12. Structural ordering tendencies in the new ferromagnetic Ni-Co-Fe-Ga-Zn Heusler alloys

    NASA Astrophysics Data System (ADS)

    Dannenberg, Antje; Siewert, Mario; Gruner, Markus E.; Wuttig, Manfred; Entel, Peter

    In search for new ferromagnetic shape memory alloys (FSMA) we have calculated structural energy differences, magnetic exchange interaction constants and mixing energies of quaternary (X1X2)YZ Heusler alloys with X1,X2,Y =Ni,Co,Fe and Z=Ga, Zn using density functional theory. The comparison of the energy profiles of (NiCo)FeZ, (FeNi)CoZ, and (FeCo)NiZ with Z=Ga and Zn as a function of the tetragonal distortion c / a reveals that the energetically preferred ordering type is (NiCo)FeGa and (NiCo)FeZn which shows that Fe prefers to occupy the same cubic sublattice as Ga or Zn what implies that Fe favors Co and Ni as nearest neighbors, respectively. The Curie temperatures of (NiCo)FeGa and (NiCo)FeZn are high of the order of 600 K. (NiCo)FeGa, which has the same valence electron concentration (e/a=7.5) as Ni2MnGa and also possesses a high martensitic transformation temperature (>500 K), is of interest for future magnetic shape memory devices.

  13. Magnetic Compton scattering study of the Co2FeGa Heusler alloy: Experiment and theory

    NASA Astrophysics Data System (ADS)

    Deb, Aniruddha; Itou, M.; Sakurai, Y.; Hiraoka, N.; Sakai, N.

    2001-02-01

    The spin density in Co2FeGa Heusler alloy has been measured in a magnetic Compton scattering experiment using 274-keV circularly polarized synchrotron radiation at the high energy inelastic scattering beamline (BL08W) at SPring-8, Japan. A detailed band-structure calculation including hyperfine field study was performed utilizing the generalized gradient corrected full-potential linear augmented plane-wave (FLAPW-GGA) method. The magnetic Compton profiles for the [100], [110], and [111] principal directions, reported here, show anisotropy in the momentum density which is in good agreement with the FLAPW-GGA results based on ferromagnetic ground state. The conduction electrons were found to have a negative spin polarization of 0.60μB, which is at variance with the prediction of a positive moment from the recent neutron data. In the calculation, 3d spin moment at the Co and Fe site was found to be 1.20μB and 2.66μB, and their respective contribution in the eg and t2g sub-bands are in excellent agreement with the earlier reported neutron-diffraction measurements. It is also seen from our calculated results that the Co and Fe moment are mainly eg in character.

  14. Low-temperature electrical transport in Heusler-type Fe2V (AlSi) alloys

    NASA Astrophysics Data System (ADS)

    Vasundhara, M.; Srinivas, V.; Rao, V. V.

    2005-09-01

    The temperature variation of the electrical resistivity ρ and the Seebeck coefficient S of Heusler-type Fe2VAl1-xSix (0<=x<=1) alloys has been investigated. We have shown that the transport parameters are very sensitive to doping. For the x = 0 sample, high values of ρ and negative temperature coefficient of resistivity (TCR) have been observed. As the Si concentration increases, ρ decreases and the TCR changes its sign, while S shows significant changes in magnitude as well as sign when Al is replaced with Si. These changes appear to be reminiscent of a metal to semiconductor transition. It has been shown that the conventional transport theories proposed for intermetallic alloys or semiconductors cannot explain the transport behaviour in the whole temperature range of the present study. Low-temperature resistivity data of x = 0-0.02 samples could be described with a gapless semiconductor model. The strong composition dependence of S and ρ is attributed to the sharp variations in electronic density of states at the Fermi energy. It is also shown that by optimum doping one can achieve very large values of power factor (P). The estimated power factor at room temperature is observed to be highest (2.23 × 10-3 W mK-2) for x = 0.06 and comparable to that of conventional thermoelectric material. At lower temperatures P is found to be even higher than that of conventional thermoelectric material.

  15. The structural, electronic and magnetic properties of quaternary Heusler alloy TiZrCoIn

    NASA Astrophysics Data System (ADS)

    Yan, Peng-Li; Zhang, Jian-Min; Xu, Ke-Wei

    2016-04-01

    Employing the first-principles calculations, we have investigated the structural, electronic and magnetic properties of quaternary Heusler alloy TiZrCoIn. The TiZrCoIn alloy with type (I) configuration is predicted to be half-metallic ferromagnet at its equilibrium lattice constant 6.525 Å with an indirect band gap of 0.930 eV in minority spin channel. The total magnetic moment is 2 μB/f.u., following the Slater-Pauling rule μt=Zt-18. Moreover, the negative formation energy indicates the thermodynamical stability of this alloy. The band gap of minority spin channel is determined by the bonding (t2g) and antibonding (t1u) states created from the hybridizations of the d states of transition metal atoms Ti, Zr and Co. In addition, the HM, character is kept as hydrostatic strain ranged from -10% to 7.6% and tetragonal strain ranged from -19% to 27%.

  16. High spin polarization and spin splitting in equiatomic quaternary CoFeCrAl Heusler alloy

    NASA Astrophysics Data System (ADS)

    Bainsla, Lakhan; Mallick, A. I.; Coelho, A. A.; Nigam, A. K.; Varaprasad, B. S. D. Ch. S.; Takahashi, Y. K.; Alam, Aftab; Suresh, K. G.; Hono, K.

    2015-11-01

    In this paper, we investigate CoFeCrAl alloy by means of ab-initio electronic structure calculations and various experimental techniques. The alloy is found to exist in the B2-type cubic Heusler structure, which is very similar to Y-type (or LiMgPdSn prototype) structure with space group F-43m (#216). Saturation magnetization (MS) of about 2 μB/f.u. is observed at 8 K under ambient pressure, which is in good agreement with the Slater-Pauling rule. MS values are found to be independent of pressure, which is a prerequisite for half-metals. The ab-initio electronic structure calculations predict half-metallicity for the alloy with a spin slitting energy of 0.31 eV. Importantly, this system shows a high current spin polarization value of 0.67±0.02, as deduced from the point contact Andreev reflection measurements. Linear dependence of electrical resistivity with temperature indicates the possibility of reasonably high spin polarization at elevated temperatures (~150 K) as well. All these suggest that CoFeCrAl is a promising material for the spintronic devices.

  17. Half-metallicity in Heusler-type Fe2Cr1-x Co x Si alloys

    NASA Astrophysics Data System (ADS)

    Ramudu, M.; Inamdar, Swaleha; Arout Chelvane, J.; Manivel Raja, M.; Kamat, S. V.

    2016-02-01

    The effects of the substitution of Cr with Co on microstructure, phase composition, structure, magnetic, and electrical properties in \\text{F}{{\\text{e}}2}\\text{C}{{\\text{r}}1-x}\\text{C}{{\\text{o}}x}\\text{Si} (0  ⩽  x  ⩽  1) alloys was investigated to identify the compositions with the potential to exhibit half-metallicity. The microstructural and structural studies revealed that only \\text{F}{{\\text{e}}2}\\text{C}{{\\text{r}}1-x}\\text{C}{{\\text{o}}x}\\text{Si} alloys with x  ⩾  0.5 exhibited the desired single phase L21 full Heusler alloy structure. Both the saturation magnetization (M s) and Curie temperature (T C) were found to increase with the increase in Co concentration. The experimentally measured M s values are in good agreement with the Slater-Pauling rule. The electrical resistivity measurements in the temperature range 10-300 K gives indirect evidence of half-metallic behaviour in these alloys at low temperatures. The temperature range in which the half-metallic behaviour was observed also increased with an increase in Co concentration.

  18. Room-temperature tetragonal non-collinear Heusler antiferromagnet Pt2MnGa

    NASA Astrophysics Data System (ADS)

    Singh, Sanjay; D'Souza, S. W.; Nayak, J.; Suard, E.; Chapon, L.; Senyshyn, A.; Petricek, V.; Skourski, Y.; Nicklas, M.; Felser, C.; Chadov, S.

    2016-08-01

    Antiferromagnetic spintronics is a rapidly growing field, which actively introduces new principles of magnetic storage. Despite that, most applications have been suggested for collinear antiferromagnets. In this study, we consider an alternative mechanism based on long-range helical order, which allows for direct manipulation of the helicity vector. As the helicity of long-range homogeneous spirals is typically fixed by the Dzyaloshinskii-Moriya interactions, bi-stable spirals (left- and right-handed) are rare. Here, we report a non-collinear room-temperature antiferromagnet in the tetragonal Heusler group. Neutron diffraction reveals a long-period helix propagating along its tetragonal axis. Ab-initio analysis suggests its pure exchange origin and explains its helical character resulting from a large basal plane magnetocrystalline anisotropy. The actual energy barrier between the left- and right-handed spirals is relatively small and might be easily overcome by magnetic pulse, suggesting Pt2MnGa as a potential candidate for non-volatile magnetic memory.

  19. High tunneling magnetoresistance ratio in perpendicular magnetic tunnel junctions using Fe-based Heusler alloys

    SciTech Connect

    Wang, Yu-Pu; Lim, Sze-Ter; Han, Gu-Chang; Teo, Kie-Leong

    2015-12-21

    Heulser alloys Fe{sub 2}Cr{sub 1−x}Co{sub x}Si (FCCS) with different Co compositions x have been predicted to have high spin polarization. High perpendicular magnetic anisotropy (PMA) has been observed in ultra-thin FCCS films with magnetic anisotropy energy density up to 2.3 × 10{sup 6 }erg/cm{sup 3}. The perpendicular magnetic tunnel junctions (p-MTJs) using FCCS films with different Co compositions x as the bottom electrode have been fabricated and the post-annealing effects have been investigated in details. An attractive tunneling magnetoresistance ratio as high as 51.3% is achieved for p-MTJs using Fe{sub 2}CrSi (FCS) as the bottom electrode. The thermal stability Δ can be as high as 70 for 40 nm dimension devices using FCS, which is high enough to endure a retention time of over 10 years. Therefore, Heusler alloy FCS is a promising PMA candidate for p-MTJ application.

  20. Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys.

    PubMed

    Cugini, F; Porcari, G; Fabbrici, S; Albertini, F; Solzi, M

    2016-08-13

    We report a complete structural and magneto-thermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy versus temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'. © 2016 The Author(s).

  1. Room-temperature tetragonal non-collinear Heusler antiferromagnet Pt2MnGa

    PubMed Central

    Singh, Sanjay; D'Souza, S. W.; Nayak, J.; Suard, E.; Chapon, L.; Senyshyn, A.; Petricek, V.; Skourski, Y.; Nicklas, M.; Felser, C.; Chadov, S.

    2016-01-01

    Antiferromagnetic spintronics is a rapidly growing field, which actively introduces new principles of magnetic storage. Despite that, most applications have been suggested for collinear antiferromagnets. In this study, we consider an alternative mechanism based on long-range helical order, which allows for direct manipulation of the helicity vector. As the helicity of long-range homogeneous spirals is typically fixed by the Dzyaloshinskii–Moriya interactions, bi-stable spirals (left- and right-handed) are rare. Here, we report a non-collinear room-temperature antiferromagnet in the tetragonal Heusler group. Neutron diffraction reveals a long-period helix propagating along its tetragonal axis. Ab-initio analysis suggests its pure exchange origin and explains its helical character resulting from a large basal plane magnetocrystalline anisotropy. The actual energy barrier between the left- and right-handed spirals is relatively small and might be easily overcome by magnetic pulse, suggesting Pt2MnGa as a potential candidate for non-volatile magnetic memory. PMID:27561795

  2. Influence of Si substitution on structural, electronic and magnetic properties of Fe2MnGa Heusler compound

    NASA Astrophysics Data System (ADS)

    Zhu, Xing-Feng; Wang, Lu

    2017-04-01

    We investigate the electronic and magnetic properties of Fe2MnGa1-xSix alloy (x = 0, 0.25, 0.5, 0.75, and 1) using first-principles density functional theory within the generalized gradient approximation method. The lattice constant decreases linearly whereas bulk modulus increases with increasing Si content. The total magnetic moment varies linearly with increasing Si content, which follows the Slater-Pauling rule. Electronic band structure calculations indicate that the Fe2MnGa1-xSix exhibits half-metallic character for all the concentrations studied and the spin polarization and the spin-down band gap both increase with the Si content. Based on the magnetic properties calculations, the Heisenberg exchange coupling parameters give Fe-Mn ferromagnetic coupling and Mn-Mn antiferromagnetic coupling. The TC first decreases and then increases with Si content, which is in well agreement with the experimental results.

  3. Magnetocrystalline anisotropy and its electric-field-assisted switching of Heusler-compound-based perpendicular magnetic tunnel junctions

    NASA Astrophysics Data System (ADS)

    Bai, Zhaoqiang; Shen, Lei; Cai, Yongqing; Wu, Qingyun; Zeng, Minggang; Han, Guchang; Feng, Yuan Ping

    2014-10-01

    Employing density functional theory combined with the non-equilibrium Green's function formalism, we systematically investigate the structural, magnetic and magnetoelectric properties of the Co2FeAl(CFA)/MgO interface, as well as the spin-dependent transport characteristics of the CFA/MgO/CFA perpendicular magnetic tunnel junctions (p-MTJs). We find that the structure of the CFA/MgO interface with the oxygen-top FeAl termination has high thermal stability, which is protected by the thermodynamic equilibrium limit. Furthermore, this structure is found to have perpendicular magnetocrystalline anisotropy (MCA). Giant electric-field-assisted modifications of this interfacial MCA through magnetoelectric coupling are demonstrated with an MCA coefficient of up to 10-7 erg V-1 cm. In addition, our non-collinear spin transport calculations of the CFA/MgO/CFA p-MTJ predict a good magnetoresistance performance of the device.

  4. Suppression of the ferromagnetic order in the Heusler alloy Ni50Mn35In15 by hydrostatic pressure

    NASA Astrophysics Data System (ADS)

    Salazar Mejía, C.; Mydeen, K.; Naumov, P.; Medvedev, S. A.; Wang, C.; Hanfland, M.; Nayak, A. K.; Schwarz, U.; Felser, C.; Nicklas, M.

    2016-06-01

    We report on the effect of hydrostatic pressure on the magnetic and structural properties of the shape-memory Heusler alloy Ni50Mn35In15. Magnetization and x-ray diffraction experiments were performed at hydrostatic pressures up to 5 GPa using diamond anvil cells. Pressure stabilizes the martensitic phase, shifting the martensitic transition to higher temperatures, and suppresses the ferromagnetic austenitic phase. Above 3 GPa, where the martensitic-transition temperature approaches the Curie temperature in the austenite, the magnetization shows no longer indications of ferromagnetic ordering. We further find an extended temperature region with a mixture of martensite and austenite phases, which directly relates to the magnetic properties.

  5. First-principle study on some new spin-gapless semiconductors: The Zr-based quaternary Heusler alloys

    NASA Astrophysics Data System (ADS)

    Gao, Qiang; Xie, Huan-Huan; Li, Lei; Lei, Gang; Deng, Jian-Bo; Hu, Xian-Ru

    2015-09-01

    Employing first-principle calculations, we have investigated electronic and magnetic properties of the Zr-based quaternary Heusler alloys: ZrCoVIn, ZrFeVGe, ZrCoFeP, ZrCoCrBe and ZrFeCrZ (Z = In and Ga). Our calculation results show that all the alloys are (or nearly) spin-gapless semiconductors. The Slater-Pauling behaviours of these alloys are discussed as well. The cohesion energy and formation energy of these alloys have also been discussed, and the results indicate the studied alloys are stable.

  6. Structural phase transition, electronic structure and optical properties of half Heusler alloys LiBeZ (Z = As, Sb)

    SciTech Connect

    Amudhavalli, A.; Rajeswarapalanichamy, R.

    2016-05-23

    Ab initio calculations are performed to investigate the structural stability, electronic structure, mechanical properties and optical properties of half Heusler alloys (LiBeAs and LiBeSb) for three different phases of zinc blende crystal structure. Among the considered phases, α- phase is found to be the most stable phase for these alloys at normal pressure. A pressure induced structural phase transition from α-phase to β- phase is observed for LiBeAs. The electronic structure reveals that these alloys are semiconductors. The optical properties confirm that these alloys are semiconductor in nature.

  7. Investigation of structural, electronic and magnetic properties of 1:1:1:1 stoichiometric quaternary Heusler alloys YCoCrZ (Z=Si, Ge, Ga, Al): An ab-initio study

    NASA Astrophysics Data System (ADS)

    Rasool, M. Nasir; Mehmood, Salman; Sattar, M. Atif; Khan, Muhammad Azhar; Hussain, Altaf

    2015-12-01

    Full potential linearized augmented plane wave method (FPLAPW) has been employed to probe the structural, electronic and magnetic properties of equiatomic yttrium based quaternary Heusler alloys YCoCrZ (Z=Si, Ge, Ga, Al). These calculations have been carried out via ab -initio simulations based on density functional theory (DFT) approach coded by Wien2K. The generalized gradient approximation of Perdew-Burke-Ernzerhof 96 scheme is engaged for calculations in all alloys under investigation. Equilibrium lattice constants are studied by structural optimization performed by computing total energies versus volumes. Structural optimization demonstrates that Y(3/4,3/4,3/4)Co(0,0,0)Cr(1/2,1/2,1/2)Z(1/4,1/4,1/4) (Type-1) configuration is the most stable one. The calculated electronic and magnetic properties based on type-1, indicate that YCoCrZ alloys are half-metallic ferromagnetic. The calculation of spin polarization is also made and further their total magnetic moments follow the Slater Pauling rule of Mtot=NVE-18 conceding the integer value i.e. 4.00μB and 3.00μB for YCoCrSi, Ge and YCoCrGa, Al respectively. The results of density of states (DOS) revealed that yttrium based quaternary Heusler alloys exhibit excellent band gaps i.e. 0.70, 0.65, 0.46 and 0.35 eV for YCoCrSi, Ge, Ga and Al respectively. The formation of band gaps owing to hybridization effect is also described. The half-metallic gaps of these compounds comprising the order YCoCrGa>YCoCrSi>YCoCrAl>YCoCrGe by size, is also manipulated. The incredible spin gapless semiconductor (SGS) type character of YCoCrGa and YCoCrAl having bantam DOS in spin up version is also discoursed. The optimised results of these compounds signpost that these are suitable candidates for spintronics applications.

  8. First-principles study of the Hf-based Heusler alloys: Hf2CoGa and Hf2CoIn

    NASA Astrophysics Data System (ADS)

    Hu, Yan; Zhang, Jian-Min

    2017-01-01

    The electronic structures and magnetic properties of the new Heusler alloys Hf2CoGa and Hf2CoIn have been studied by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). Both Hf2CoGa and Hf2CoIn Heusler alloys have the half-metallic character and completely (100%) spin polarization at the Fermi level (EF) and the indirect band gaps of 0.733 eV and 0.654 eV, respectively, in the minority spin channel. The total magnetic moments μt are all 2μB per formula unit, linearly scaled with the total number of valence electrons (Zt) by μt=Zt-18 and the atomic magnetic moments have localized character due to less affected by deformations. The origin of the indirect band gaps for these two new Heusler alloys is well understood. These two new Heusler alloys are the ideal candidates for spintronic devices.

  9. Synthesis and characterization of Fe-Ti-Sb intermetallic compounds: Discovery of a new Slater-Pauling phase

    NASA Astrophysics Data System (ADS)

    Naghibolashrafi, N.; Keshavarz, S.; Hegde, Vinay I.; Gupta, A.; Butler, W. H.; Romero, J.; Munira, K.; LeClair, P.; Mazumdar, D.; Ma, J.; Ghosh, A. W.; Wolverton, C.

    2016-03-01

    Compounds of Fe, Ti, and Sb were prepared using arc melting and vacuum annealing. Fe2TiSb , expected to be a full Heusler compound crystallizing in the L 21 structure, was shown by XRD and SEM analyses to be composed of weakly magnetic grains of nominal composition Fe1.5TiSb with iron-rich precipitates in the grain boundaries. FeTiSb, a composition consistent with the formation of a half-Heusler compound, also decomposed into Fe1.5TiSb grains with Ti-Sb rich precipitates and was weakly magnetic. The dominant Fe1.5TiSb phase appears to crystallize in a defective L 21 -like structure with iron vacancies. Based on this finding, a first-principles DFT-based binary cluster expansion of Fe and vacancies on the Fe sublattice of the L 21 structure was performed. Using the cluster expansion, we computationally scanned >103 configurations and predict a novel, stable, nonmagnetic semiconductor phase to be the zero-temperature ground state. This new structure is an ordered arrangement of Fe and vacancies, belonging to the space group R 3 m , with composition Fe1.5TiSb , i.e., between the full- and half-Heusler compositions. This phase can be visualized as alternate layers of L 21 phase Fe2TiSb and C 1b phase FeTiSb, with layering along the [111] direction of the original cubic phases. Our experimental results on annealed samples support this predicted ground-state composition, but further work is required to confirm that the R 3 m structure is the ground state.

  10. Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials

    PubMed Central

    Fu, Chenguang; Bai, Shengqiang; Liu, Yintu; Tang, Yunshan; Chen, Lidong; Zhao, Xinbing; Zhu, Tiejun

    2015-01-01

    Solid-state thermoelectric technology offers a promising solution for converting waste heat to useful electrical power. Both high operating temperature and high figure of merit zT are desirable for high-efficiency thermoelectric power generation. Here we report a high zT of ∼1.5 at 1,200 K for the p-type FeNbSb heavy-band half-Heusler alloys. High content of heavier Hf dopant simultaneously optimizes the electrical power factor and suppresses thermal conductivity. Both the enhanced point-defect and electron–phonon scatterings contribute to a significant reduction in the lattice thermal conductivity. An eight couple prototype thermoelectric module exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 W cm−2 at a temperature difference of 655 K. These findings highlight the optimization strategy for heavy-band thermoelectric materials and demonstrate a realistic prospect of high-temperature thermoelectric modules based on half-Heusler alloys with low cost, excellent mechanical robustness and stability. PMID:26330371

  11. Evolution of the electronic structure and physical properties of Fe2MeAl (Me = Ti, V, Cr) Heusler alloys

    NASA Astrophysics Data System (ADS)

    Shreder, E.; Streltsov, S. V.; Svyazhin, A.; Makhnev, A.; Marchenkov, V. V.; Lukoyanov, A.; Weber, H. W.

    2008-01-01

    We present the results of experiments on the optical, electrical and magnetic properties and electronic structure and optical spectrum calculations of the Heusler alloys Fe2TiAl, Fe2VAl and Fe2CrAl. We find that the drastic transformation of the band spectrum, especially near the Fermi level, when replacing the Me element (Me = Ti, V, Cr), is accompanied by a significant change in the electrical and optical properties. The electrical and optical properties of Fe2TiAl are typical for metals. The abnormal behavior of the electrical resistivity and the optical properties in the infrared range for Fe2VAl and Fe2CrAl are determined by electronic states at the Fermi level. Both the optical spectroscopic measurements and the theoretical calculations demonstrate the presence of low-energy gaps in the band spectrum of the Heusler alloys. In addition, we demonstrate that the formation of Fe clusters may be responsible for the large enhancement of the total magnetic moment in Fe2CrAl.

  12. Design of Heusler Precipitation Strengthened NiTi- and PdTi-Base SMAs for Cyclic Performance

    NASA Astrophysics Data System (ADS)

    Frankel, Dana J.; Olson, Gregory B.

    2015-06-01

    For a wide range of actuation applications, the performance of NiTi-based shape memory alloys is limited by cyclic instability associated with accommodation slip. For medical applications, low-Ni compositions are also desirable. Increasing yield strength via precipitation of a coherent nanoscale Ni2TiAl-type Heusler phase from a supersaturated B2 matrix is an effective approach for eliminating slip in order to improve the stability of the functional response and increase the structural fatigue life. Quaternary additions that partition into the L21 Heusler phase, such as Zr or Pd, are favorable for reducing interphase misfit and maintaining coherency during aging. Phase relations and precipitation kinetics in quaternary Ni(TiZrAl), low-Ni (PdNi)(TiAl), and Ni-free (PdFe)(TiAl) systems are summarized from TEM and atom probe tomography data in the literature. Strengthening behavior during isothermal aging is compared in the NiTiZrAl and PdNiTiAl systems, and recent work characterizing a high-strength, low-Ni "Hybrid" (PdNi)(TiZrAl) alloy is presented. A systems design approach is taken in which an optimal microstructure for peak strengthening is identified while other property objectives such as transformation temperature, misfit, radiopacity, and biocompatibility are satisfied.

  13. Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials.

    PubMed

    Fu, Chenguang; Bai, Shengqiang; Liu, Yintu; Tang, Yunshan; Chen, Lidong; Zhao, Xinbing; Zhu, Tiejun

    2015-09-02

    Solid-state thermoelectric technology offers a promising solution for converting waste heat to useful electrical power. Both high operating temperature and high figure of merit zT are desirable for high-efficiency thermoelectric power generation. Here we report a high zT of ∼1.5 at 1,200 K for the p-type FeNbSb heavy-band half-Heusler alloys. High content of heavier Hf dopant simultaneously optimizes the electrical power factor and suppresses thermal conductivity. Both the enhanced point-defect and electron-phonon scatterings contribute to a significant reduction in the lattice thermal conductivity. An eight couple prototype thermoelectric module exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 W cm(-2) at a temperature difference of 655 K. These findings highlight the optimization strategy for heavy-band thermoelectric materials and demonstrate a realistic prospect of high-temperature thermoelectric modules based on half-Heusler alloys with low cost, excellent mechanical robustness and stability.

  14. Consecutive magnetic and magnetocaloric transitions in herringbone nanostructured Heusler Mn50Ni41Sn9 alloy.

    PubMed

    Prasanna, A A; Ram, S; Fecht, H J

    2013-08-01

    A herringbone nanostructured Mn-rich Heusler Mn50Ni50-Sndelta (8 - 9) alloy exhibits tailored magnetocaloric properties in the martensite and ferro <-> paramagnetic transitions concur in a narrow temperature window. In a Sn --> Ni substitution 8 - 9, the martensite (M) <-- austenite (A) transition up-lifts adequately well above the room temperature - 310.5 K in the DSC thermogram and magnetization scanned with temperature. A noninterrupted heating following a cooling in DSC at a given rate gives a smaller enthalpy change deltaH(M <-- A) - deltaH(M --> A approximately equal to 282 mJ/g (deltaC(P)(M <-- A) - deltaC(P)(M --> A) approximately equal to 0.025 mJ/g-K in the heat capacity), i.e., the M <-> A transition process lacks a complete reversibility. Warming a zero-field cooled sample retains lower magnetization (sigma) at low fields B, e.g., by 58% over the field cooled value at 5 mT, wherein merely low field magnetic susceptibility imparts the magnetization process. A reversible thermal hysteresis thus the transition traces in cooling and heating. The field diminishes difference in two sigma-values progressively, e.g., only - 12% lasts at 5 T. The two curves bifurcate below 160 K (B-5 mT) and the gap grows exponentially over lower temperatures before sigma(M <-- A) gets steady near 60 K in a superparamagnetic (SPM) behavior. The SPM feature (follows the Langevin model) below a paramagnetic regime begins (> or = 250 K) before a ferromagnetic A-state lines-up the successive transitions. Temperature and frequency dependence ac and dc susceptibilities describe the surface spins dynamics.

  15. Theoretical investigation of the structural, magnetic and band structure characteristics of Co2FeGe1- x Si x ( x = 0, 0.5, 1) full-Heusler alloys

    NASA Astrophysics Data System (ADS)

    Amari, S.; Dahmane, F.; Omran, S. Bin; Doumi, B.; Yahiaoui, I. E.; Tadjer, A.; Khenata, R.

    2016-11-01

    In this study, the structural, magnetic and electronic properties of the Co2FeGe1- x Si x ( x = 0, 0.5, 1) Heusler compounds have been calculated using the full-potential linearized augmented plane-wave method based on the spin density functional theory within the generalized gradient approximation of Perdew-Burke-Ernzerhof. In order to take into account the correlation effects, we have also performed GGA + U calculations, where the Hubbard on-site Coulomb interaction correction U is calculated by using the constraint local density approximation for the Co and the Mn atoms. The Cu2MnAl-type structure is found to be energetically more favorable than the Hg2CuTitype structure for both the Co2FeSi and the Co2FeGe compounds. The calculated atomic resolved densities of states of Co2FeSi and Co2FeGe indicate nearly half-metallic behaviors with small spindown electronic densities of states at the Fermi level. This behavior is corrected by including the Hubbard Coulomb energy U term. The Coulomb exchange correlation U confirms the halfmetallic property in both the Co2FeSi and the Co2FeGe compounds. We also discuss the electronic structures, the total and the partial densities of states, and the local magnetic moments. The Co2FeGe0.5Si0.5 compound shows a nearly half-metallic behavior with a small spin-down electronic density of states at the Fermi level in both the GGA and GGA+ U approximations.

  16. CO2-based mixtures as working fluids for geothermal turbines.

    SciTech Connect

    Wright, Steven Alan; Conboy, Thomas M.; Ames, David E.

    2012-01-01

    Sandia National Laboratories is investigating advanced Brayton cycles using supercritical working fluids for application to a variety of heat sources, including geothermal, solar, fossil, and nuclear power. This work is centered on the supercritical CO{sub 2} (S-CO{sub 2}) power conversion cycle, which has the potential for high efficiency in the temperature range of interest for these heat sources and is very compact-a feature likely to reduce capital costs. One promising approach is the use of CO{sub 2}-based supercritical fluid mixtures. The introduction of additives to CO{sub 2} alters the equation of state and the critical point of the resultant mixture. A series of tests was carried out using Sandia's supercritical fluid compression loop that confirmed the ability of different additives to increase or lower the critical point of CO{sub 2}. Testing also demonstrated that, above the modified critical point, these mixtures can be compressed in a turbocompressor as a single-phase homogenous mixture. Comparisons of experimental data to the National Institute of Standards and Technology (NIST) Reference Fluid Thermodynamic and Transport Properties (REFPROP) Standard Reference Database predictions varied depending on the fluid. Although the pressure, density, and temperature (p, {rho}, T) data for all tested fluids matched fairly well to REFPROP in most regions, the critical temperature was often inaccurate. In these cases, outside literature was found to provide further insight and to qualitatively confirm the validity of experimental findings for the present investigation.

  17. The half-metallicity of LiMgPdSn-type quaternary Heusler alloys FeMnScZ (Z=Al, Ga, In): A first-principle study

    SciTech Connect

    Gao, Y. C.; Gao, X.

    2015-05-15

    Based on the first-principles calculations, quaternary Heusler alloys FeMnScZ (Z=Al, Ga, In) including its phase stability, band gap, the electronic structures and magnetic properties has been studied systematically. We have found that, in terms of the equilibrium lattice constants, FeMnScZ (Z=Al, Ga, In) are half-metallic ferrimagnets, which can sustain the high spin polarization under a very large amount of lattice distortions. The half-metallic band gap in FeMnScZ (Z=Al, Ga, In) alloys originates from the t{sub 1u}-t{sub 2g} splitting instead of the e{sub u}-t{sub 1u} splitting. The total magnetic moments are 3μB per unit cell for FeMnScZ (Z=Al, Ga, In) alloys following the Slater–Pauling rule with the total number of valence electrons minus 18 rather than 24. According to the study, the conclusion can be drawn that all of these compounds which have a negative formation energy are possible to be synthesized experimentally.

  18. High-pressure and high-temperature physical properties of half-metallic full-Heusler alloy Mn2RuSi by first-principles and quasi-harmonic Debye model

    NASA Astrophysics Data System (ADS)

    Song, Ting; Ma, Qin; Sun, Xiao-Wei; Liu, Zi-Jiang; Wei, Xiao-Ping; Tian, Jun-Hong

    2017-02-01

    First-principles calculations based on density functional theory and quasi-harmonic Debye model are used to investigate the high-pressure and high-temperature physical properties, including the lattice constant, magnetic moment, density of states, pressure-volume-temperature relationship, bulk modulus, thermal expansivity, heat capacity, and Grüneisen parameter for the new Mn-based full-Heusler alloy Mn2RuSi in CuHg2Ti-type structure. The optimized equilibrium lattice constant is consistent with experimental and other theoretical results. The calculated total spin magnetic moment remains an integral value of 2.0 μB in the lattice constant range of 5.454-5.758 Å, and then decreases very slowly with the decrease of lattice constant to 5.333 Å. By the spin resolved density of states calculations, we have shown that Mn2RuSi compound presents half-metallic ferrimagnetic properties under the equilibrium lattice constant. The effects of temperature and pressure on bulk modulus, thermal expansivity, heat capacity, and Grüneisen parameter are opposite, which are consistent with a compression rate of volume. Furthermore, the results show that the effect of temperature is larger than pressure for heat capacity and the effect of high temperature and pressure on thermal expansion coefficient is small. All the properties of Mn2RuSi alloy are summarized in the pressure range of 0-100 GPa and the temperature up to 1200 K.

  19. Large enhancement of bulk spin polarization by suppressing Co{sub Mn} anti-sites in Co{sub 2}Mn(Ge{sub 0.75}Ga{sub 0.25}) Heusler alloy thin film

    SciTech Connect

    Li, S.; Takahashi, Y. K.; Sakuraba, Y. Furubayashi, T.; Tsuji, N.; Tajiri, H.; Chen, J.; Hono, K.

    2016-03-21

    We have investigated the structure and magneto-transport properties of Co{sub 2}Mn(Ge{sub 0.75}Ga{sub 0.25}) (CMGG) Heusler alloy thin films with near-stoichiometric and Mn-rich compositions in order to understand the effect of Co-Mn anti-sites on bulk spin polarization. Anomalous x-ray diffraction measurements using synchrotron radiated x-rays confirmed that Co{sub Mn} anti-sites easily form in the near-stoichiometric CMGG compound at annealing temperature higher than 400 °C, while it can be suppressed in Mn-rich CMGG films. Accordingly, large enhancement in negative anisotropic magnetoresistance of CMGG films and giant magnetoresistance (GMR) in current-perpendicular-to-plane (CPP) pseudo spin valves were observed in the Mn-rich composition. A large resistance-area product change (ΔRA) of 12.8 mΩ μm{sup 2} was demonstrated in the CPP-GMR pseudo spin valves using the Mn-rich CMGG layers after annealing at 600 °C. It is almost twice of the maximum output observed in the CPP-GMR pseudo spin valves using the near-stoichiometric CMGG. These indicate that the spin polarization of CMGG is enhanced in the Mn-rich composition through suppressing the formation of Co{sub Mn}-antisites in CMGG films, being consistent with first-principle calculation results.

  20. The half-metallicity of LiMgPdSn-type quaternary Heusler alloys FeMnScZ (Z=Al, Ga, In): A first-principle study

    NASA Astrophysics Data System (ADS)

    Gao, Y. C.; Gao, X.

    2015-05-01

    Based on the first-principles calculations, quaternary Heusler alloys FeMnScZ (Z=Al, Ga, In) including its phase stability, band gap, the electronic structures and magnetic properties has been studied systematically. We have found that, in terms of the equilibrium lattice constants, FeMnScZ (Z=Al, Ga, In) are half-metallic ferrimagnets, which can sustain the high spin polarization under a very large amount of lattice distortions. The half-metallic band gap in FeMnScZ (Z=Al, Ga, In) alloys originates from the t1u-t2g splitting instead of the eu-t1u splitting. The total magnetic moments are 3μB per unit cell for FeMnScZ (Z=Al, Ga, In) alloys following the Slater-Pauling rule with the total number of valence electrons minus 18 rather than 24. According to the study, the conclusion can be drawn that all of these compounds which have a negative formation energy are possible to be synthesized experimentally.

  1. Topological phase, structural, electronic, thermodynamic and optical properties of XPtSb (X=Lu, Sc) compounds

    NASA Astrophysics Data System (ADS)

    Narimani, Mitra; Nourbakhsh, Zahra

    2017-03-01

    The electronic, thermodynamic and optical properties of XPtSb (X=Lu, Sc) half Heusler compounds are studied based on density functional theory. The calculations are carried out in the presence of spin orbit interaction. The exchange correlation part of total energy is calculated within local density approximation, generalized gradient approximation, Engel-Vosco generalized gradient approximation and modified Becke and Johnson exchange potential with the correlation potential of the generalized gradient approximation. The effect of pressure on the electron density of states and linear coefficient of the electronic specific heat is studied. Using the band structure calculations at different pressures, the band inversion strength and topological phase transition of these compounds are investigated. Some thermodynamic properties of XPtSb compounds by different thermal models using the non-equilibrium Gibbs function are studied and compared with experiment. Furthermore the effect of pressure on dielectric function of XPtSb (X=Lu, Sc) compounds is investigated.

  2. Thermoelectrical properties of the compounds ScMVIIISb and YMVIIISb (MVIII = Ni, Pd, Pt)

    NASA Astrophysics Data System (ADS)

    Oestreich, J.; Probst, U.; Richardt, F.; Bucher, E.

    2003-02-01

    The research into new materials with good thermoelectric properties has revealed new compounds consisting of metallic elements (Bando Y, Suemitsu T, Takagi K, Tokushima H, Echizen Y, Katoh K, Umeo K, Maeda Y and Takabatake T 2000 J. Alloys Compounds 313 1-6, Ghelani N, Loo S, Chung D, Sportouch S, Nardi S, Kanatzidis M, Hogan T and Nolas G 2000 Mater. Res. Soc. 626 Z8.6.1). The half-Heusler compound ZrNiSn, in particular, shows promising thermoelectric properties and has been studied by many scientists during recent years (Uher C, Hu S, Yang J, Meisner G P and Morelli D T 1997 Proc. ICT'97: 16th Int. Conf. on Thermoelectrics pp 485-8, Romaka L P, Stadnyk Yu V, Goryn A M, Gorelenko Yu K and Skolozdra R V 1997 Proc. ICT'97: 16th Int. Conf. on Thermoelectrics pp 516-19, Hohl H, Ramirez A P, Goldmann C, Ernst G, Wölfing B and Bucher E 1998 J. Phys.: Condens. Matter 11 1697-709, Oestreich J, Käfer W, Richardt F, Probst U and Bucher E 1999 Proc. 5th European Workshop on Thermoelectrics pp 192-5). In an effort to find new thermoelectric materials, the half-Heusler compounds of the groups ScMVIIISb and YMVIIISb (MVIII = Ni, Pd, Pt) were synthesized by arc melting and the thermoelectric properties were examined by standard characterization methods. Doping experiments showed that it is possible to change the electrical properties of the compounds while retaining the half-Heusler structure. Within the two groups, YPtSb showed the best thermoelectrical properties. At a temperature of 400 K the electrical conductivity of YPtSb is 748Omega-1 cm-1 and the Seebeck coefficient is 116.3muV K-1. The thermal conductivity at 400 K extrapolated using the Wiedemann-Franz law is 2.87 W K-1 m-1. This leads to a dimensionless figure of merit of 0.14.

  3. Magnetostructural martensitic transformations with large volume changes and magneto-strains in all-d-metal Heusler alloys

    NASA Astrophysics Data System (ADS)

    Wei, Z. Y.; Liu, E. K.; Li, Y.; Han, X. L.; Du, Z. W.; Luo, H. Z.; Liu, G. D.; Xi, X. K.; Zhang, H. W.; Wang, W. H.; Wu, G. H.

    2016-08-01

    The all-d-metal Mn2-based Heusler ferromagnetic shape memory alloys Mn50Ni40-xCoxTi10 (x = 8 and 9.5) are realized. With a generic comparison between d-metal Ti and main-group elements in lowering the transformation temperature, the magnetostructural martensitic transformations are established by further introducing Co to produce local ferromagnetic Mn-Co-Mn configurations. A 5-fold modulation and (3, -2) stacking of [00 10] of martensite are determined by X-ray diffraction and HRTEM analysis. Based on the transformation, a large magneto-strain of 6900 ppm and a large volume change of -2.54% are observed in polycrystalline samples, which makes the all-d-metal magnetic martensitic alloys of interest for magnetic/pressure multi-field driven applications.

  4. First-principles investigation of magnetocrystalline anisotropy at the L 21 full Heusler |MgO interfaces and tunnel junctions

    NASA Astrophysics Data System (ADS)

    Vadapoo, Rajasekarakumar; Hallal, Ali; Yang, Hongxin; Chshiev, Mairbek

    2016-09-01

    Magnetocrystalline anisotropy at Heusler alloy |MgO interfaces has been studied using first-principles calculations. It has been found that Co-terminated Co2FeAl |MgO interfaces show perpendicular magnetic anisotropy up to 1.31 mJ/m2, while those with FeAl termination exhibit in-plane magnetic anisotropy. Atomic layer-resolved analysis indicates that the origin of perpendicular magnetic anisotropy in Co2FeAl |MgO interfaces can be attributed to the out-of-plane orbital contributions of interfacial Co atoms. At the same time, Co2MnGe and Co2MnSi interfaced with MgO tend to favor in-plane magnetic anisotropy for all terminations.

  5. Perpendicularizing magnetic anisotropy of full-Heusler Co2FeAl films by cosputtering with terbium

    NASA Astrophysics Data System (ADS)

    Li, X. Q.; Xu, X. G.; Zhang, D. L.; Miao, J.; Zhan, Q.; Jalil, M. B. A.; Yu, G. H.; Jiang, Y.

    2010-04-01

    In this letter, we fabricated Co2FeAl films with perpendicular-to-plane magnetic anisotropy by cosputtering with terbium (Tb). The as-prepared (Tb+Co2FeAl) films (TCFA) consists of nanocrystalline L21 Co2FeAl and amorphous alloy of Tb(Co, Fe, and Al). The coercivity field (Hc) of the TCFA films is adjustable from 200 to 800 Oe. After annealing, the Hc decreases to 70 Oe. A perpendicularly magnetized spin valve with the TCFA films as free and reference layers shows a current-perpendicular-to-plane magnetoresistance of 1.8% at room temperature. Our result opens a way to fabricate perpendicularly magnetized full-Heusler alloys and makes it possible to realize faster and simple structured magnetic storage bits in the future.

  6. Large half-metallic gap in ferromagnetic semi-Heusler alloys CoCrP and CoCrAs

    SciTech Connect

    Yao Zhongyu; Zhang, Y. S.; Yao, K. L.

    2012-08-06

    We investigate the electronic structure and magnetism of semi-Heusler alloys CoCrP and CoCrAs using the full-potential linearized augmented plane wave method. The calculations reveal that CoCrP and CoCrAs are half-metallic (HM) ferromagnets with the same magnetic moment of 2.00 {mu}{sub B} per formula unit. Both alloys have large half-metallic gaps (up to 0.50 eV) and wide band gaps (above 1 eV). The half-metallicity of CoCrP and CoCrAs can be retained even when their lattice constants are changed by -4.8% to 6.6% and -7.7% to 4.5%, respectively. The two alloys show great promise in the applications of spin valve and magnetic tunnel junction.

  7. Effect of compositional and antisite disorder on the electronic and magnetic properties of Ni-Mn-In Heusler alloy.

    PubMed

    Borgohain, Parijat; Sahariah, Munima B

    2015-05-08

    A systematic study has been done on the electronic and magnetic properties of metamagnetic Ni-Mn-In Heusler alloy with compositional and structural (anti-site) disorder at high temperature austenite phase. The electronic structure calculation shows an increasing Mn-Ni hybridization which occurs due to the decrease in Mn-Ni bond length as the system approaches martensite phase. The results obtained from magnetic moment calculations follow a similar trend to the previous experimental and theoretical results. The magnetic coupling parameters, Jij, obtained from the ab initio calculation explains the presence of competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions in the system and the dominating AFM interactions nearer to the martensite phase.

  8. Magnetostructural martensitic transformations with large volume changes and magneto-strains in all-d-metal Heusler alloys

    SciTech Connect

    Wei, Z. Y.; Liu, E. K. Xi, X. K.; Zhang, H. W.; Wang, W. H.; Wu, G. H.; Li, Y.; Han, X. L.; Du, Z. W.; Luo, H. Z.; Liu, G. D.

    2016-08-15

    The all-d-metal Mn{sub 2}-based Heusler ferromagnetic shape memory alloys Mn{sub 50}Ni{sub 40−x}Co{sub x}Ti{sub 10} (x = 8 and 9.5) are realized. With a generic comparison between d-metal Ti and main-group elements in lowering the transformation temperature, the magnetostructural martensitic transformations are established by further introducing Co to produce local ferromagnetic Mn-Co-Mn configurations. A 5-fold modulation and (3, −2) stacking of [00 10] of martensite are determined by X-ray diffraction and HRTEM analysis. Based on the transformation, a large magneto-strain of 6900 ppm and a large volume change of −2.54% are observed in polycrystalline samples, which makes the all-d-metal magnetic martensitic alloys of interest for magnetic/pressure multi-field driven applications.

  9. Enhanced thermoelectric properties of n-type NbCoSn half-Heusler by improving phase purity

    DOE PAGES

    He, Ran; Huang, Lihong; Wang, Yumei; ...

    2016-06-01

    In this paper, we report the thermoelectric properties of NbCoSn-based n-type half-Heuslers (HHs) that were obtained through arc melting, ball milling, and hot pressing process. With 10% Sb substitution at the Sn site, we obtained enhanced n-type properties with a maximum power factor reaching ~35 μW cm-1 K-2 and figure of merit (ZT) value ~0.6 in NbCoSn0.9Sb0.1. The ZT is doubled compared to the previous report. In addition, the specific power cost ($ W-1) is decreased by ~68% comparing to HfNiSn-based n-type HH because of the elimination of Hf.

  10. Pulsed high-magnetic-field experiments: New insights into the magnetocaloric effect in Ni-Mn-In Heusler alloys

    SciTech Connect

    Salazar Mejía, C. Nayak, A. K.; Felser, C.; Nicklas, M.; Ghorbani Zavareh, M.; Wosnitza, J.; Skourski, Y.

    2015-05-07

    The present pulsed high-magnetic-field study on Ni{sub 50}Mn{sub 35}In{sub 15} gives an extra insight into the thermodynamics of the martensitic transformation in Heusler shape-memory alloys. The transformation-entropy change, ΔS, was estimated from field-dependent magnetization experiments in pulsed high magnetic fields and by heat-capacity measurements in static fields. We found a decrease of ΔS with decreasing temperature. This behavior can be understood by considering the different signs of the lattice and magnetic contributions to the total entropy. Our results further imply that the magnetocaloric effect will decrease with decreasing temperature and, furthermore, the martensitic transition is not induced anymore by changing the temperature in high magnetic fields.

  11. Static and dynamic magnetic properties of epitaxial Co{sub 2}FeAl Heusler alloy thin films

    SciTech Connect

    Ortiz, G.; Gabor, M. S.; Petrisor, T. Jr.; Boust, F.; Issac, F.; Tiusan, C.; Hehn, M.; Bobo, J. F.

    2011-04-01

    Structural and magnetic properties of epitaxial Co{sub 2}FeAl Heusler alloy thin films were investigated. Films were deposited on single crystal MgO (001XS) substrates at room temperature, followed by an annealing process at 600 deg. C. MgO and Cr buffer layers were introduced in order to enhance crystalline quality, and improve magnetic properties. Structural analyses indicate that samples have grown in the B2 ordered epitaxial structure. VSM measures show that the MgO buffered sample displays a magnetization saturation of 1010 {+-} 30 emu/cm{sup 3}, and Cr buffered sample displays a magnetization saturation of 1032 {+-} 40 emu/cm{sup 3}. Damping factor was studied by strip-line ferromagnetic resonance measures. We observed a maximum value for the MgO buffered sample of about 8.5 x 10{sup -3}, and a minimum value of 3.8 x 10{sup -3} for the Cr buffered one.

  12. Ferromagnetism and electronic structures of nonstoichiometric Heusler-alloy Fe3-xMnxSi Epilayers grown on Ge(111).

    PubMed

    Hamaya, K; Itoh, H; Nakatsuka, O; Ueda, K; Yamamoto, K; Itakura, M; Taniyama, T; Ono, T; Miyao, M

    2009-04-03

    For the study of ferromagnetic materials which are compatible with group-IV semiconductor spintronics, we demonstrate control of the ferromagnetic properties of Heusler-alloy Fe3-xMnxSi epitaxially grown on Ge(111) by tuning the Mn composition x. Interestingly, we obtain L2(1)-ordered structures even for nonstoichiometric atomic compositions. The Curie temperature of the epilayers with x approximately 0.6 exceeds 300 K. Theoretical calculations indicate that the electronic structures of the nonstoichiometric Fe3-xMnxSi alloys become half-metallic for 0.75 < or = x < or = 1.5. We discuss the possibility of room-temperature ferromagnetic Fe(3-x)Mn(x)Si/Ge epilayers with high spin polarization.

  13. Enhanced thermoelectric properties of n-type NbCoSn half-Heusler by improving phase purity

    NASA Astrophysics Data System (ADS)

    He, Ran; Huang, Lihong; Wang, Yumei; Samsonidze, Georgy; Kozinsky, Boris; Zhang, Qinyong; Ren, Zhifeng

    2016-10-01

    Here we report the thermoelectric properties of NbCoSn-based n-type half-Heuslers (HHs) that were obtained through arc melting, ball milling, and hot pressing process. With 10% Sb substitution at the Sn site, we obtained enhanced n-type properties with a maximum power factor reaching ˜35 μW cm-1 K-2 and figure of merit (ZT) value ˜0.6 in NbCoSn0.9Sb0.1. The ZT is doubled compared to the previous report. In addition, the specific power cost ( W-1) is decreased by ˜68% comparing to HfNiSn-based n-type HH because of the elimination of Hf.

  14. Soft x-ray magnetic circular dichroism of L21-type Co2FeGa Heusler alloy

    NASA Astrophysics Data System (ADS)

    Umetsu, R. Y.; Nakamura, T.; Kobayashi, K.; Kainuma, R.; Sakuma, A.; Fukamichi, K.; Ishida, K.

    2010-03-01

    Spin and orbital magnetic moments of the L21-type Co2FeGa Heusler alloy have been investigated using x-ray magnetic circular dichroism spectra in the soft x-ray region. From the spectra of the L2,3-edge of Co and Fe, the ratios of the orbital magnetic moment to the spin magnetic moment Morb/Mspin are estimated to be 0.06 for Co and 0.02 for Fe, in agreement with the available theoretical values. The orbital magnetic moments of these two elements are small in line with theoretical results, reflecting the high symmetry of the L21-type crystal structure. Furthermore, it has been confirmed that the magnetic moment of Ga is induced in the present alloy.

  15. B2+L2{sub 1} ordering in Co{sub 2}MnAl Heusler alloy

    SciTech Connect

    Vinesh, A.; Sudheesh, V. D.; Lakshmi, N.; Venugopalan, K.

    2014-04-24

    Magnetic and structural properties of B2 ordered Co{sub 2}MnAl Heusler alloy have been studied by X-ray diffraction and DC magnetization techniques. X-ray diffractogram shows the structure is of B2 type with preferential site disorder between Mn and Al atoms and presence of a small L2{sub 1} phase. DC magnetization studies at low temperature establish that the antiferromagnetic nature arises mainly due to the antiparallel coupling of spin moments of 3d electrons of Co with Mn atoms. Curie temperature (T{sub c}) is 733 K which is close to T{sub c} of the L2{sub 1} phase.

  16. Growth temperature dependent structural and magnetic properties of epitaxial Co{sub 2}FeAl Heusler alloy films

    SciTech Connect

    Qiao, Shuang; Nie, Shuaihua; Zhao, Jianhua; Zhang, Xinhui

    2013-06-21

    The structural and magnetic properties of a series of Co{sub 2}FeAl Heusler alloy films grown on GaAs(001) substrate by molecular beam epitaxy have been studied. The epitaxial Co{sub 2}FeAl films with an ordered L{sub 21} structure have been successfully obtained at growth temperature of 433 K, with an in-plane cubic magnetic anisotropy superimposed with an unusual uniaxial magnetic anisotropy. With increasing growth temperature, the ordered L{sub 21} structure degrades. Meanwhile, the uniaxial anisotropy decreases and eventually disappears above 673 K. The interfacial bonding between As and Co or Fe atom is suggested to be responsible for the additional uniaxial anisotropy.

  17. Bending strain-tunable magnetic anisotropy in Co2FeAl Heusler thin film on KaptonxAE

    NASA Astrophysics Data System (ADS)

    Gueye, M.; Wague, B. M.; Zighem, F.; Belmeguenai, M.; Gabor, M. S.; Petrisor, T.; Tiusan, C.; Mercone, S.; Faurie, D.

    2014-08-01

    Bending effect on the magnetic anisotropy in 20 nm Co2FeAl Heusler thin film grown on Kapton® has been studied by ferromagnetic resonance and glued on curved sample carrier with various radii. The results reported in this Letter show that the magnetic anisotropy is drastically changed in this system by bending the thin films. This effect is attributed to the interfacial strain transmission from the substrate to the film and to the magnetoelastic behavior of the Co2FeAl film. Moreover, two approaches to determine the in-plane magnetostriction coefficient of the film, leading to a value that is close to λCFA= 14 × 10-6, have been proposed.

  18. The realization of ferro-ferrimagnetic transition and half-metallicity in half-Heusler CoMnGa alloy

    NASA Astrophysics Data System (ADS)

    Wang, L. Y.; Dai, X. F.; Wang, X. T.; Liu, X. F.; Li, P. P.; Cui, Y. T.; Liu, E. K.; Wang, W. H.; Wu, G. H.; Liu, G. D.

    2014-11-01

    We theoretically predicted that half-Heusler CoMnGa alloy to be half-metallic ferrimagnet at the equilibrium lattice parameter. With the lattice expansion, a local energy minimum occurs at a larger lattice parameter where CoMnGa alloy is in a metastable ferromagnetic state. However, a ferro-ferrimagnetic transition (Fo-Fi-T) is not observed in experiment. We found the Co-Mn antisites can induce the Fo-Fi-T by adjusting the driving force of magnetic transition and the energy barrier. The antisites are sensitive to the preparation methods and annealing temperatures. The highly ordered CoMnGa is achieved by annealing at 1073 K. The Fo-Fi-T occurs in a sample annealed at 1083 K.

  19. Enhancement of magnetocaloric properties near room temperature in Ga-doped Ni50Mn34.5In15.5 Heusler-type alloy

    NASA Astrophysics Data System (ADS)

    Takeuchi, A. Y.; Guimarães, C. E.; Passamani, E. C.; Larica, C.

    2012-05-01

    A martensitic Ni50Mn34.5In15.5 Heusler-type alloy doped with Ga was studied by x-ray diffractometry and magnetization measurements. Ga-doping does not affect the austenitic phase transition but shifts the martensitic phase transformation towards room temperature, producing an enhancement of the magnetic entropy change (ΔSM) in that temperature region. Large ΔSM-values in the Ga-doped samples are attained for an applied field of 30 kOe as opposed to the field of 50 kOe commonly found for the un-doped cases. These effects (enhancement of ΔSM-values, shift to temperatures close to 300 K, and large ΔSM-values at lower applied fields) make the Ga-doped Ni50Mn34.5In15.5 Heusler-type alloys good candidates for technological applications as a solid refrigerant.

  20. Simultaneous laser excitation of backward volume and perpendicular standing spin waves in full-Heusler Co2FeAl0.5Si0.5 films

    NASA Astrophysics Data System (ADS)

    Chen, Zhifeng; Yan, Yong; Li, Shufa; Xu, Xiaoguang; Jiang, Yong; Lai, Tianshu

    2017-02-01

    Spin-wave dynamics in full-Heusler Co2FeAl0.5Si0.5 films are studied using all-optical pump-probe magneto-optical polar Kerr spectroscopy. Backward volume magnetostatic spin-wave (BVMSW) mode is observed in films with thickness ranging from 20 to 100 nm besides perpendicular standing spin-wave (PSSW) mode, and found to be excited more efficiently than the PSSW mode. The field dependence of the effective Gilbert damping parameter appears especial extrinsic origin. The relationship between the lifetime and the group velocity of BVMSW mode is revealed. The frequency of BVMSW mode does not obviously depend on the film thickness, but the lifetime and the effective damping appear to do so. The simultaneous excitation of BVMSW and PSSW in Heusler alloy films as well as the characterization of their dynamic behaviors may be of interest for magnonic and spintronic applications.

  1. Simultaneous laser excitation of backward volume and perpendicular standing spin waves in full-Heusler Co2FeAl0.5Si0.5 films

    PubMed Central

    Chen, Zhifeng; Yan, Yong; Li, Shufa; Xu, Xiaoguang; Jiang, Yong; Lai, Tianshu

    2017-01-01

    Spin-wave dynamics in full-Heusler Co2FeAl0.5Si0.5 films are studied using all-optical pump-probe magneto-optical polar Kerr spectroscopy. Backward volume magnetostatic spin-wave (BVMSW) mode is observed in films with thickness ranging from 20 to 100 nm besides perpendicular standing spin-wave (PSSW) mode, and found to be excited more efficiently than the PSSW mode. The field dependence of the effective Gilbert damping parameter appears especial extrinsic origin. The relationship between the lifetime and the group velocity of BVMSW mode is revealed. The frequency of BVMSW mode does not obviously depend on the film thickness, but the lifetime and the effective damping appear to do so. The simultaneous excitation of BVMSW and PSSW in Heusler alloy films as well as the characterization of their dynamic behaviors may be of interest for magnonic and spintronic applications. PMID:28195160

  2. Electronic structures, magnetic properties and half-metallicity in Heusler alloys Zr2CoZ (Z=Al, Ga, In, Sn)

    NASA Astrophysics Data System (ADS)

    Yan, Peng-Li; Zhang, Jian-Min; Xu, Ke-Wei

    2015-10-01

    The electronic structures, magnetic properties, and half-metallicity of full-Heusler alloys Zr2 CoZ (Z=Al, Ga, In, Sn) with the Hg2 CuTi -type structure have been studied by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). The Zr2 CoZ (Z=Al, Ga, In, Sn) are found to be half-metallic ferrimagnets within a certain range of the lattice constant. The total magnetic moments (μt) of the Zr2 CoZ alloys are calculated to be 2 for Z=Al, Ga, In and 3 for Z=Sn, linearly scaled with the total number of valence electrons (Zt) by μt =Zt - 18 . The origin of the band gap for these half-metallic alloys is well understood. These new Zr-based Heusler alloys are the ideal candidates for spintronic devices.

  3. Fabrication of highly ordered Co2Fe0.4Mn0.6Si Heusler alloy films on Si substrates

    NASA Astrophysics Data System (ADS)

    Koike, Takeo; Oogane, Mikihiko; Ono, Atsuo; Ando, Yasuo

    2016-08-01

    The structural and magnetic properties of Si(100)/MgO/Co2Fe0.4Mn0.6Si (CFMS) Heusler alloy thin films were systematically investigated. Highly B2-ordered CFMS Heusler films with an ordering parameter of ca. 70-80% were obtained by both the insertion of a very thin Mg layer into the Si/MgO interfaces to prevent oxidation of the Si surface and the optimization of the annealing temperature for the CFMS films. The prepared CFMS films exhibited high magnetization close to that of the CFMS bulk. Such highly B2-ordered CFMS films are very useful for realizing high spin injection efficiency in Si because of the half-metallicity of the CFMS films.

  4. Half-metallicity at the Heusler alloy Co(2)Cr(0.5)Fe(0.5)Al(001) surface and its interface with GaAs(001).

    PubMed

    Zarei, Sareh; Javad Hashemifar, S; Akbarzadeh, Hadi; Hafari, Zohre

    2009-02-04

    Electronic and magnetic properties of the Heusler alloy Co(2)Cr(0.5)Fe(0.5)Al(001) surfaces and its interfaces with GaAs(001) are studied within the framework of density functional theory by using the plane-wave pseudopotential approach. The phase diagram obtained by ab initio atomistic thermodynamics shows that the CrAl surface is the most stable (001) termination of this Heusler alloy. We discuss that, at the ideal surfaces and interfaces with GaAs, half-metallicity of the alloy is lost, although the CrAl surface keeps high spin polarization. The energy band profile of the stable interface is investigated and a negative p Schottky barrier of -0.78 eV is obtained for this system.

  5. Enhancement of current-perpendicular-to-plane giant magnetoresistance by insertion of amorphous ferromagnetic underlayer in Heusler alloy-based spin-valve structures

    NASA Astrophysics Data System (ADS)

    Choi, Young-suk; Nakatani, Tomoya; Read, John C.; Carey, Matthew J.; Stewart, Derek A.; Childress, Jeffrey R.

    2017-01-01

    We report an improved method for depositing Heusler alloy thin films, which reduces the B2-ordering temperature, and demonstrate its effect on improving spin-polarization and ΔR/R in CPP-GMR sensors. The insertion of a CoFeBTa or CoBTi amorphous ferromagnetic underlayer induced the formation of an amorphous Co2(Mn,Fe)Ge Heusler alloy film, reducing the B2-ordering temperature to ∼220 °C, which is significantly lower than the value of 500 °C for an epitaxial system and 400 °C for a polycrystalline system. This novel approach allows the fabrication of spin-valve sensor structures with ΔR/R of 18% after post-deposition annealing at temperatures less than 300 °C and is thus compatible with standard recording read-head sensor production.

  6. Effect of graphene tunnel barrier on Schottky barrier height of Heusler alloy Co2MnSi/graphene/n-Ge junction

    NASA Astrophysics Data System (ADS)

    Gui-fang, Li; Jing, Hu; Hui, Lv; Zhijun, Cui; Xiaowei, Hou; Shibin, Liu; Yongqian, Du

    2016-02-01

    We demonstrate that the insertion of a graphene tunnel barrier between Heusler alloy Co2MnSi and the germanium (Ge) channel modulates the Schottky barrier height and the resistance-area product of the spin diode. We confirm that the Fermi level is depinned and a reduction in the electron Schottky barrier height (SBH) occurs following the insertion of the graphene layer between Co2MnSi and Ge. The electron SBH is modulated in the 0.34 eV-0.61 eV range. Furthermore, the transport mechanism changes from rectifying to symmetric tunneling following the insertion. This behavior provides a pathway for highly efficient spin injection from a Heusler alloy into a Ge channel with high electron and hole mobility. Project supported by the National Natural Science Foundation of China (Grant No. 61504107) and the Fundamental Research Funds for the Central Universities, China (Grant Nos. 3102014JCQ01059 and 3102015ZY043).

  7. TRIFLUOROMETHYL COMPOUNDS OF GERMANIUM

    DTIC Science & Technology

    FLUORIDES, *GERMANIUM COMPOUNDS, *HALIDES, *ORGANOMETALLIC COMPOUNDS, ALKYL RADICALS, ARSENIC COMPOUNDS, CHEMICAL BONDS, CHEMICAL REACTIONS ...CHLORIDES, CHLORINE COMPOUNDS, HYDROLYSIS, IODIDES, METHYL RADICALS, POTASSIUM COMPOUNDS, PYROLYSIS, STABILITY, SYNTHESIS, TIN COMPOUNDS.

  8. High-field magnetization of heusler alloys Fe2 XY ( X = Ti, V, Cr, Mn, Fe, Co, Ni; Y = Al, Si)

    NASA Astrophysics Data System (ADS)

    Kourov, N. I.; Marchenkov, V. V.; Korolev, A. V.; Belozerova, K. A.; Weber, H. W.

    2015-10-01

    The magnetization curves of ferromagnetic Heusler alloys Fe2 XY (where X = Ti, V, Cr, Mn, Fe, Co, Ni are transition 3 d elements and Y = Al, Si are the s and p elements of the third period of the Periodic Table) have been measured at T = 4.2 K in the field range H ≤ 70 kOe. It has been shown that the high-field ( H ≥ 20 kOe) magnetization is described within the Stoner model.

  9. Thermoelectric properties of fine-grained FeVSb half-Heusler alloys tuned to p-type by substituting vanadium with titanium

    SciTech Connect

    Zou, Minmin; Li, Jing-Feng; Kita, Takuji

    2013-02-15

    Fine-grained Ti-doped FeVSb half-Heusler alloys were synthesized by combining mechanical alloying and spark plasma sintering and their thermoelectric properties were investigated with an emphasis on the influences of Ti doping and phase purity. It was found that substituting V with Ti can change the electrical transport behavior from n-type to p-type due to one less valence electron of Ti than V, and the sample with nominal composition FeV{sub 0.8}Ti{sub 0.4}Sb exhibits the largest Seebeck coefficient and the maximum power factor. By optimizing the sintering temperature and applying annealing treatment, the power factor is significantly improved and the thermal conductivity is reduced simultaneously, resulting in a ZT value of 0.43 at 500 Degree-Sign C, which is relatively high as for p-type half-Heusler alloys containing earth-abundant elements. - Graphical abstract: Fine-grained Ti-doped FeVSb alloys were prepared by the MA-SPS method. The maximum ZT value reaches 0.43 at 500 Degree-Sign C, which is relatively high for p-type half-Heusler alloys. Highlights: Black-Right-Pointing-Pointer Ti-doped FeVSb half-Heusler alloys were synthesized by combining MA and SPS. Black-Right-Pointing-Pointer Substituting V with Ti changes the electrical behavior from n-type to p-type. Black-Right-Pointing-Pointer Thermoelectric properties are improved by optimizing sintering temperature. Black-Right-Pointing-Pointer Thermoelectric properties are further improved by applying annealing treatment. Black-Right-Pointing-Pointer A high ZT value of 0.43 is obtained at 500 Degree-Sign C for p-type Ti-doped FeVSb alloys.

  10. Structural, electronic and magnetic properties of CoFeTiGa1-xSbx compounds

    NASA Astrophysics Data System (ADS)

    Zhang, Y. J.; Liu, Z. H.; Liu, G. D.; Ma, X. Q.

    2017-01-01

    Structural, electronic and magnetic properties of Heusler compounds CoFeTiGa1-xSbx have been studied. X-ray diffraction patterns show that CoFeTiGa1-xSbx (x=0.25, 0.5, 0.75, 1) crystallize into LiMgPdSn-type structure. First-principles calculations indicate that CoFeTiGa1-xSbx (x=0.25, 0.5, 0.75, 1) are half-metal with considerable minority band gaps. The substitution of Sb for Ga gives rise to the spin moments of the compounds, and makes them to be half-metallic ferrimagnets.

  11. Hybridization gap in the semiconducting compound SrIr4In2Ge4

    DOE PAGES

    Calta, Nicholas P.; Im, Jino; Fang, Lei; ...

    2016-11-18

    Here, large single crystals of SrIr4In2Ge4 were synthesized using the In flux method. This compound is a hybridization gap semiconductor with an experimental optical band gap of Eg = 0.25(3) eV. It crystallizes in the tetragonal EuIr4In2Ge4 structure type with space group 1more » $$\\overline{4}$$2m and unit cell parameters a = 6.9004(5) Å and c = 8.7120(9) Å. The electronic structure is very similar to both EuIr4In2Ge4 and the parent structure Ca3Ir4Ge4, suggesting that these compounds comprise a new family of hybridization gap materials that exhibit indirect gap, semiconducting behavior at a valence electron count of 60 per formula unit, similar to the Heusler alloys.« less

  12. First-Principles Study on the Structural, Electronic, Magnetic and Thermodynamic Properties of Full Heusler Alloys Co2VZ (Z = Al, Ga)

    NASA Astrophysics Data System (ADS)

    Bentouaf, Ali; Hassan, Fouad H.; Reshak, Ali H.; Aïssa, Brahim

    2017-01-01

    We report on the investigation of the structural and physical properties of the Co2VZ (Z = Al, Ga) Heusler alloys, with L21 structure, through first-principles calculations involving the full potential linearized augmented plane-wave method within density functional theory. These physical properties mainly revolve around the electronic, magnetic and thermodynamic properties. By using the Perdew-Burke-Ernzerhof generalized gradient approximation, the calculated lattice constants and spin magnetic moments were found to be in good agreement with the experimental data. Furthermore, the thermal effects using the quasi-harmonic Debye model have been investigated in depth while taking into account the lattice vibrations, the temperature and the pressure effects on the structural parameters. The heat capacities, the thermal expansion coefficient and the Debye temperatures have also been determined from the non-equilibrium Gibbs functions. An application of the atom in molecule theory is presented and discussed in order to analyze the bonding nature of the Heusler alloys. The focus is on the mixing of the metallic and covalent behavior of Co2VZ (Z = Al, Ga) Heusler alloys.

  13. The effect of disorder on electronic and magnetic properties of quaternary Heusler alloy CoFeMnSi with LiMgPbSb-type structure

    NASA Astrophysics Data System (ADS)

    Feng, Yu; Chen, Hong; Yuan, Hongkuan; Zhou, Ying; Chen, Xiaorui

    2015-03-01

    Thin films based on Heusler alloy often lost their theoretical predicted ultra-high spin polarization owing to the appearance of disorder. Using the first-principles calculations within density functional theory (DFT), we investigate the effect of disorder including antisite and swap on electronic and magnetic properties of quaternary Heusler alloy CoFeMnSi with LiMgPbSb-type structure. Twelve kinds of antisites and six kinds of swap disorders are proposed and studied comprehensively. In our calculations, Co(Fe)-, Mn(Fe)-, Si(Mn)-antisite and Co-Fe swap disorders are most favorable due to their lowest formation energies. Moreover, the positive binding energies of Co-Fe, Co-Si, Fe-Si and Mn-Si swap disorders with respect to their corresponding antisite disorders indicate that these complex swap disorders are more stable compared with their corresponding isolated antisite disorders. The investigations on density of states (DOS) show that the spin down energy gap of disordered structures suffers contraction and their DOS entirely move towards lower zone. Besides, the 100% spin polarization is maintained in all structures with antisite and swap disorders except for those with Co(Mn)-, Co(Si)-antisite and Co-Mn, Co-Si swap disorders. Therefore, the half-metallicity of quaternary Heusler alloy CoFeMnSi is quite robust against interfering effects such as Si(Mn), Co(Fe) and Co-Fe disorders most possibly formed in the growth.

  14. Synthesis and characterization of rare Earth based Fe2CeAg Heusler alloy nanoparticles for ultra-soft magnetic applications

    NASA Astrophysics Data System (ADS)

    Saravanan, G.; Asvini, V.; Ravichandran, K.

    2017-05-01

    Heusler Alloy based Fe2CeAg nanoparticles were synthesized by using coprecipitation method and reducing a mixture of precursors having FeCl2.4H2O, Ce(NO3)3.6H2O and AgNO3.8H2O under N2 atmosphere with various annealing temperatures and time durations. X-ray diffractions studies were used to characterize the crystal structure of Fe2CeAg nanoparticles and magnetic properties were studied using VSM. XRD data analysis confirms the Heusler alloy phase showing the L21 structure. Magnetic properties are measured for synthesized samples and annealed at various temperatures exhibiting an ultra-soft magnetic property, at 700°C annealed for 5h in N2 atmosphere sample having the lowest coercivity (Hc = 58 (Oe), and high saturation magnetization value (Ms = 1.77 emu/g) in comparison with 800°C and 850°C annealed for 5h in N2 atmosphere sample. At 700°C annealed for 5h in N2 atmosphere sample shows a promising material in Heusler alloy formation and also can possible to use in spintronics, magnetic sensors and transducer applications.

  15. Ab initio calculations and synthesis of the off-stoichiometric half-Heusler phase Ni{sub 1-x}Mn{sub 1+x}Sb

    SciTech Connect

    Ekholm, M.; Larsson, P.; Alling, B.; Helmersson, U.; Abrikosov, I. A.

    2010-11-15

    We perform a combined theoretical and experimental study of the phase stability and magnetism of the off-stoichiometric Ni{sub 1-x}Mn{sub 1+x}Sb in the half-Heusler crystal phase. Our work is motivated by the need for strategies to engineer the magnetism of potentially half-metallic materials, such as NiMnSb, for improved performance at elevated temperatures. By means of ab initio calculations we investigate Ni{sub 1-x}Mn{sub 1+x}Sb over the whole composition range 0{<=}x{<=}1 of Ni replacing Mn and show that at relevant temperatures, the half-Heusler phase should be thermodynamically stable up to at least x=0.20 with respect to the competing C38 structure of Mn{sub 2}Sb. Furthermore we find that half-Heusler Ni{sub 1-x}Mn{sub 1+x}Sb retains half-metallic band structure over the whole concentration range and that the magnetic moments of substitutional Mn{sub Ni} atoms display magnetic exchange interactions an order of magnitude larger than the Ni-Mn interaction in NiMnSb. We also demonstrate experimentally that the alloys indeed can be created by synthesizing off-stoichiometric Ni{sub 1-x}Mn{sub 1+x}Sb films on MgO substrates by means of magnetron sputtering.

  16. Suppression of interfacial intermixing between MBE-grown Heusler alloy Ni2MnIn and (0 0 1)InAs or InAs-HEMT structures

    NASA Astrophysics Data System (ADS)

    Bohse, S.; Zolotaryov, A.; Kreuzpaintner, W.; Lott, D.; Kornowski, A.; Stemmann, A.; Heyn, Ch.; Hansen, W.

    2011-05-01

    This paper reports on the application of a thin MgO interlayer as a diffusion barrier between a Ni2MnIn Heusler film and the substrate consisting of either (0 0 1)InAs or a high electron mobility transistor structure with an InAs channel layer. The functionality of the MgO interlayers is studied in dependence of their layer thicknesses. Our studies reveal that MgO interlayers are effective diffusion barriers, which in conjunction with post-growth annealing significantly improve the structural and magnetic properties of the Heusler films. For all as-grown samples, a Curie temperature of 170 K was found indicating that the Ni2MnIn films are crystallized in the B2 phase. Post-growth annealing for 15 h at 350 °C of samples with MgO layer thicknesses smaller than 3 nm leads to a strong decrease in magnetisation. This film degradation may be attributed to the intermixing of the Heusler films with substrate material through not-completely closed MgO films. For samples with a MgO interlayer thickness of 3 nm, the Curie temperature increases up to 300 K. This Curie temperature is close to the value reported for bulk Ni2MnIn films in the desired L21 phase. Furthermore, an increase in saturation magnetisation by a factor of 2.4 was observed.

  17. Structure and magnetic properties of Co2(Cr1-xFex)Al, (0 ≤ x ≤ 1) Heusler alloys prepared by mechanical alloying

    NASA Astrophysics Data System (ADS)

    Srivastava, Yogesh; Vajpai, Sanjay Kumar; Srivastava, Sanjay

    2017-07-01

    In the present study, a series of nanocrystalline Co2(Cr1-xFex)Al Heusler alloy powders were successfully prepared by high energy ball milling and the effect of substitution of Fe for Cr on the microstructure and magnetic properties was investigated in detail. The Co2CrAl alloy powder consisted of only A2 type disordered structure whereas the substitution of Cr by Fe led to the appearance of increasing amounts of B2 type disordered structure along with A2 type structure. All the Co2(Cr1-xFex)Al Heusler alloy powders demonstrated high spontaneous magnetization together with a very small hysteresis losses. The saturation magnetization, remanence, coercivity, and Curie temperature increased with increasing Fe content. The increasing magnetization with increasing Fe content was attributed to the replacement of antiferromagnetic Cr by strongly ferromagnetic Fe and an increasing amounts of relatively more ordered, atomically as well as ferromagnetically, B2 structure as compared to that of A2 phase. The increment in remanence and coercivity with increasing Fe content were associated with the variation in microstructural characteristics, such as grain size, lattice defects, and the presence of small amounts of magnetic/nonmagnetic secondary phases. The increment in Curie temperature with increasing Fe content was attributed to the enhancement of d-d exchange interaction due to the possible occupancy of vacant sites by Fe atoms. All the Heusler alloys indicated extremely low magnetic anisotropy and the relative anisotropy decreased with increasing Fe content.

  18. Lattice thermal conductivity of TixZryHf1 -x -yNiSn half-Heusler alloys calculated from first principles: Key role of nature of phonon modes

    NASA Astrophysics Data System (ADS)

    Eliassen, Simen N. H.; Katre, Ankita; Madsen, Georg K. H.; Persson, Clas; Løvvik, Ole Martin; Berland, Kristian

    2017-01-01

    In spite of their relatively high lattice thermal conductivity κℓ, the X NiSn (X =Ti , Zr, or Hf) half-Heusler compounds are good thermoelectric materials. Previous studies have shown that κℓ can be reduced by sublattice alloying on the X site. To cast light on how the alloy composition affects κℓ, we study this system using the phonon Boltzmann-transport equation within the relaxation time approximation in conjunction with density functional theory. The effect of alloying through mass-disorder scattering is explored using the virtual crystal approximation to screen the entire ternary TixZryHf1 -x -yNiSn phase diagram. The lowest lattice thermal conductivity is found for the TixHf1 -xNiSn compositions; in particular, there is a shallow minimum centered at Ti0.5Hf0.5NiSn with κℓ taking values between 3.2 and 4.1 W/mK when the Ti content varies between 20% and 80%. Interestingly, the overall behavior of mass-disorder scattering in this system can only be understood from a combination of the nature of the phonon modes and the magnitude of the mass variance. Mass-disorder scattering is not effective at scattering acoustic phonons of low energy. By using a simple model of grain boundary scattering, we find that nanostructuring these compounds can scatter such phonons effectively and thus further reduce the lattice thermal conductivity; for instance, Ti0.5Hf0.5NiSn with a grain size of L =100 nm experiences a 42% reduction of κℓ compared to that of the single crystal.

  19. Panoscopically optimized thermoelectric performance of a half-Heusler/full-Heusler based in situ bulk composite Zr(0.7)Hf(0.3)Ni(1+x)Sn: an energy and time efficient way.

    PubMed

    Bhardwaj, A; Chauhan, N S; Sancheti, Bhagyashree; Pandey, G N; Senguttuvan, T D; Misra, D K

    2015-11-28

    All scale hierarchical architecturing, matrix/inclusion band alignment and intra-matrix electronic structure engineering, the so called panoscopic approach for thermoelectric materials has been demonstrated to be an effective paradigm for optimizing high ZT. To achieve such hierarchically organized microstructures, composition engineering has been considered to be an efficient strategy. In this work, such a panoscopic concept has been extended to demonstrate for the first time in the case of half-Heusler based thermoelectric materials via a composition engineering route. A series of new off-stoichiometric n-type Zr0.7Hf0.3Ni1+xSn (0 ≤x≤ 0.10) HH compositions have been modified to derive HH(1 -x)/full-Heusler (FH)(x) composite with an all scale hierarchically modified microstructure with FH inclusions within the matrix to study the temperature dependent thermoelectric properties. The structural analysis employing XRD, FE-SEM and HR-TEM of these materials reveal a composite of HH and FH, with hierarchically organized microstructures. In such a submicron/nano-composite, the electronic properties are observed to be well optimized yielding a large power factor; α(2)σ (∼30.7 × 10(-4) W m(-1) K(-2) for Zr0.7Hf0.3Ni1.03Sn) and reduced thermal conductivity (∼2.4 W m(-1) K(-1) for Zr0.7Hf0.3Ni1.03Sn) yielding a high ZT∼ 0.96 at 773 K for composition Zr0.7Hf0.3Ni1.03Sn which is ∼250% larger than the normal HH Zr0.7Hf0.3NiSn (ZT∼ 0.27 at 773 K). The enhancement in ZT of these composites has been discussed in terms of primary electron filtering, electron injection and several phonon scattering mechanisms such as alloy scattering, point defect scattering, and grain boundary scattering. The Bergman and Fel model is used to calculate effective thermoelectric parameters of these composites for comparing the experimental results.

  20. Polybenzimidazole compounds

    DOEpatents

    Klaehn, John R.; Peterson, Eric S.; Wertsching, Alan K.; Orme, Christopher J.; Luther, Thomas A.; Jones, Michael G.

    2010-08-10

    A PBI compound that includes imidazole nitrogens, at least a portion of which are substituted with an organic-inorganic hybrid moiety. At least 85% of the imidazole nitrogens may be substituted. The organic-inorganic hybrid moiety may be an organosilane moiety, for example, (R)Me.sub.2SiCH.sub.2--, where R is selected from among methyl, phenyl, vinyl, and allyl. The PBI compound may exhibit similar thermal properties in comparison to the unsubstituted PBI. The PBI compound may exhibit a solubility in an organic solvent greater than the solubility of the unsubstituted PBI. The PBI compound may be included in separatory media. A substituted PBI synthesis method may include providing a parent PBI in a less than 5 wt % solvent solution. Substituting may occur at about room temperature and/or at about atmospheric pressure. Substituting may use at least five equivalents in relation to the imidazole nitrogens to be substituted or, preferably, about fifteen equivalents.

  1. Polybenzimidazole compounds

    SciTech Connect

    Klaehn, John R; Peterson, Eric S; Orme, Christopher J; Jones, Michael G; Wertsching, Alan K; Luther, Thomas A; Trowbridge, Tammy L

    2011-11-22

    A PBI compound includes imidazole nitrogens at least a portion of which are substituted with a moiety containing a carbonyl group, the substituted imidazole nitrogens being bonded to carbon of the carbonyl group. At least 85% of the nitrogens may be substituted. The carbonyl-containing moiety may include RCO--, where R is alkoxy or haloalkyl. The PBI compound may exhibit a first temperature marking an onset of weight loss corresponding to reversion of the substituted PBI that is less than a second temperature marking an onset of decomposition of an otherwise identical PBI compound without the substituted moiety. The PBI compound may be included in separatory media. A substituted PBI synthesis method may include providing a parent PBI in a less than 5 wt % solvent solution. Substituting may use more than 5 equivalents in relation to the imidazole nitrogens to be substituted.

  2. Structural and magnetic properties of epitaxial thin films of the equiatomic quaternary CoFeMnSi Heusler alloy

    NASA Astrophysics Data System (ADS)

    Bainsla, Lakhan; Yilgin, Resul; Okabayashi, Jun; Ono, Atsuo; Suzuki, Kazuya; Mizukami, Shigemi

    2017-09-01

    We report the structural and magnetic properties of CoFeMnSi equiatomic quaternary Heusler alloy thin films. The epitaxial growth of the single-crystalline films with full B 2 and partial L 21 ordering on the Cr-buffered MgO(001) substrate was achieved using the in situ postannealing at temperature (Ta) of 500-600°C. A saturation magnetization value of about 3.5 μB/f .u . (where f.u. represents formula unit) was obtained at room temperature for the sample with Ta=600°/C, which is very close to the value of 3.7 μB/f .u . reported previously for a bulk sample. Ferromagnetic resonance unveiled that negligible extrinsic relaxation due to magnetic softness as well as small intrinsic spin relaxation, i.e., the Gilbert damping constant of 0.005, which is smaller than that of permalloy. The spin magnetic moments for Co, Fe, and Mn atoms for the sample with Ta=600°C deduced from the x-ray magnetic circular dichroism were in agreement with those previously reported in the bulk sample, the effects of Fe-Mn swapping and Mn antisite disorder were discussed.

  3. Vortex spin-torque oscillator using Co2FexMn1 -xSi Heusler alloys

    NASA Astrophysics Data System (ADS)

    Yamamoto, Tatsuya; Seki, Takeshi; Takanashi, Koki

    2016-09-01

    We show spin-transfer-torque-driven vortex oscillations in current-perpendicular-to-plane giant magnetoresistance junctions using epitaxially grown Co2FexMn1 -xSi (CFMS) Heusler alloy thin films. The soft magnetic property and high spin polarization of CFMS enable us to realize vortex oscillation emitting large microwave power with a low threshold current. The output power is maximized for a certain Fe-Mn composition ratio associated with a reduction of the threshold current for the oscillation, which is in agreement with a general model for spin-torque oscillation. Through comparison with an analytical theory that describes the translational motion of a vortex core, we show that the vortex core motion excited in the present device is inhomogeneous along the thickness direction. In spite of the inhomogeneity, the gyration radius at the CFMS/spacer interface region was estimated to be ˜75 % of the actual ferromagnetic layer radius, which indicates that the CFMS-based all-metallic junction is useful for achieving large-amplitude vortex core motion. This comprehensive investigation would also be useful for designing high-performance all-metallic nano-oscillators based on magnetic vortex dynamics.

  4. Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure.

    PubMed

    Berry, T; Ouardi, S; Fecher, G H; Balke, B; Kreiner, G; Auffermann, G; Schnelle, W; Felser, C

    2017-01-04

    The thermoelectric properties of the n-type semiconductor TiNiSn were optimized by partial substitution with metallic MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the Ti1-xMnxNiSn1-xSbx system. The alloys were prepared by arc-melting and annealed at temperatures obtained from differential thermal analysis and differential scanning calorimetry results. The phases were characterized using powder X-ray diffraction patterns, energy-dispersive X-ray spectroscopy, and differential scanning calorimetry. After annealing, the majority phase was TiNiSn with some Ni-rich sites, and the minority phases were primarily Ti6Sn5, Sn and MnSn2. The Ni-rich sites were caused by Frenkel defects; this led to metal-like behavior in the semiconductor specimens at low temperature. For x ≤ 0.05 the samples showed an activated conduction, whereas for x > 0.05 they showed metallic character. The figure of merit for x = 0.05 was increased by 61% (zT = 0.45) in comparison with the pure TiNiSn.

  5. Tuning the magnetocaloric properties of the Ni2+xMn1-xSn Heusler alloys

    NASA Astrophysics Data System (ADS)

    Mukadam, M. D.; Yusuf, S. M.; Bhatt, Pramod

    2013-05-01

    We report the effect of Ni substitution on the magnetic properties of polycrystalline Ni2+xMn1-xSn (x = 0, 0.05, and 0.1) Heusler alloys using the magnetization and neutron diffraction measurement techniques. The paramagnetic to ferromagnetic transition temperature (Curie temperature, TC) has been tuned with the substitution of Ni at the Mn sites (TC≈ 349, 337, and 317 K for x = 0, 0.05, and 0.1 samples, respectively) without a significant reduction in the magnetic entropy change -ΔSM. For a magnetic field change from 0 to 5 T, -ΔSM of 2.9, 2.5, and 2.2 J kg-1 K-1 have been observed for x = 0, 0.05, and 0.1 samples, respectively. From the neutron diffraction study, it has been found that with increasing x, the Mn site ordered moment decreases. -ΔSM as a function of changing magnetic field and Curie temperature follows the molecular mean field model. The studied Ni2+xMn1-xSn alloys, with their nontoxic constituent elements and low-cost, can be used for magnetic cooling over a wide temperature range of 278-379 K covering room temperature.

  6. First-principles study of martensitic transformation and magnetic properties of carbon doped Ni-Mn-Sn Heusler alloys

    NASA Astrophysics Data System (ADS)

    Xiao, Haibo; Yang, Changping; Wang, Ruilong; Xu, Linfang; Liu, Guozhen; Marchenkov, V. V.

    2016-10-01

    The magnetic properties, structural stabilities and martensitic transformation of carbon doped Ni-Mn-Sn Heusler alloys are investigated by means of ab initio calculations in framework of the density functional theory. The results of calculations have shown that the martensitic transformation can be realized in all series of carbon doped Ni2Mn1.5Sn0.5 - xCx alloys with tetragonal ratio of 1.34, 1.40,1.42 and 1.44, respectively for x = 0.125 , 0.25 , 0.375 and 0.5. The DOS peak at the Fermi level almost disappearing in the tetragonal phase near the Fermi level is the evidence of triggering martensitic transformation which is due to the structural Jahn-Teller effect. We have also found that the difference between the austenitic and martensitic phases increases with increasing carbon content, which implies an enhancement of the martensitic phase transition temperature (TM). Besides, the electron density difference shows the enhancement of bonding between Mn and carbon atoms with the distortion taken place.

  7. Large field-induced irreversibility in Ni-Mn based Heusler shape-memory alloys: A pulsed magnetic field study

    NASA Astrophysics Data System (ADS)

    Nayak, A. K.; Mejia, C. Salazar; D'Souza, S. W.; Chadov, S.; Skourski, Y.; Felser, C.; Nicklas, M.

    2014-12-01

    We present a pulsed magnetic field study on the magnetic and magnetostriction properties of Ni-Mn-Z (Z =In , Sn, and Sb) based Heusler shape-memory alloys. These materials generally display a field-induced magnetostructural transition that could lead to an irreversible phase transition, when measured near the martensitic transition temperature. Here, we show that independently of the transition temperature, the critical field for the phase transition sensitively depends on the main-group element in the sample. Irrespective of their compositions, all samples display a magnetization of around 2 μB/f .u . in the martensite phase and about 6 μB/f .u . in the cubic austenite phase. Our magnetic and magnetostriction measurements at low temperatures exhibit a partial or complete arrest of the high-field austenite phase below the reverse martensitic transition. This results in a large irreversibility with a hysteresis width as high as 24 T. We introduce a theoretical model to discuss the experimental results.

  8. Galvanomagnetic properties of Fe{sub 2}YZ (Y = Ti, V, Cr, Mn, Fe, Ni; Z = Al, Si) heusler alloys

    SciTech Connect

    Kourov, N. I. Marchenkov, V. V.; Belozerova, K. A.; Weber, H. W.

    2015-11-15

    The Hall effect and the magnetoresistance of Fe{sub 2}YZ Heusler alloys, where Y = Ti, V, Cr, Mn, Fe, and Ni, are the 3d transition metals and Z = Al and Si are the s, p elements of the third period of the periodic table, are studied at T = 4.2 K in magnetic fields H ≤ 100 kOe. It is shown that, in the high-field limit (H > 10 kOe), the value and the sign of the normal (R{sub 0}) and anomalous (R{sub s}) Hall coefficients change anomalously during transition from paramagnetic (Y = Ti, V) to ferromagnetic (Y = Cr, Mn, Fe, Ni) alloys. These coefficients have different signs for all alloys. Constant R{sub s} in the ferromagnetic alloys is positive, proportional to the residual resistivity ratio (R{sub s} ∝ ρ{sub 0}{sup 3.1}), and inversely proportional to spontaneous magnetization. The magnetoresistance of the alloys is a few percent and has a negative sign. A positive addition to transverse magnetoresistance is only detected in high magnetic fields, H > 10 kOe.

  9. Effect of Fe substitution by Co on off-stoichiometric Ni-Fe-Co-Mn-Sn Heusler alloy ribbons

    NASA Astrophysics Data System (ADS)

    Mishra, S. S.; Mukhopadhyay, Semanti; Yadav, T. P.; Yadav, R. M.; Radhakrishnan, Sruthi; Vajtai, R.; Ajayan, P. M.; Mukhopadhyay, N. K.; Singh, H. K.; Srivastava, O. N.

    2017-08-01

    We have synthesized Ni45Fe5-X Co X Mn40Sn10 Heusler alloy with different Co doping and studied the effect on the structural and magnetic properties of Ni45Fe5-X Co X Mn40Sn10 (at. X  =  0, 2.5, 5) ribbons. X-ray diffraction, scanning and transmission electron microscopic characterization reveal the structural/microstructural features in melt-spun ribbons of different compositions. A significant transformation in the crystal structure has been observed in Fe substituted ribbons. The crystal structure changes from cubic L21 phase to bi-phasic 4O  +  L21 and 10M  +  L21 modulated phases for the partial and complete substitution of Fe by Co specimens respectively. Williamson-Hall analysis of x-ray diffraction data was used to compute the crystallite size and residual elastic strain. Magnetic properties and magnetic field-induced structural transformation of melt-spun alloy ribbons over a large temperature range of 10 K  ⩽  T  ⩽  500 K were examined. Our results revealed that Fe substitution by Co causes a change in the magnetic behavior which could be ascribed to the increase in the lattice strain as well as a magnetic strain due to high antiferromagnetic fraction.

  10. Galvanomagnetic properties of Fe2YZ (Y = Ti, V, Cr, Mn, Fe, Ni; Z = Al, Si) heusler alloys

    NASA Astrophysics Data System (ADS)

    Kourov, N. I.; Marchenkov, V. V.; Belozerova, K. A.; Weber, H. W.

    2015-11-01

    The Hall effect and the magnetoresistance of Fe2YZ Heusler alloys, where Y = Ti, V, Cr, Mn, Fe, and Ni, are the 3 d transition metals and Z = Al and Si are the s, p elements of the third period of the periodic table, are studied at T = 4.2 K in magnetic fields H ≤ 100 kOe. It is shown that, in the high-field limit ( H > 10 kOe), the value and the sign of the normal ( R 0) and anomalous ( R s ) Hall coefficients change anomalously during transition from paramagnetic (Y = Ti, V) to ferromagnetic (Y = Cr, Mn, Fe, Ni) alloys. These coefficients have different signs for all alloys. Constant R s in the ferromagnetic alloys is positive, proportional to the residual resistivity ratio ( R s ∝ ρ 0 3.1 ), and inversely proportional to spontaneous magnetization. The magnetoresistance of the alloys is a few percent and has a negative sign. A positive addition to transverse magnetoresistance is only detected in high magnetic fields, H > 10 kOe.

  11. Coherent tunnelling conductance in magnetic tunnel junctions of half-metallic full Heusler alloys with MgO barriers.

    PubMed

    Miura, Yoshio; Uchida, Hirohisa; Oba, Yoshihiro; Nagao, Kazutaka; Shirai, Masafumi

    2007-09-12

    We have carried out electronic structure and transport calculations for magnetic tunnel junctions (MTJ) composed of MgO and a half-metallic full Heusler alloy Co(2)MnSi on the basis of the density functional theory and the Landauer formula. We find that the density of states of Co atoms at the Co(2)MnSi/MgO(001) interface shifts toward the higher energy side due to the reduced symmetry, leading to a reduction of the spin polarization at the interface. Furthermore, we show that the majority-spin transmittance as a function of the in-plane wavevector [Formula: see text] has a broad peak centred at [Formula: see text] due to the tunnelling from the Δ(1) channel of Co(2)MnSi, while the transmittance from the Δ(5) channel is three orders of magnitude smaller than that of the Δ(1) channel. These results indicate that coherent tunnelling through the Δ(1) band is dominant also in an MTJ with Co(2)MnSi and an MgO barrier, like in Fe/MgO/Fe(001) MTJ and related systems.

  12. Correlations Between Structural and Magnetic Properties of Co2 FeSi Heusler-Alloy Thin Films

    NASA Astrophysics Data System (ADS)

    Zhu, Weihua; Wu, Di; Zhao, Bingcheng; Zhu, Zhendong; Yang, Xiaodi; Zhang, Zongzhi; Jin, Q. Y.

    2017-09-01

    The structural and magnetic properties are the most important parameters for practical applications of Co-based Heusler alloys. The correlations between the crystallization degree, chemical order, magnetic coercivity, saturation magnetization (MS ), and in-plane magnetic anisotropies are systematically investigated for Co2FeSi (CFS) films fabricated at different temperatures (TS ). XRD shows that the CFS layer changes progressively from a disordered crystal structure into a chemically disordered A 2 structure and further into a chemically ordered B 2 and even L 21 structures when increasing TS up to 480 °C . Meanwhile, the static angular remanence magnetization curves show a clear transition of magnetic anisotropy from twofold to fourfold symmetry, due to the competition effect between the uniaxial anisotropy field HU and biaxial anisotropy field HB . The HU value is found to be weakly dependent on TS , while HB shows a continuous enhancement at TS>300 °C , implying that the enhancement of the L 21 ordering degree would not weaken the biaxial anisotropy. The varying trend of HB is similar to MS , which can be respectively attributed to the improved crystal structure and chemical order. The anisotropic fields and their variation behaviors determined by a vibrating sample magnetometer are highly consistent with the results by a time-resolved magneto-optical Kerr effect study. Our findings provide a better understanding of the structural ordering and magnetic anisotropy, which will be helpful for designing advanced spintronic devices.

  13. Enhancement of magnetic properties of Co2MnSi Heusler alloy prepared by mechanical alloying method

    NASA Astrophysics Data System (ADS)

    Rabie, Naeemeh; Gordani, Gholam Reza; Ghasemi, Ali

    2017-07-01

    Ferromagnetic Heusler alloys of Co2MnSi were synthesized by mechanical alloying method at low temperature. The effect of milling time and annealing process on structural and magnetic properties of ferromagnetic alloy samples were studied by X-ray diffraction, scanning electron microscopy and vibration sample magnetometer methods, respectively. Structural characteristics such as crystallite size, phase percentage, and lattice parameter determined using the Rietveld method. The values of these parameters were obtained 362.9 nm, 5.699 Å and 98.7%, respectively for annealed sample. Magnetization studies show that the Co2MnSi phase is formed at 15 h of milling and is optimized after 20 h of milling. VSM results showed that saturation magnetization (Ms) of milled samples reduces from 112 to 75 (emu/g) with increasing milling time and then increased gradually to 95 emu/g. The effect of post-annealing on the structural and magnetic properties of milled samples was also investigated. The saturation magnetization of annealed sample (120 emu/g) is higher than the optimum milled sample (95 emu/g) due to increasing preferential ordered L21 structure.

  14. Multi-temperature synchrotron PXRD and physical properties study of half-Heusler TiCoSb.

    PubMed

    Skovsen, I; Bjerg, L; Christensen, M; Nishibori, E; Balke, B; Felser, C; Iversen, B B

    2010-11-14

    Phase pure samples of the half-Heusler material TiCoSb were synthesised and investigated. Multi-temperature synchrotron powder X-ray diffraction (PXRD) data measured between 90 and 1000 K in atmospheric air confirm the phase purity, but they also reveal a decomposition reaction starting at around 750 K. This affects the high temperature properties since TiCoSb is semiconducting, whereas CoSb is metallic. Between 90 K and 300 K the linear thermal expansion coefficient is estimated to be 10.5 × 10(-6) K(-1), while it is 8.49 10(-6) K(-1) between 550 K and 1000 K. A fit of a Debye model to the Atomic Displacement Parameters obtained from Rietveld refinement of the PXRD data gives a Debye temperature of 395(4) K. The heat capacity was measured between 2 K and 300 K and a Debye temperature of 375(5) K was obtained from modelling of the data. Coming from low temperatures the electrical resistivity shows a metallic to semiconducting transition at 113 K. A relatively high Seebeck coefficient of ∼-250 μV K(-1) was found at 400 K, but the substantial thermal conductivity (∼10 W mK(-1) at 400 K) leads to a moderate thermoelectric figure of merit of 0.025 at 400 K.

  15. The chiral anomaly and thermopower of Weyl fermions in the half-Heusler GdPtBi.

    PubMed

    Hirschberger, Max; Kushwaha, Satya; Wang, Zhijun; Gibson, Quinn; Liang, Sihang; Belvin, Carina A; Bernevig, B A; Cava, R J; Ong, N P

    2016-11-01

    The Dirac and Weyl semimetals are unusual materials in which the nodes of the bulk states are protected against gap formation by crystalline symmetry. The chiral anomaly, predicted to occur in both systems, was recently observed as a negative longitudinal magnetoresistance (LMR) in Na3Bi (ref. ) and in TaAs (ref. ). An important issue is whether Weyl physics appears in a broader class of materials. We report evidence for the chiral anomaly in the half-Heusler GdPtBi. In zero field, GdPtBi is a zero-gap semiconductor with quadratic bands. In a magnetic field, the Zeeman energy leads to Weyl nodes. We have observed a large negative LMR with the field-steering properties specific to the chiral anomaly. The chiral anomaly also induces strong suppression of the thermopower. We report a detailed study of the thermoelectric response function αxx of Weyl fermions. The scheme of creating Weyl nodes from quadratic bands suggests that the chiral anomaly may be observable in a broad class of semimetals.

  16. The chiral anomaly and thermopower of Weyl fermions in the half-Heusler GdPtBi

    NASA Astrophysics Data System (ADS)

    Hirschberger, Max; Kushwaha, Satya; Wang, Zhijun; Gibson, Quinn; Liang, Sihang; Belvin, Carina A.; Bernevig, B. A.; Cava, R. J.; Ong, N. P.

    2016-11-01

    The Dirac and Weyl semimetals are unusual materials in which the nodes of the bulk states are protected against gap formation by crystalline symmetry. The chiral anomaly, predicted to occur in both systems, was recently observed as a negative longitudinal magnetoresistance (LMR) in Na3Bi (ref. ) and in TaAs (ref. ). An important issue is whether Weyl physics appears in a broader class of materials. We report evidence for the chiral anomaly in the half-Heusler GdPtBi. In zero field, GdPtBi is a zero-gap semiconductor with quadratic bands. In a magnetic field, the Zeeman energy leads to Weyl nodes. We have observed a large negative LMR with the field-steering properties specific to the chiral anomaly. The chiral anomaly also induces strong suppression of the thermopower. We report a detailed study of the thermoelectric response function αxx of Weyl fermions. The scheme of creating Weyl nodes from quadratic bands suggests that the chiral anomaly may be observable in a broad class of semimetals.

  17. Epitaxial growth of Heusler alloy Co{sub 2}MnSi/MgO heterostructures on Ge(001) substrates

    SciTech Connect

    Li Guifang; Taira, Tomoyuki; Matsuda, Ken-ichi; Arita, Masashi; Uemura, Tetsuya; Yamamoto, Masafumi

    2011-06-27

    We prepared Heusler alloy Co{sub 2}MnSi/MgO heterostructures on single-crystal Ge(001) substrates through magnetron sputtering for Co{sub 2}MnSi and electron beam evaporation for MgO as a promising candidate for future generation spin-based functional devices. Structural investigations showed that the Co{sub 2}MnSi/MgO heterostructure was grown epitaxially on a Ge(001) substrate with extremely smooth and abrupt interfaces and showed the L2{sub 1} structure for the Co{sub 2}MnSi film. Furthermore, a sufficiently high saturation magnetization ({mu}{sub s}) value of 5.1 {mu}{sub B}/f.u. at 10 K, which is close to the theoretically predicted {mu}{sub s} of 5.0 {mu}{sub B}/f.u. for half-metallic Co{sub 2}MnSi, was obtained for prepared Co{sub 2}MnSi films.

  18. Investigation of the structural, electronic, and magnetic properties of Ni-based Heusler alloys from first principles

    NASA Astrophysics Data System (ADS)

    Qawasmeh, Yasmeen; Hamad, Bothina

    2012-02-01

    Density functional theory (DFT) calculations are performed to investigate the structural, electronic, magnetic, and elastic properties of Ni2MnZ (Z = B, Al, Ga, In) and Ni2FeZ (Z = Al, Ga) full Heusler alloys. The alloys are found to be metallic ferromagnets with total magnetic moments of about 4μB/f.u. and 3μB/f.u for Ni2MnZ and Ni2FeZ alloys, respectively. The Ni2MnAl and Ni2MnIn alloys are found to be stable at L21 phase, while the other alloys are more stable in the tetragonal phase with c/a ratios of 1.38 and 1.27 for Ni2MnB and Ni2MnGa, respectively and 1.35 for both Ni2FeAl and Ni2FeGa. The Ni2MnB alloy exhibits the highest electron spin polarization in its tetragonal phase, which is about 88% greater than that of L21 structure. However, the Ni2MnGa, Ni2FeAl, and Ni2FeGa alloys exhibit lower spin polarizations in their tetragonal phase than those at the L21. The most contribution of the total magnetic moments comes from Mn or Fe atoms, whereas Ni atoms exhibit much smaller magnetic moments. However, Z atoms have small induced magnetic moments, which are coupled antiferromagnetically with Ni, Mn and Fe.

  19. Structure, Magnetic, and Electrical Properties of Heusler-Type Fe3- x Co x Si Ferromagnetic Alloys

    NASA Astrophysics Data System (ADS)

    Raja, M. Manivel; Kamat, S. V.

    2015-10-01

    The effect of substitution of Co for Fe on structure, magnetic, and electrical resistivity of Heusler-type Fe3- x Co x Si (0 ≤ x ≤ 1) alloys was investigated using X-ray powder diffraction, 57Fe Mössbauer spectroscopy, magnetic, and electrical transport measurements. The results revealed that these alloys consist of ordered DO3 phase and some L21 phase up to x ≤ 0.5. However, for x > 0.5, the alloys consisted of L21 ordered phase and B2 disordered phase. The magnetization value was close to that predicted from Slater-Pauling rule for x ≥ 0.5 alloys. The Curie temperature increased from 832 K (559 °C) for x = 0 (Fe3Si) alloy to 1016 K (743 °C) for x = 1 (Fe2CoSi) alloy. Electrical transport studies revealed the presence of half-metallic behavior at low temperatures in x ≥ 0.5 alloys. No half-metallic behavior was observed for x = 0 and 0.25 alloys; however, a high resistivity with ferromagnetism was observed in these alloys, which is desirable for ferromagnetic metal/semiconductor spintronic devices.

  20. Martensitic transition, magnetic, magnetocaloric and exchange bias properties of Fe-substituted Mn-Ni-Sn Heusler alloys

    NASA Astrophysics Data System (ADS)

    Sharma, Jyoti; Suresh, K. G.

    2016-12-01

    In this report, effect of Fe substitution on martensitic transition, magnetic, magnetocaloric and exchange bias (EB) properties of Mn50Ni40-xFexSn10 (x=0, 0.5, 1, 1.5, 2 and 3) Heusler alloys series has been investigated systematically. Fe substitution has been found to affect the ferromagnetic/antiferromagnetic interactions significantly in both the martensite and austenite phases. Martensitic transition temperature decreases with increasing Fe content, which is attributed to the decrease in number of average valence electrons per atom (e/a ratio) of these alloys. Large magnetic entropy change (ΔSM) and refrigerant capacity (RC) have been observed in these alloys, as a maximum ΔSM of 12.6 J/kg. K is observed for composition x=0.5. Present alloys have also been found to show large exchange bias properties, as maximum exchange bias fields (HEB) of 890 Oe and 810 Oe are observed for x=0 and 0.5, respectively at 5 K. Composition and temperature dependencies of EB are associated with the change in exchange anisotropy at interfaces of competing magnetic phases. Study of minor loop and training effect also corroborates with the presence of EB in these alloys.