Phase transitions in Heusler alloys with exchange inversion

Phase diagrams of the Heusler Ni–Mn–X (X=In, Sn, Sb) alloys with exchange inversion are investigated. It is shown that a few different types of the phase diagrams depending on the signs of phenomenological parameters of the Ginzburg–Landau functional are obtained. In the diagrams there are thermodynamic paths, which can explain the experimentally observed sequences of the phase transitions in Ni–Mn–X alloys.     

Exchange bias in bulk Ni-Mn-In-based Heusler alloys

The crystal structure and exchange bias of the bulk Heusler alloys Ni₅₀Mn_50−xIn_x with 14.5?x?15.2 have been investigated using X-ray diffraction and magnetization measurements, respectively. Magnetic measurements were performed with SQUID magnetometry after samples were zero-field cooled and field cooled (FC) in positive magnetic fields up to H=50 kOe, from a temperature T=380 K. Three temperatures of the phase transitions, T₁<T_M<T_C, and a shift of the FC (50 kOe) magnetic hysteresis loops up to 120 Oe at 5 K have been detected for all samples. The exchange bias field (H_E) was almost constant for intermediate In concentrations 14.8<x<15.2, and sharply decreased to about 20 Oe on the borders of this concentration interval (x?14.5; 15.2?x). The changes of H_E have been related to changes in the ratio of T₁ to T_M: the overlapping of transitions at T₁ and T_M (for x=14.8, and 15.2) results in a decrease in H_E.     

Magnetic entropy change involving martensitic transition in NiMn-based Heusler alloys

Our recent progress on magnetic entropy change (ΔS) involving martensitic transition in both conventional and metamagnetic NiMn-based Heusler alloys is reviewed. For the conventional alloys, where both martensite and austenite exhibit ferromagnetic (FM) behavior but show differentmagnetic anisotropies, a positive ΔS as large as 4.1 J·kg^-1·K^-1 under a field change of 0–0.9 T was first observed at martensitic transition temperature T_M ~ 197 K. Through adjusting the Ni:Mn:Ga ratio to affect valence electron concentration e/a, T_M was successfully tuned to room temperature, and a large negative ΔS was observed in a single crystal. The -ΔS attained 18.0 J·kg-1·K-1 under a field change of 0–5 T. We also focused on the metamagnetic alloys that show mechanisms different from the conventional ones. It was found that post-annealing in suitable conditions or introducing interstitial H atoms can shift the T_M across a wide temperature range while retaining the strong metamagnetic behavior, and hence, retaining large magnetocaloric effect (MCE) and magnetoresistance (MR). The melt-spun technique can disorder atoms and make the ribbons display a B2 structure, but the metamagnetic behavior, as well as the MCE, becomes weak due to the enhanced saturated magnetization of martensites. We also studied the effect of Fe/Co co-doping in Ni₄₅(Co_1-xFe_x)5Mn36.6In13.4 metamagnetic alloys. Introduction of Fe atoms can assist the conversion of the Mn–Mn coupling from antiferromagnetic to ferromagnetic, thus maintaining the strong metamagnetic behavior and large MCE and MR. Furthermore, a small thermal hysteresis but significant magnetic hysteresis was observed around T_M in Ni₅₁Mn_49-xIn_x metamagnetic systems, which must be related to different nucleation mechanisms of structural transition under different external perturbations.     

Grain oriented NiMnSn and NiMnIn Heusler alloys ribbons produced by melt spinning: Martensitic transformation and magnetic properties

We outline the microstructural, martensitic transformation and magnetic properties of Heusler alloys with starting compositions Ni₅₀Mn₃₇Sn₁₃, Ni₅₀Mn₃₆In₁₄, and Mn₅₀Ni₄₀In₁₀, produced by melt spinning. The ribbons were obtained in argon environment at a high wheel linear speed of 48 m s⁻¹ (typical dimensions: 1.2-2.0 mm in width, 4-12 mm in length, and 7-12 μm in thickness). EDS microanalysis showed that the resulting average elemental chemical composition is slightly shifted with respect to the starting one. Ribbons are fully crystalline and tend to show a highly ordered columnar-like microstructure with grains running through the entire ribbon thickness; the larger dimension of the grains is perpendicular to the ribbon plane. As-spun alloys were single-phase with ferromagnetic bcc L2₁ austenite as high-temperature parent phase. At low temperatures austenite transforms into a structurally modulated martensite with a lattice symmetry that depends on the system (7 M orthorhombic for Ni₅₀Mn₃₇Sn₁₃, 10 M monoclinic for Ni₅₀Mn₃₆In₁₄, and 14 M monoclinic for Mn₅₀Ni₄₀In₁₀). Magnetization isotherms measured in the temperature interval where martensite thermally transforms into austenite confirmed the occurrence of field-induced reverse martensitic transition in the alloys studied.     

Heusler合金NiCoMnSn中的磁场驱动马氏体相变、超自旋玻璃和交换偏置

合成了一系列Ni_50-xCo_xMn₃₉Sn₁₁ (8≤x≤10) 样品, 并对它们的结构和磁性进行了研究. 发现随Co含量的增加, 样品的饱和磁化强度逐渐增强, 并在Ni₄₂Co₈Mn₃₉Sn₁₁中实现了磁场诱发马氏体相变. 另外, 在Co大于8.0的成分中探测到了超自旋玻璃, 并且观察到交换偏置现象. 证实了超自旋玻璃的马氏体相和铁磁奥氏体母相共存, 这也是产生交换偏置的原因.我们猜测超自旋玻璃的形成可能是来源于Mn-Mn团簇的存在, 这和之前报道的Mn₂Ni_1.6Sn_0.4 的结果相一致^[1]. 关键词： NiCoMnSn Heusler合金 超自旋玻璃 交换偏置     

Evidence of pressure dependence tricritical point at the nematic–smectic-A transition

We examine the effect of pressure on the nematic to smectic-A N–SmA) transition within the Landau phenomenological theory. The influence of pressure on the N–SmA transition is discussed by varying the coupling between the orientational and translational order parameters. The possibility of the tricritical point at the N–SmA transition is discussed in a phenomenological way. The theoretical predictions are found to be in good qualitative agreement with experimental results.     

Global hysteresis in ferroelectrics with incommensurate phases

A review of the basic experimental results connected with the formation of long-living metastable states in the incommensurate phases of ferroelectric crystals and related phenomena is given. The interpretation and appropriate physical mechanisms underlying the global hysteresis phenomena associated with these states are considered.     

Magnetic hysteresis and refrigeration capacity of Ni-Mn-Ga alloys near Martensitic transformation

This paper studies the magnetic hysteresis and refrigeration capacity of Ni-Mn-Ga alloys in detail during heating and cooling isothermal magnetisation processes. The Ni-Mn-Ga alloys show larger magnetic hysteresis when they transform from austenite to martensite, but smaller magnetic hysteresis when they transform from martensite to austenite. This behaviour is independent of either the pure Ni-Mn-Ga alloys or the alloys doped with other elements. Because of the existence of the magnetic hysteresis, the relation between the magnetic entropy change and refrigeration capacity is not simply linear. For practical consideration, magnetocaloric effect of Ni-Mn-Ga alloys should be investigated both on cooling and heating processes.     

CoNiZ系列合金的结构和马氏体相变性质

利用X射线衍射研究了CoNiZ(Z＝Si，Sb，Sn，Ga 等)合金在不同热处理条件下的相组成.当Z元素为Sn，Sb时，材料是完全的B2结构；但Z为Si时，材料变成面心立方的γ相.形成B2还是γ相由电子浓度和原子尺寸效应两种因素共同决定.而CoNiGa的研究结果表明，在合金中除了形成B2结构的同时还容易形成γ相，常表现出两相共存的特性.对材料进行不同方式的热处理可以使合金中两相的含量有所消长,γ相含量的多少对CoNiGa合金的马氏体相变有很大的影响.分析指出，两相共存及其所带来的物性变化是CoNiGa铁磁性形状记忆合金非常有利用价值的物理性质. 关键词： Heusler合金 马氏体相变 γ相     

铁基Heusler合金Fe2Co1-xCrxSi的结构、磁性和输运性质的研究

基于多种实验手段和能带计算的方法, 对四元合金Fe₂Co_1-xCr_xSi的晶体结构、 磁性、输运性质及能带结构进行了研究. 研究发现, 随着Cr的增加, 合金Fe₂Co_1-xCr_xSi保持了高度有序结构, 逐渐从Hg₂CuTi结构的Fe₂CoSi 过渡到L2₁结构的Fe₂CrSi; 由于次晶格网络的破坏, 居里温度逐渐下降; 系列合金的分子磁矩呈现线性下降, 符合半金属特性; 剩余电阻比率与原子占位有序程度密切相关, 呈现两端大、 中间小的特点. 在Cr替代Co的过程中, 材料半金属能隙逐渐打开, 表现半金属特征. 同时费米能级从Fe₂CoSi半金属能隙的价带顶上移至Fe₂CrSi能隙的导带底. 最大的能隙宽度出现在x= 0.75处, 这表明四元合金有可能成为具有更高自旋极化率和更强抗干扰能力的自旋电子学材料.     

Electronic structure and half-metallicity in Heusler alloys Fe2YB (Y=Ti, V, Mn, Cr)

The electronic structure and magnetic properties of B-based Heusler alloys Fe₂YB (Y=Ti, V, Cr and Mn) have been studied theoretically. These alloys are all ferrimagnets except for Fe₂VB. The latter has 24 valence electrons and is a paramagnetic semimetal. Fe₂CrB is predicted to be half-metals at equilibrium lattice constant. The spin polarization of Fe₂MnB is also quite high. The calculated total moments are 1.00 μ_B for Fe₂CrB and 2.04 μ_B for Fe₂MnB. In Fe₂CrB and Fe₂MnB, the total moments are mainly determined by the partial moment of Cr or Mn. The Fe moment is relatively small and antiparallel to that of Cr or Mn. Under uniform lattice distortion, the half-metallicity of Fe₂CrB is more stable than Fe₂MnB, which is related to the detailed DOS structure of them near E_F.     

Electronic structure and half-metallicity of the new Heusler alloys PtZrTiAl,PdZrTiAl and Pt0.5Pd0.5ZrTiAl

The electronic structure and magnetic properties of the PtZrTiAl, PdZrTiAl and Pt_0.5Pd_0.5ZrTiAl Heusler alloys were investigated using the full-potential linearized augmented plane wave (FPLAPW) within the generalized gradient approximation (GGA). For the PtZrTiAl, and PdZrTiAl alloys, the results showed that these Heusler alloys were stable in the Type I structure. The (Pt, Pd)ZrTiAl Heusler alloys are found to exhibit half-metallic ferromagnetism for both the Type I and Type II structure. The total magnetic moments of the PtZrTiAl and PdZrTiAl alloys were obtained to be 3 μ_B per formula unit, which are in agreement with the Slater-Pauling rule m_tot = (Nv ? 18). The half-metalliciy characteristic exists in the relatively wide ranges of 6.06–6.78 Å, and 6.13–6.73 Å for the PtZrTiAl and PdZrTiAl alloys, respectively. To complete the fundamental characteristics of these alloys, Pt_0.5Pd_0.5ZrTiAl is predicted to be a half-metallic ferromagnet with an energy gap of 0.90 eV in the minority spin and a complete spin polarization at the Fermi level. These new Heusler alloys may become ideal candidate material for future spintronic applications.     

哈斯勒合金Ni-Co-Mn-Sn的马氏体相变及其磁热效应研究

通过结构和磁性测量,研究了四元哈斯勒合金Ni_50-xCo_xMn₃₈Sn₁₂（x=1, 2, 4, 6, 8）的晶体结构和相变特征.结果表明,Co原子的掺杂不但没有影响三元哈斯勒合金Ni-Mn-Sn的原有结构,而且还增强了样品在奥氏体相的铁磁交换作用.此外,通过Maxwell方程计算了其中三种成分样品（x= 2, 4, 6）的磁熵变ΔS_{M关键词： 哈斯勒合金 Ni-Co-Mn-Sn 马氏体相变 磁热效应}     

Direct and Indirect Determination of the Magnetocaloric Effect in the Heusler Compound Ni1.7Pt0.3MnGa

Magnetic shape-memory materials are potential magnetic refrigerants, due the caloric properties of their magnetic-field-induced martensitic transformation. The first-order nature of the martensitic transition may be the origin of hysteresis effects that can hinder practical applications. Moreover, the presence of latent heat in these transitions requires direct methods to measure the entropy and to correctly analyze the magnetocaloric effect. Here, we investigated the magnetocaloric effect in the Heusler material Ni1.7Pt0.3MnGa by combining an indirect approach to determine the entropy change from isofield magnetization curves and direct heat-flow measurements using a Peltier calorimeter. Our results demonstrate that the magnetic entropy change ΔS in the vicinity of the first-order martensitic phase transition depends on the measuring method and is directly connected with the temperature and field history of the experimental processes.     

Martensitic transformation in Ni-Mn-Ga alloys

Single crystals of different compositions in shape memory alloys Ni-Mn-Ga have been studied by electron and low temperature X-ray diffraction as well as by differential scanning calorimetry. It is shown that the cooling-induced martensitic phases are long-periodic ones modulated along the (110) directions by a transverse wave of atomic shifts with 5 and 7 atomic layers periodicity for the alloys studied exhibiting a martensitic transformation at 180 K and 446 K, respectively. The transformation heats appeared to be about 10 times different for both alloys.     

LiMgPbSb型四元Heusler合金CoFeTiSb的半金属性及其无序效应

基于密度泛函理论的GGA计算,我们研究了LiMgPbSb型Heusler合金CoFeTiSb电子结构,发现CoFeTiSb在费米面处存在100%的自旋极化率,并且具有2μB的原胞总磁矩.此外,我们考虑了Co-Fe、Co-Ti和Fe-Ti交换无序对CoFeTiSb合金电子结构的影响,发现这三种交换无序均使得CoFeTiSb完全丧失了半金属性,在Co-Fe交换无序下,CoFeTiSb合金具有85%的自旋极化率,而在Co-Ti无序下,CoFeTiSb合金的自旋极化率只有5%.     

Magnetostructural transition in NiCuCoMnGa alloys

The effects of the addition of Co on the martensitic transformation and Curie transition temperatures of polycrystalline Ni46-xCu4CoxMn33.sGa16.5 （x = 0, 1, 3, 5） alloys are investigated. An abrupt decrease in the martensitic transformation temperature and an obvious increase in the Curie transition temperature of austenite （TA） are observed when Co is doped in the NiCuMnGa alloy. As a result, the composition range for obtaining the magnetostructural transition is extended. Furthermore, the effect of a strong magnetic field on the magnetostructural transition is analyzed. This study offers a possible method to extend the composition range for obtaining magnetostructural transition in Heusler alloys.     

Ultrafast optical probe of coherent acoustic phonons in Co_2MnAl Heusler film

In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co_2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, probe wavelength,external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co_2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of(3.85±0.1)×10~3m/s at 800 nm for the epitaxial Co_2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co_2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.     

Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L21结构Co2FeSi高有序合金之间的Co50Fe50-xSix合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似（GGA＋U）方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co2FeSi合金的分子磁矩和居里温度分别达到5.92μB和777℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co2FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内.     

哈斯勒合金Ni-Mn-Sn的马氏体相变与反磁热性质

通过结构以及磁性测量,研究了哈斯勒合金Ni₅₀Mn_25+xSn_25-x (x=11,12)的马氏体相变和磁热性质.结果表明,与样品在奥氏体相的磁性不同,由于在马氏体相中反铁磁交换作用的增强,导致铁磁和反铁磁在马氏体状态下共存.此外,通过Maxwell方程,研究了两样品在不同磁场变化下马氏体相变温度附近的反磁热性质,并阐明了该系列合金产生大的正磁熵变(ΔS_M)不仅与其在降温过程中发生马氏体相变所导致的磁跃变(ΔM)有关,而且与发生马氏体相变所经历的温度区间有密切的联系. 关键词： 哈斯勒合金 Ni-Mn-Sn 马氏体相变 正磁熵变