Heusler合金Ni2MnGa磁性微观机理的第一性原理

采用量子力学计算软件包研究了单晶Heusler合金Ni2MnGa的结构参数、磁矩、四方变形以及磁性微观机理,并将计算结果与实验值和其它理论计算值进行了比较.计算所得的结构参数和磁矩相对于局域自旋密度近似值而言,与实验值符合得更好.四方变形中,发现磁矩在c/a=1附近有一个尖锐的最小值,在c/a≈0.94处有一个局域最大值,此磁矩的局域最大值对应着一个稳定的马氏体.总磁矩随c/a的变化主要来自Ni原子:由于一个原胞中有两个Ni原子对其总磁矩作了贡献,因而屏蔽了Mn原子的贡献所致.对总态密度的分析表明,自旋向上的态密度位于费米面以下,而自旋向下的态密度有两个主峰,分别位于费米面两侧,这是磁有序合金的一个典型特性.且合金磁性主要源于Mn原子的d-eg和d-t2g亚带,这与实验结果一致.     

热处理工艺对Heusler型Ni2MnGa合金马氏体相变的影响

采用电弧熔炼法熔炼Ni48Mn31Ga21和Ni50Mn255Ga25两种合金,并对这两种合金进行了热处理,借助交流磁化率测定、金相显微镜观察、X射线衍射等手段研究了热处理工艺条件对非化学计量比的Ni48Mn31Ga21合金马氏体相变和结构有序度的影响.结果表明(1)Ni48Mn31Ga21合金相变特征温度为Ms=305K,Mf=297K,As=313K,Af=318K,居里温度Tc=366K;(2)与空冷态相比,淬火态的马氏体转变量降低,经过400℃退火后,随着保温时间t的延长,马氏体转变量上升,当t=50h时达到最大值;(3)Ni48Mn31Ga21合金经过900℃×4d+400℃×50h热处理后,有序度明显提高.     

Modeling of the magnetocaloric effect in Heusler Ni2MnGa alloy: Ab initio calculations and Monte Carlo simulations

The ab initio calculations, based on Density Functional Theory(DFT) approach and Full potential Linear Augmented Plane Wave (FLAPW) method, are used to investigate the electronic and magnetic properties of Heusler Ni₂MnGa alloy. The magnetic moment considered to lie along (001) axes is computed. The obtained value of magnetic moment is used as input for Monte Carlo simulations to calculate the magnetic parameters. The Monte Carlo simulation is also used to study the magnetic properties of Ni₂MnGa such as the Curie temperature T_C(K). The phase transition becomes second-order at this point of T_C(K). The heat specific and magnetic entropy are given. The magnetic entropy change and the adiabatic temperatures are deduced for a several magnetic field. Finally, we have determined the relative cooling power.     

FMR study of thin films of Co2Cr0.6Fe0.4Al and Co2MnSi Heusler alloys

The method of ferromagnetic resonance (FMR) was used to study magnetic properties of thin films of half-metallic ferromagnetic Heusler alloys Co₂Cr_0.6Fe_0.4Al and Co₂MnSi depending on the film thickness and the presence or absence of a vanadium buffer layer. It is shown that the FMR method is a highly efficient technique for studying nanoscale magnetic properties of thin films, especially for the investigation of their magnetic inhomogeneities and anisotropy. Samples of Co₂Cr_0.6Fe_0.4Al and Co₂MnSi were prepared by magnetron-sputtering deposition on substrates of single-crystal silicon dioxide (SiO₂) with an orientation (100). It has been shown that the magnetic properties of thin Co₂Cr_0.6Fe_0.4Al films strongly depend on both the film thickness (25 or 100 nm) and the presence of an intermediate vanadium layer (50 nm). Well-resolved spin-wave modes were observed in the sample 100 nm thick without a vanadium buffer layer, which made it possible to determine the parameter of spin stiffness D for this ferromagnet. Two series of thin films of Co₂MnSi have also been studied, which were prepared on a buffer layer of vanadium (42 nm thick): (1) with various thicknesses (4–100 nm) and a fixed annealing temperature (450°C) and (2) with a fixed thickness (80 nm) and various annealing temperatures (425–550°C). It has been shown that in the series of Co₂MnSi films with a variable thickness (4–100 nm) the greatest value of magnetization is reached for a film with a thickness of 61 nm. The investigations of the other series of films, which were annealed at various temperatures, show that to achieve both a greater magnetization and a better structural homogeneity, annealing at temperatures T ≥ 450°C is required. In addition, low-intensity spin-waves were observed in some samples with thicknesses of 100 and 61 nm, which made it possible to estimate the spin-stiffness parameter D for the Co₂MnSi Heusler alloy as well.     

Self-diffusion in Ni2MnGa

Lattice self-diffusion of ⁶³Ni, ⁶⁷Ga, and ⁵⁴Mn tracers was studied in the ferromagnetic Ni₂MnGa shape-memory alloy. The composition of the monocrystalline alloy was slightly Ni-rich (Ni_53.7Mn_21.1Ga_25.2) and the direction of diffusion was perpendicular to the (100) plane. In the B2′ and L2₁ structures, the diffusion coefficients of the transition metal components are close to each other within a factor of 2. Gallium diffuses in the L2₁ structure 2–4 orders of magnitude slower. At about 1100 K, a change of the slope was detected on the Arrhenius plots of the transitional metal components. This sharp change is due to the L2₁–B2′ transformation. The Ga diffusivity seems not to be affected by the transformation. Interestingly, the diffusion activation energy of Mn and Ni is definitely higher in the B2′ phase. The diffusion trends observed in B2′ and L2₁ structures are explained qualitatively in the framework of a six-jump-cycle (6JC) mechanism. In the L2₁ structure one can define Ga-migrating and transition metal migrating cycles. These cycles result in rather different activation energies for the diffusion of transition metal atoms and for Ga atoms in the L2₁ structure. In the B2′ structure, however, all components (Ni, Mn, Ga) should take part in every type of six-jump cycle, resulting in similar diffusion parameters for all 3 components.     

Deformation twinning in Ni2MnGa

Deformation twinning is investigated in the martensitic phase of a Ni_46.75Mn₃₄Ga_19.25 (at.%) alloy. X-ray and electron diffraction are used to establish the crystallography of the non-modulated tetragonal martensite, and transmission electron microscopy is employed to deduce the twinning parameters. It is convenient to define the twinning parameters with respect to a “monoclinic” unit cell, designated 2M: then K₁, η₁, K₂, and η₂ are (0 0 1), [1 0 0], (1 0 0), and [0 0 1] respectively. The Burgers vector of the active twinning disconnections is close to 1/6[1 0 0] and the disconnections are associated with steps of height d₍₀₀₂₎. These defects are expected to be highly mobile since their motion does not require atomic shuffling. It is shown that periodic arrangements of two layer twins produce modulated crystal structures, such as 14M.     

Microstructure and spin polarization of quaternary Co2Cr1−xVxAl,Co2V1−xFexAl and Co2Cr1−xFexAl Heusler alloys

The microstructures, magnetic properties and spin polarization of quaternary Co₂Cr_1−xV_xAl, Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys were investigated. Phase separation into A2 and B2/L2₁ structure occurs in Co₂CrAl and Co₂Cr_0.6Fe_0.4Al, whereas Co₂FeAl exhibits a single-phase B2 structure. The ordered L2₁ structure becomes more stable with increasing vanadium concentration (x ⩾ 0.35). The saturation magnetization measured at 5 K for Co₂Cr_1−xV_xAl alloy changes from 1.4 to 2.0 μ_B when x increases from 0.0 to 0.5 and then becomes 1.4 μ_B for x = 1.0. This behavior can be attributed to the variation in the local magnetic moment of Co atoms. The saturation magnetization of Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys increases with increasing Fe concentration. The spin polarization decreases from 0.62 to 0.56 with increasing x for Co₂Cr_1−xFe_xAl alloy. Also, the spin polarization decreases with increasing x for Co₂Fe_1−xV_xAl and Co₂Cr_1−xV_xAl alloys. Possible reasons for the reduced spin polarization in these alloys are discussed.     

First-principle study of magnetic,elastic and thermal properties of full Heusler Co2MnSi

In this work, first principles calculation of structural, electronic magnetic and elastic properties of the half-metallic ferromagnetic Heusler compound Co₂MnSi are presented. We have applied the full-potential linearized augmented plane waves plus local orbitals (FP-L/APW+lo) method based on the density functional theory (DFT). For the exchange and correlation potential generalized-gradient approximation (GGA) is used. The computed equilibrium lattice parameters agree well with the available theoretical and experimental data. Elastic constants and their pressure dependence are also calculated. The calculated total magnetization of 5 μ_B is in excellent agreement with recent experiments. We also presented the thermal effects using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. Temperature and pressure effects on the structural parameters, heat capacities, entropy, thermal expansion coefficient, and Debye temperatures are determined from the non-equilibrium Gibbs functions.     

Solvothermal synthesis and thermoelectric properties of skutterudite compound Fe（0.25）Ni（0.25）Co（0.5）Sb3

Nanostructured skutterudite-related compound Fe0.25Ni0.25Co0.5Sb3 was synthesized by a solvothermal method using FeCl3, NiCl2, CoCl2, and SbCl3 as the precursors and NaBH4 as the reductant. The soivothermally synthesized powders consisted of free granules with an average particle size of tens of nanometers. The bulk material was prepared by hot pressing the powders. Transport property measurements indicated a heavily doped semiconductor behavior with n-type conduction. The thermal conductivity is about 1.83 W.m-1·K-1 at room temperature and decreases to 1.57 W.m-1·K-1 at 673 K. The low thermal conductivity is attributed to small grain size and high porosity. A maximum dimensionless figure of merit of 0.15 is obtained at 673 K.     

铁磁性形状记忆合金Ni_2MnGa

Ｎｉ2 ＭｎＧａ合金是一种具有Ｈｅｕｓｌｅｒ型有序晶格的铁磁性金属间化合物 ,其马氏体相变开始温度和终了温度分别为 2 0 7Ｋ和 193Ｋ ;其逆相变开始和终了温度分别为 2 0 9Ｋ和 2 2 1Ｋ ;居里点为 352Ｋ。非化学计量的Ｎｉ2 -ｘＭｎ1-ｘＧａ(ｘ =0～ 0 19) ,随Ｎｉ含量的增高其相变温度提高 ,而居里点降低。因此 ,可通过变成分来获得所要求的相变温度和居里点。Ｎｉ2 ＭｎＧａ合金具有较大的磁致伸缩 (孪晶磁致伸缩 ) ,为了改善这一合金的脆性 ,当前已开发了几种方法 ,有热处理法 ,放电等离子烧结制取大块材料 ,单辊快淬薄带等方法。日…     

Synthesis and Structural Characterization of Sb-Doped TiFe<Subscript>2</Subscript>Sn Heusler Compounds

Ternary Heusler compounds form a numerous class of intermetallics, which include two families with general compositions ABC and AB₂C, usually referred to as half- and full-Heusler compounds, respectively. Given their tunable electronic properties, made possible by adjusting the chemical composition, these materials are currently considered for the possible use in sustainable technologies such as solar energy and thermoelectric conversion. According to theoretical predictions, Sb substitution in the TiFe₂Sn full-Heusler compound is thought to yield band structure modifications that should enhance the thermoelectric power factor. In this work, we tested the phase stability and the structural and microstructural properties of such heavily doped compounds. We synthesized polycrystalline TiFe₂Sn_1?xSb_x samples, with x?=?0, 0.1, 0.2 and 1.0 by arc melting, followed by an annealing treatment. The structural characterization, performed by x-ray powder diffraction and microscopy analyses, confirmed the formation of the pseudo-ternary Heusler structure (cF16, Fm-3m, prototype: MnCu₂Al) in all samples, with only few percent amounts of secondary phases and only slight deviations from nominal stoichiometry. With increasing Sb substitution, we found a steady decrease in the lattice parameter, confirming that the replacement takes place at the Sn site. Quite unusually, the as-cast samples exhibited a higher lattice contraction than the annealed ones. The fully substituted x?=?1.0 compound, again adopting the MnCu₂Al structure, does not form as stoichiometric phase and turned out to be strongly Fe deficient. The physical behavior at room temperature indicated that annealing with increasing temperature is beneficial for electrical and thermoelectrical transport. Moreover, we measured a slight improvement in electrical and thermoelectrical properties in the x?=?0.1 sample and a suppression in the x?=?0.2 sample, as compared to the undoped x?=?0 sample.     

Half-metallic ferrimagnetism in the full-Heusler compound Co2ScSb

The AlCu₂Mn-type structure is found to be preferable energy wise than the CuHg₂Ti-type structure for the full-Heusler Co₂ScSb and exhibits half-metallic ferrimagnetism based on first-principles density-functional calculations. Calculations show that the total spin moment is 2 μ_B at the equilibrium lattice constant a₀ = 6.21 Å, which agrees with the Slater–Pauling rule. The spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.38 eV, and the spin-flip gap is of 0.24 eV. Half-metallicity is stable for a wide range of lattice constants.     

Phase composition and the mechanical properties of a Heusler alloy Ni<Subscript>2</Subscript> MnGa alloyed by iron and tungsten

Two series of samples of a Heusler alloy Ni₂MnGa have been studied. The samples alloyed with iron (to 1.6 at %) and with tungsten (0.5 at %) are single-phase (L2₁). Alloying leads to a strengthening of the samples; during the compression tests, values of σ_0.2 and σ_u have been found to grow. The deformation of the samples during compression increases from 2 to 5.5% in the alloy with 1.6 at % Fe and to 6.0% in the alloy with 0.5 at % W. At the concentration of 0.5 at %, the alloying with tungsten strengthens the alloy to a larger degree than alloying with iron. At the concentration 0.8 at %, no complete dissolution of tungsten in Ni₂MnGa occurs. Pileups of particles of the second phase are observed, which represents a solid solution on the basis of tungsten. Temperatures of the magnetic and martensitic transformation have been determined.     

Half-metallic properties and the robustness of Co2MnGe Heusler alloy under pressure: Ab-initio calculation

To investigate the structural, mechanical, electronic and magnetic properties of full Heusler Co₂MnGe in L2₁ structure, we used first-principle calculations. We made use of all-electron full potential linearized muffin-tin orbital method (FP-LMTO) within the density functional theory. The generalized gradient approximation GGA allows describing the exchange correlation energy. The obtained lattice parameters, bulk modulus and its derivate are in good agreement with experimental and theoretical values. We have also calculated the elastic constants and their related parameters. GGA + U approach allows to process electronic properties. For our compound, the band structure and magnetic moments are treated at equilibrium lattice constant. At Fermi level, the material has a semi metallic behavior with 100% spin polarization ratio. Computed magnetic exchange constants are used to evaluate the Curie temperature from mean field-approximation (MFA). Finally the elastic constants, electronic structure and magnetic properties are investigated at high-pressure. We noticed that half-metal to metal transition was around 102 GPa. The increasing pressure does not affect-the total magnetic moment and the overall shape of the band structure which indicates the robustness of electronic structure of this system.     

Structure and magnetoresistive properties of current-perpendicular-to-plane pseudo-spin valves using polycrystalline Co2Fe-based Heusler alloy films

We report current-perpendicular-to-plane giant magnetoresistance (CPP–GMR) of pseudo-spin valves (PSVs) with polycrystalline Co₂Fe(Al_0.5Si_0.5) (CFAS) and Co₂Fe(Ga_0.5Ge_0.5) (CFGG) Heusler alloy films. Strongly [0 1 1] textured polycrystalline Heusler alloy films grew on the Ta/Ru/Ag underlayer. Relatively large CPP–GMR values of ΔRA up to 4 mΩ μm² and ΔR/R up to 10% were obtained with 5 nm thick Heusler alloy films and Ag spacer layer by annealing CFAS PSV at 450 °C and CFGG PSV at 350 °C. Transmission electron microscopy revealed a flat and sharp interface between the [0 1 1] textured CFAS layers and the [1 1 1] textured Ag spacer layer. Annealing above an optimal temperature for each PSV led to reductions in MR values as a result of the thickening of the spacer layer induced by the Ag diffusion from the outer Ag layers.     

Effect of GaSb Addition and Sb Doping on the Thermoelectric Properties of Mg2Si0.5Sn0.5 Solid Solutions

利用B2O3助熔剂法结合热压法制备了Mg2Si0.487-2xSn0.5(GaSb)xSb0.013 (0.04 ≤ x ≤ 0.10)固溶体。X射线衍射结果表明样品呈单相。Sb掺杂有效提高了样品的电导率。随温度升高,Mg2Si0.487-2xSn0.5(GaSb)xSb0.013 (0.04 ≤ x ≤ 0.10)样品的电导率降低而塞贝克系数升高。随GaSb含量的增多,样品的电导率呈现出先增大后减小的变化趋势。所有样品中Mg2Si0.287Sn0.5(GaSb)0.1Sb0.013具有最低晶格热导率,其室温晶格热导率比Mg2Si0.5Sn0.5[11]低15%。由于电导率较高使Mg2Si0.327Sn0.5(GaSb)0.08Sb0.013具有最高热电优值,在720 K达到0.61,显著高于基体Mg2Si0.5Sn0.5[11]的最高热电优值0.019     

Interaction of Co3Sn2 and CoSn compounds with metallic bismuth

Phase diagrams of polythermal Co₃Sn₂-Bi (quasi-binary system) and CoSn-Bi (non-quasi-binary system) sections are constructed on the basis of results of complex methods of physicochemical analysis. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 3–5, April, 2006.