L21-type Structure Research Articles

Significant off-stoichiometry effect leading to the N-type conduction and ferromagnetic properties in titanium doped Fe2VAl thin films

Thermoelectric thin films are of great interest for self-powering Internet of Things (IoT) nodes via energy harvesting. Heusler-type Fe2VAl attracts some attention for room-temperature thermoelectric applications. However, the large overall thermal conductivity leads to a poor performance that restricts the current investigations. Herein, we report a comprehensive study of structural, thermoelectric, and magnetic properties of Fe2V0.9Ti0.1Al thin films on quartz substrate, grown by magnetron sputtering. It is found that high-temperature annealing is more effective than the long-time annealing at the low temperatures to stabilize the L21 type structure of Fe2VAl, leading to a much larger Seebeck coefficient and slightly higher thermal conductivity. More importantly, the significant off-stoichiometry phenomenon, namely Fe-rich composition, contributes to n-type conduction of thin films. Both the picosecond and nanosecond time-domain thermoreflectance techniques were employed to successfully measure the thin-film thermal conductivities, merely one-third of bulk material, demonstrating the effectiveness of utilizing nanostructuring to reduce the thermal conductivity of Fe2VAl. Moreover, magnetic susceptibility measurements reveal that thin films are ferromagnets, while the bulk sample is characterized by paramagnetism. Our finding sheds new light on the composition-structure-property relationship of Fe2VAl thin films for future related applications.

Site preference, magnetic and electronic properties of half-metallic Vanadium-based full Heusler alloys

We reported a new atomic site preference occupation for Vanadium-based X2YZ full Heusler alloys. It tells that alloys with less 24 valence electrons form the L21-type structure, while others form the XA-type structure. This fact means that the site preference rule for Vanadium-based alloys depends on the total number of valence electrons instead of the electropositive between X and Y atoms. The magnetic moments of XA-type alloys follow the Slater-Pauling rules, with two forms, Mt = NV − 24 and Mt = NV − 18 corresponding to different origins of the band gaps. XA-type V2YZ (Y = Cr, Mn and Fe; Z = Ga, Ge and As), V2CoGa, V2CoGe and V2NiGa alloys have half-metallic property. L21-type V2YZ (Y = Cu and Zn; Z = Ga, Ge and As) alloys show no spin polarization and no half-metallic property due to the structural symmetry and the non-magnetism of copper and zinc atoms.

Reobservations on ordering, precipitation and polymorphic phase transformation phenomena during annealing of a severely cold rolled magnetic Fe-Co-10V alloy

Despite extensive investigations conducted on magnetic Fe-Co-V alloys, some aspects of microstructural evolution during annealing of cold rolled sheets have not yet been clarified yet. Various techniques such as dilatometry, X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM, SEM) as well as ThermoCalc predictions were employed to study ordering, precipitation and austenite transformation in an 86% cold-rolled 40Fe-50Co-10 V alloy. Dilatometric results revealed atomic ordering initiated from disordered martensite at 350 °C, while XRD patterns ascertained the completion of the process at 600 °C and its disappearance close to 750 °C. X-ray and electron diffraction patterns exhibited the B2 type ordered structure in this temperature range. Rod-shaped precipitates with HCP crystalline structure and the constant composition of (Co,Fe)3 V were detected by SEM within the deformed ferrite grains after annealing in the temperature range of 500–700 °C. The percentage of the precipitates decreased by raising the annealing temperature and they vanished above 700 °C, in accordance with ThermoCalc prediction. Austenite grains formed below 750 °C had an ordered L12-type structure as revealed by TEM and XRD patterns. A combination of different techniques demonstrated that the two-phase ferrite-austenite region exists in the temperature range of 495–840 °C. Vanadium partitioning into austenite during isothermal holding in addition to austenite grain size were evaluated as the main parameters affecting austenite stability during quenching from temperatures between 500 and 800 °C. As a final result, most of the existing disagreements and ambiguities in previous works could be explained.

Electronic structures, magnetic properties, and martensitic transformation in all-d-metal Heusler-like alloys Cd2MnTM (TM = Fe, Ni, Cu)**Project supported by the Natural Science Foundation of Zhejiang Province, China (Grant No. LQ19E010006), the National Natural Science Foundation of China (Grant Nos. 51671048 and 91963123), the Ten Thousand Talents Plan of Zhejiang Province, China (Grant No. 2018R52003), and the Fundamental Research Funds for

The electronic structures, magnetic properties, and martensitic transformation in all-d-metal Heusler-like alloys Cd2MnTM (TM = Fe, Ni, Cu) were investigated by the first-principles calculations based on density-functional theory. The results indicate that all three alloys are stabilized in the ferromagnetic L21-type structure. The total magnetic moments mainly come from Mn and Fe atoms for Cd2MnFe, whereas, only from Mn atoms for Cd2MnNi and Cd2MnCu. The magnetic moment at equilibrium lattice constant of Cd2MnFe (6.36 μB) is obviously larger than that of Cd2MnNi (3.95 μB) and Cd2MnCu (3.82 μB). The large negative energy differences (ΔE) between martensite and austenite in Cd2MnFe and Cd2MnNi under tetragonal distortion and different uniform strains indicate the possible occurrence of ferromagnetic martensitic transformation (FMMT). The minimum total energies in martensitic phase are located with the c/a ratios of 1.41 and 1.33 for Cd2MnFe and Cd2MnNi, respectively. The total moments in martensitic state still maintain large values compared with those in cubic state. The study is useful to find the new all-d-metal Heusler alloys with FMMT.

Structure stability and magnetic properties of Fe-doped Co2VGa Heusler alloys: Insights from first-principles calculations

In this work, we report an interesting phenomenon with respect to the coexistence of martensite phase transformation (MPT) and half-metallic properties in Fe doped Co2VGa Heusler alloys, which is predicted theoretically according to the ab initial calculations based on density functional theory (DFT). The effects of Fe partial substitution for Co, V or Ga atoms on the structural stability, MPT, electronic structure and magnetic properties in Co-V-Ga system alloys have been systematically investigated. It was found that Co2-xFexVGa (x = 0, 0.25, 0.5, 0.75, 1), Co2V0.75Fe0.25Ga and Co2VFe0.25Ga0.75 alloys are stable in highly ordered L21-type structure. The magnetic analysis indicates that the magnetic moment decreases with the increase of Fe content in Co2-xFexVGa (x = 0, 0.25, 0.5, 0.75, 1) alloys, whereas it increases with the increase of Fe content for V or Ga partially replaced by Fe atoms. According to the calculated results, MPT can take place in Co2V0.5Fe0.5Ga and Co2VFe0.5Ga0.5 Heusler alloys. All the studied alloys exhibit high spin polarizability, especially in the alloys of Co or V partially replaced with Fe.

Synthesis and characterization of Pt(Cu0.67Sn0.33)

Pt(Cu0.67Sn0.33) has recently been found in a natural sample. In order to be able to characterize this new ternary compound, we synthesized it from the elements. Samples were characterized by X-ray powder diffraction, differential scanning calorimetry, thermal relaxation calorimetry, and scanning electron microscopy studies. Density functional theory-based model calculations complemented the experimental studies. Pt(Cu0.67Sn0.33) was already formed at a relatively low temperature of 773 K. Rietveld refinement of Pt(Cu0.67Sn0.33) has been carried out in CuAu-type or L10-type structure, space group P4∕mmm, with Pt on 0,0,0 and disordered Cu and Sn on 12, 12, 12 and Z = 1. The lattice parameters are a = 2.823(1) Å, c = 3.64(1) Å, and V = 29.00(4) Å 3; which are in good agreement with values obtained earlier on the natural sample and with the results of DFT calculations. The vibrational entropy for Pt(Cu0.67Sn0.33) is S298.15vib = 79.9(7) J mol−1 K−1. The pressure dependence up to 36(2) GPa of the unit-cell volume and the lattice parameters and unit-cell volume have been obtained by synchrotron based powder diffraction using a diamond anvil cell. A fit of a 3rd-order Birch–Murnaghan equation of state to the Pt(Cu0.67Sn0.33) (p,V)-data results in a bulk modulus of B0 = 215(27) GPa and B′ = 5(2).

The mechanism of magnetostructural transition in Heusler alloy Co2V1.5Ga0.5

Recently, magnetic field induced low magnetization tetragonal martensite to high magnetization cubic austenite transformation has been realized in Heusler alloy Co50V34Ga16 (Applied Physics Letters112, 211903 (2018)). In this paper, first-principles calculations have been performed on Heusler alloy Co2V1.5Ga0.5 to reveal the mechanism for magnetostructural transition. It has been found that the alloy prefers to crystallize into ferromagnetic L21 type structure at austenite, with Co atoms tending to occupy the Wyckoff sites A (0, 0, 0) and C (0.5, 0.5, 0.5), V atoms occupying at site B (0.25, 0.25, 0.25), Ga and the extra V atoms entering D (0.75, 0.75, 0.75) site. Moments of Co and V atoms parallel to each other and a total formula moment of 2.95 μB is achieved. A potential of tetragonal distortion from ferromagnetic cubic structure to non-magnetic tetragonal structure has been predicted from the view of energetically favorable state. The stability of tetragonal Co2V1.5Ga0.5 is further confirmed by the phonon spectrum. The peaks of dx2-y2 and dz2 states for Co and V(D) 3d states near the Fermi level for the cubic structure split at the tetragonal structure, implying the structural transition is mainly attributed to the band Jahn-Teller effect. The hybridization between the Co 3d states and the 3d states of V at D site plays an important role in the martensitic transformation. A volume contraction of 1.3% is obtained accompanying with the magnetostructural transition.

Structural configuration and phase stability in full Heusler alloys Cr2ZnSi and Cr2ZnGe

In Heusler compounds, structural configuration and phase stability are essential for determining their properties. Under this framework, we have systematically investigated the atomic configuration in the full Heusler compounds Cr2ZnSi and Cr2ZnGe and their electronic, magnetic, mechanical and dynamic properties. Both L21-type and XA-type structural configurations have been investigated and the corresponding equilibrium lattice constants in cubic phase have been determined, which are in good consistence with previous studies. Mechanical and dynamic properties have been further investigated and results show that the L21-type structures for both Cr2ZnSi and Cr2ZnGe are both mechanically and dynamically stable whereas the XA-type structures are not. And, strong elastic anisotropy is also revealed with the calculated directional dependent Young’s modulus and shear modulus. Afterwards, the tetragonal phase has been considered and results show that tetragonal XA-type structure can further reduce the total energy with regard to the cubic phase. The minimum total energies under tetragonal phases at c/a ratio of 1.31 and 1.41 are even lower than that of the cubic L21-type structure for both Cr2ZnSi and Cr2ZnGe, respectively, and their mechanical and dynamic stabilities have also been confirmed. This detailed investigation about the structural configuration and phase stability in full Heusler alloys Cr2ZnSi and Cr2ZnGe from different perspectives can be important and helpful for the related studies and inspire other relative works.

Site-Preference, Electronic, Magnetic, and Half-Metal Properties of Full-Heusler Sc2VGe and a Discussion on the Uniform Strain and Tetragonal Deformation Effects

A hypothetical full-Heusler alloy, Sc2VGe, was analyzed, and the comparison between the XA and L21 structures of this alloy was studied based on first-principles calculations. We found that the L21-type structure was more stable than the XA one. Further, the electronic structures of both types of structure were also investigated based on the calculated band structures. Results show that the physical nature of L21-type Sc2VGe is metallic; however, XA-type Sc2VGe is a half-metal (HM) with 100% spin polarization. When XA-type Sc2VGe is at its equilibrium lattice parameter, its total magnetic moment is 3 μ B , and its total magnetism is mainly attributed to the V atom. The effects of uniform strain and tetragonal lattice distortion on the electronic structures and half-metallic states of XA-type Sc2VGe were also studied. All the aforementioned results indicate that XA-type Sc2VGe would be an ideal candidate for spintronics studies, such as spin generation and injection.

Theoretical Study of the Electronic and Magnetic Properties and Phase Stability of the Full Heusler Compound Pd2CoAl

Based on first principles calculation, a systematical investigation has been performed to study the electronic, magnetic, dynamic, and mechanical properties of the full Heusler compound Pd2CoAl. It is found that the L21-type structure is energetically more stable than the XA-type due to the lower total energy. The obtained lattice constant in cubic ground state is 6.057 Å, which matches well with previous study. The calculated electronic band structure reveals the metallic nature of Pd2CoAl and its total magnetic moment of 1.78 μB is mainly contributed by Co atom from strong spin splitting effect, as indicated with the distinctive distributions of the density of states in two spin directions. Under uniform strains from −5% to +5%, the variation of total magnetic moment has been obtained and it is still caused by the much larger change from Co atom, compared with Pd and Al atoms. The tetragonal structure has further been analyzed and we found that there is possible martensitic phase transformation because the total energy can be further reduced when the cubic structure is varied into the tetragonal one. The large energy difference of 0.165 eV between the tetragonal and cubic phases is found at the c/a ratio of 1.30. The total density of states has been compared between the cubic and tetragonal phases for Pd2CoAl and results show tetragonal phase transformation could reduce the states at the Fermi energy level in both directions. In addition, the dynamic and mechanical stabilities have also been evaluated for Pd2CoAl in both cubic and tetragonal structures and results confirm that the tetragonal phase shows good stability against the cubic phase, which further verifies that the tetragonal phase transformation is highly expected. In the end, the strong elastic anisotropy in the tetragonal structure has been clearly shown with the calculated directional dependence of the Young’s modulus and shear modulus.

Short-range ordering in a commercial Ni-Cr-Al-Fe precision resistance alloy

Short-range order has been thought to be the primary reason of the abnormal increase of electrical resistivity after aging treatment in the Ni-20Cr based precision resistance alloys. However, there is no report on the microstructures of this alloy, to say nothing of the lattice type of the short-range order. In this work, the microstructure of a commercial Ni-20Cr-3.2Al-2.5Fe precision resistance alloy was investigated using X-ray diffraction, transmission electron microscopy and atom probe tomography techniques. A random solid solution was initially formed after quenching treatment. Short-range order with the L12-type structure, which was formed due to aging treatment, resulted in a decrease in lattice parameter and an increase in electrical resistivity. The short-range order microdomains distributed in a random matrix coherently with the same composition.

Coexistence of half-metallicity and martensitic transition in Co2VGa1−xSbx (x = 0, 0.25 and 0.5) Heusler alloys: First-principles calculations combined with structural experiments

Herein, we have investigated, theoretically and experimentally, the influence of Sb-doping, at Ga-site, on crystal structure, martensitic transition (MT) and half-metallicity of Co2VGa1−xSbx (x = 0, 0.25 and 0.5) Heusler alloys. The results reveal that the Co-V-Ga-Sb alloys with x = 0 and 0.25 exhibit typical half-metallicity, and at x = 0.25 the density of states (DOS) show a wider band gap near the Fermi level in minority states, which implies that Co2VGa0.75Sb0.25 is an efficient material with 100% spin polarization. Moreover, Co2VGa0.5Sb0.5 exhibits a martensitic transformation from parent phase with L21-type cubic structure to the D022-type tetragonal structure at ∼347 K. In addition, the DOS of Co2VGa0.5Sb0.5 also exhibit half-metallicity after expanding the lattice parameter by 2%. The unique features of Co2VGa1−xSbx Heusler alloys imply that these alloys are promising for high-efficiency spintronics and shape memory devices.

Calculation of magnetic moments and lattice parameters Co-based Heusler alloys with determination of their energy favorable structure

In this work we present results of ab-initio investigation atomic structures and magnetic properties of Cobalt-based full-Heusler compounds with composition Co2YZ by using VASP software package. The lattice parameters of Co2FeA1(CFA), Co2FeSi(CFS), Co2FeAl0.5Si0.5 (CFAS) Heusler alloys depending on the parameters of convergence are calculated. Investigation of CFA and CFS alloys in B2-type and L21-type structures with determination of their energy efficiency of the formation are carried out. The values of the magnetic moment of CFAS atoms in the bulk and in CFAS/Ag(100)/CFAS multilayer structure taking into account the relaxation effects are determined.

Competition of L21 and L10 ordering in Pd2MnSe, Pd2FeSe, Pd2MnTe and Pd2FeTe Heusler alloys

Abstract Based on first-principles method, for Pd2MnSe, Pd2FeSe, Pd2MnTe and Pd2FeTe, we investigated the competition between cubic L21-type and their tetragonal L10-type ordering. Firstly, low-energy tetragonal ground states are found in these alloys during the tetragonal deformation. By regulating the c/a ratio under the fixed volume of equilibrium cubic phase, we obtain two minimums in total energy-c/a curve: a shallow one in c/a 1. Meanwhile the uniform strain is also taken into consideration in this work, and we found that ΔEM and Vopt + X%Vopt are negatively correlated, that is, with the volume increases from Vopt − 3%Vopt to Vopt + 3%Vopt, the absolute value of ΔEM decreases accordingly. Secondly, the origin of the tetragonal ground states of Pd2 [Fe/Mn][Se/Te] are also be explained by the density of states (DOS) figures. From the valley-and-peak structure, we can find the DOS at or in the vicinity of the Fermi level is much broader and shallower in tetragonal L10 state than cubic L21 state, indicating the phase stability of tetragonal L10 phases of these Pd2-based alloys. Moreover, to further study the competition of L21 and L10 ordering in these alloys and verify the stability of the tetragonal L10 state, elastic constants were introduced in this work, C11, C12 and C44 for cubic state while C11, C12, C13, C33, C44, C66 for tetragonal L10-type structure. According to the judgment basis, all alloys exhibit L10-stable, satisfying our conclusions about the stability of tetragonal states. We hope our work can provide a guidance for researchers to further explore and study new magnetic functional tetragonal materials among the full-Heusler alloys.

Magnetic and martensitic transformation properties under ambient and hydrostatic pressures of Ni50Mn25.2Fe2.8Ga22 ferromagnetic shape memory alloy

The magnetic and martensitic transformation properties of Ni50Mn25.2Fe2.8Ga22 ferromagnetic shape memory alloy were systematically investigated. The Curie temperature TC and the martensitic transformation temperature TM were determined. The temperature variations of the lattice parameters were measured by using an in situ X-ray powder diffractometer. This alloy crystallizes in cubic L21-type structure for the parent phase and the martensitic transformation was observed to start at 294 K with a small volume change ΔV. The effect of hydrostatic pressure on TC and TM was investigated by measuring the temperature dependence of the initial permeability at various pressures up to 9.4 kbar. It was found that TM increases with increasing pressure. By use of the Clausius-Clapeyron equation, the values of dTM/dp and ΔV determined in this study were found to be consistent with the result of the entropy change ΔS, which was estimated by thermoanalysis. On the other hand, TC was found to slightly decrease with increasing pressure, which is a unique behavior compared to other Ni2MnZ (Z = Ga, In, Sn, Sb) alloys. On the basis of the experimental results, the exchange mechanism of the Ni50Mn25.2Fe2.8Ga22 alloy is discussed.

171 Scandium-based full Heusler compounds: A comprehensive study of competition between XA and L21 atomic ordering

The site-preference rule (SPR), which is widely used to design and study the properties of full-Heusler alloys X2YZ, is applied to determine the positions of different transition-metal elements with various numbers of valence electrons in Heusler alloys. The scandium-based full Heusler alloys should form an XA structure according to the SPR, because the transition-metal element Sc with fewer valence electrons tends to occupy the Wyckoff sites A(0, 0, 0) and B(0.25, 0.25, 0.25). As opposed to previous investigations, L21-type scandium-based full Heusler alloys, which do not follow the SPR, are also considered in this work. Theoretical computations are used to study the atomic-site preferences of 171 scandium-based full Heusler alloys, and the results indicate that most of these alloys form L21-type structures rather than XA-type structures. Two possible orderings are also studied in terms of their difference in charge density. Furthermore, the ground state, electronic structure, and magnetic characteristics of these alloys are also investigated in detail in both XA and L21 structures. The results reveal dramatic differences between XA- and L21-type alloys. This study thus presents a counterexample of the SPR rule and should provide significant guidance for the future design of full-Heusler alloys.

Lattice dynamics of Ru2FeX (X = Si, Ge) Full Heusler alloys

In present work, the lattice dynamics of Ru2FeX (X = Si, Ge) full Heusler alloys are investigated using density functional theory (DFT) within generalized gradient approximation (GGA) in a plane wave basis, with norm-conserving pseudopotentials. Phonon dispersion curves and phonon density of states are obtained using first-principles linear response approach of density functional perturbation theory (DFPT) as implemented in Quantum ESPRESSO code. Phonon dispersion curves indicates for both Heusler alloys that there is no imaginary phonon in whole Brillouin zone, confirming dynamical stability of these alloys in L21 type structure. There is a considerable overlapping between acoustic and optical phonon modes predicting no phonon band gap exists in dispersion curves of alloys. The same result is shown by phonon density of states curves for both Heusler alloys. Reststrahlen band for Ru2FeSi is found smaller than Ru2FeGe.

A new tetragonal phase in CoFeCrGe Heusler alloy

CoFeCrGe ribbons were prepared by melt spinning and then annealed at 300°C for 4h and 500°C for 4h, respectively. The as-spun ribbons of CoFeCrGe have the B2 type structure with a=0.287nm. The CoFeCrGe ribbons annealed at 300°C are composed mainly of the L21 type structure with a=0.578nm in partial disorder and the B2 type structure with a=0.289nm in further disorder case. The ribbons annealed at 500°C are composed of three compounds with average compositions of Co26.0Fe26.5Cr23.5Ge24.0, Co3.8Fe6.4Cr68.5Ge21.3 and Co53.4Fe30.4Cr8.6Ge7.6, listed in a sequence of volume percentage from major to minor. The primary compound Co26.0Fe26.5Cr23.5Ge24.0 is the L21-type structure with a=0.571nm in partial disorder and the B2 type structure with a=0.286nm in further disorder case. The secondary compound Co3.8Fe6.4Cr68.5Ge21.3 has a cubic Cr3Ge structure with a=0.461nm. The tertiary compound Co53.4Fe30.4Cr8.6Ge7.6 is a new tetragonal structure with lattice parameters a=0.76nm, c=0.284nm, which were determined by using tilt-series electron diffraction technique in this work. The tertiary (Co, Fe)-rich phase is usually embedded in the matrix of the secondary Cr-rich phase, but there is no special orientation between the two phases. The difference in the magnetic hysteresis loops of the samples annealed at 300°C and 500°C has been interpreted as the appearance of the new tetragonal crystalline phase. Element doping is the possible way for further developing the single tetragonal phase or increasing the amount of this phase.

L2₁ and XA Ordering Competition in Hafnium-Based Full-Heusler Alloys Hf₂VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb).

For theoretical designing of full-Heusler based spintroinc materials, people have long believed in the so-called Site Preference Rule (SPR). Very recently, according to the SPR, there are several studies on XA-type Hafnium-based Heusler alloys X2YZ, i.e., Hf2VAl, Hf2CoZ (Z = Ga, In) and Hf2CrZ (Z = Al, Ga, In). In this work, a series of Hf2-based Heusler alloys, Hf2VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb), were selected as targets to study the site preferences of their atoms by first-principle calculations. It has been found that all of them are likely to exhibit the L21-type structure instead of the XA one. Furthermore, we reveal that the high values of spin-polarization of XA-type Hf2VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb) alloys have dropped dramatically when they form the L21-type structure. Also, we prove that the electronic, magnetic, and physics nature of these alloys are quite different, depending on the L21-type or XA-type structures.

Uniaxial pressure effect on magnetization process of ferromagnetic domains in a tensile deformed Pt3Fe antiferromagnet

Pt3Fe antiferromagnet with L12-type structure exhibits ferromagnetism even at room temperature after plastic deformation. We have studied effects of elastic uniaxial stress along the [100] axis on magnetization hysteresis loops for a Pt3Fe single crystal with 11.6% plastic strain, in order to investigate magnetic nature of ferromagnetic domains around the {111} slip plane under uniaxial pressure. We observed an exchange bias effect below TN1 ∼ 170 K up to the measured maximum pressure of 150 MPa and found that the exchange bias field is almost constant at low uniaxial pressures, but it steeply increases above 100 MPa and shows ∼ 6% increase at 150 MPa. Correspondingly, a slight increase of coercivity above 100 MPa was observed. These observations indicate the strengthening of interfacial coupling between ferromagnetic domains and an antiferromagnetic matrix, and/or modification of deformation-induced magnetic anisotropy under uniaxial pressure.