Photoemission study of the (1 0 0) surface of Ni2MnGa and Mn2NiGa ferromagnetic shape memory alloys

The (1 0 0) surface of Ni₂MnGa and Mn₂NiGa ferromagnetic shape memory alloys have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). It is shown that by sputtering and annealing, it is possible to obtain a clean, ordered and stoichiometric surface that shows a four-fold 1 × 1 LEED pattern at room temperature. For both Ni₂MnGa and Mn₂NiGa, the surface becomes Ni-rich and Mn deficient after sputtering. However, as the annealing temperature is increased Mn segregates to the surface and at sufficiently high annealing temperature the Mn deficiency caused by sputtering is compensated. The (1 0 0) surface of Ni₂MnGa is found to have Mn-Ga termination. The valence band spectra of both Ni₂MnGa and Mn₂NiGa exhibits modifications with surface composition. For the stoichiometric surface, the origin of the spectral shape of the valence band is explained by calculations based on first principles density functional theory.     

Growth of Ni-Mn-Ga high-temperature shape memory alloy thin films by magnetron sputtering technique

Ni-Mn-Ga thin films have been fabricated by using magnetron sputtering technique under various substrate negative bias voltages. The effect of substrate negative bias voltage on the compositions and surface morphology of Ni-Mn-Ga thin films was systematically investigated by energy dispersive X-ray spectrum and atomic force microscopy, respectively. The results show that the Ni contents of the thin films increase with the increase of the substrate negative bias voltages, whereas the Mn contents and Ga contents decrease with the increase of substrate negative bias voltages. It was also found that the surface roughness and average particle size of the thin films remarkably decrease with the increase of substrate negative bias voltages. Based on the influence of bias voltages on film compositions, a Ni₅₆Mn₂₇Ga₁₇ thin film was obtained at the substrate negative bias voltage of 30 V. Further investigations indicate that the martensitic transformation start temperature of this film is up to 584 K, much higher than room temperature, and the film has a non-modulated tetragonal martensitic structure at room temperature. Transmission electron microscopy observations reveal that microstructure of the thin film exhibits an internally (1 1 1) type twinned substructure. The fabrication of Ni₅₆Mn₂₇Ga₁₇ high-temperature shape memory alloy thin film will contribute to the successful development of microactuators.     

Site Preference and Alloying Effect of Excess Ni in Ni-Mn-Ga Shape Memory Alloys

The formation energies and electronic structures of Ni-rich Ni-Mn-Ga alloys have been investigated by firstprinciples calculations using the pseudopotential plane wave method based on density functional theory. The results show that the alloying Ni prefers to occupy the Mn site directly in Ni9Mn3Ga4 and to occupy the Mn site and drive the displaced Mn atom to the Ga site in NigMn4Ga3, which is in accordance with the experimental result. According to the lattice constants and the density of states analyses, these site preference behaviours are closely related to the smaller lattice distortion and the lower-energy electronic structure when the excess Ni occupies the Mn site. The effect of Ni alloying on martensitic transformation is discussed and the enhancement of martensitic transformation temperature by Ni alloying is estimated by the calculated formation energy difference between austenite and martensite phases.     

Spin correlations and a mesoscopic structure in Ni-Mn-Ga

The mesoscopic structures of the Heusler alloys Ni_49.1Mn_29.4Ga_21.5 and Ni₂MnGa are studied by small-angle polarized neutron scattering in the temperature range 15 < T < 400 K. The characteristic temperatures of phase transformations (ferromagnetic, martensitic, and premartensitic transformations) and the characteristic sizes of mesoscopic inhomogeneities in them have been determined. Differences in the spin dynamics of these phases and magnetic-nuclear interference upon neutron scattering have been revealed. The evolutions of the mesoscopic structures in the nonstoichiometric and stoichiometric alloys are found to be substantially different.     

四元Heusler合金NiMnFeGa中Fe原子的磁性贡献

用熔炼和甩带的方法制备了组分为Ni₅₀Fe_xMn_25-xGa ₂₅(x=0—25) 的系列样品.x射线衍射实验结果表明，当Fe取代Mn的含量x＜17时，用熔炼和甩带的方法均 能合成高度有序的L2₁结构的Heusler相.而当x＞17时，普通熔炼方法只能得到 低有序度的 γ相，只有采用甩带急冷的方法才能获得高度有序的纯L2₁结构的化合物.根据 交流磁化率 和分子磁矩的测试结果，初步分析了Fe原子对化合物磁性的贡献，认为Fe原子占据了Mn原子 的位置后，具有高于一般含铁合金的原子磁矩，可达2.55—3.55μ_B. 关键词： Heusler合金 50Fe_xMn_25-xGa_{25')" href="#">Ni50FexMn25-xGa25}     

哈斯勒合金Ni-Mn-Ga的马氏体相变和磁增强双向形状记忆效应

通过研究铁磁性金属间化合物Ni_2+xMn_1-xGa(x=-0.1,0,0.08,0.13,0.18,0.2)和Ni_2-xMn_1+x/2Ga_1+x/2(x=-0.1,0,0.04,0.06,0.1)两个系列多晶样品的交流磁化率随温度的变化行为,得到了化合物在不同组分下的马氏体相变温度TM和居里温度T_C.发现随着Ni成分的增加,前者的马氏体相变温度T_m增加,而居里温度T_C降低,后者的马氏体相变温度T_m和居里温度T_C均是先增大后减小.报道了T_m在室温附近的单晶样品Ni₅₂Mn₂₄Ga₂₄的磁场增强双向形状记忆效应.发现伴随着马氏体相变,样品在[001]方向可产生1.2%的收缩.如果在该方向施加1.2T的偏磁场可以使该应变值增大到4.0%.而垂直于[001]方向施加1.2T的偏磁场时,在[001]方向产生1.6%的膨胀.阐明了产生大应变的原因并非相界移动,而是单晶的杂散内应力小和外加磁场通过孪晶界移动使马氏体变体重组的共同结果. 关键词： 形状记忆效应 马氏体相变 2MnGa')" href="#">Ni₂MnGa     

Crystal structure and physical properties of magnetic shape memory alloys Ni50 − x Cu x Mn29Ga21

The phase composition, crystal structure, and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of the stoichiometric alloy Ni₅₀Mn₂₅Ga₂₅ and nonstoichiometric alloys Ni_{50 ? x} Cu _x Mn₂₉Ga₂₁ (x = 0, 1, 2) with the thermoelastic martensitic transformation have been investigated. The influence of the chemical composition on the transformations and physical properties of the alloys has been determined.     

Transformation behavior of Ni-Mn-Ga/Si(100) thin film composites with different film thicknesses

A series of Ni_51.4Mn_28.3Ga_20.3/Si(100) thin film composites with different film thicknesses varying from 0.1 to 5 μm have been prepared by magnetron sputtering and subsequently annealed. X-ray powder diffraction patterns of the films show the features associated with the lattice-modulated martensitic phase and/or cubic austenite at room temperature. 220-fiber texture was confirmed by the X-rays measurements made at 150 °C. While the Curie temperature is almost film thickness independent, the martensitic transformation temperature shows a strong descended dependence in the submicron range. The substrate curvature measurements demonstrate that the forward and reverse martensitic transformation in the films is accompanied by the reversible relaxation and accumulation of residual stress, originally created by the thermal treatment due to the difference in thermal expansion of the film and substrate. The values of residual stresses measured by both substrate curvature and X-rays diffraction methods at constant temperatures are found to be dependent on the film thickness. This behavior appears in correlation with the thickness dependence of the transformation temperature.     

Structural and magnetic investigations in the vicinity of first-order transformations in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

Among the series of alloys derived from Ni₅₀Mn₂₉Ga₂₁ on selective substitution of Co for Ni and Mn, two alloys Ni_49.8Mn_27.2Ga_21.2Co_1.8 and Ni_46.9Mn_28.8Ga₂₁Co_3.3 referred to as CoMn-1.8 and CoNi-3.3, respectively, are found to exhibit an additional first-order transformation below their martensitic transformation temperatures. Systematic studies on temperature and field dependence of magnetic properties of these alloys are carried out, through the transformations, to understand their origin. An examination of these results in conjunction with those from structural investigations reveals that the transformation in the CoMn-1.8 alloy is an intermartensitic transformation and has a structural origin, while that in the CoNi-3.3 alloy is not of the structural origin and is attributed to local spin inversion of Co moments, which is of the magnetic origin.     

Magnetocaloric effects in Ni-Mn-X based Heusler alloys with X=Ga, Sb, In

The Mn-based Heusler alloys encompass a rich collection of useful materials from highly spin-polarized systems to shape memory alloys to magnetocaloric materials. In this work we have summarized our studies of magnetostructural transitions from paramagnetic austenite to ferromagnetic martesite phases at TMC in Ni₂MnGa-based alloys (Ni₂Mn_0.75Cu_0.25-xCo_xGa, Ni₂Mn_0.70Cu_0.30Ga_0.95Ge_0.05, Ni₂Mn_1-xCu_xGa, Ni_2+xMn_1-xGa, and Ni₂Mn_0.75-xCu_xGa), and martensitic transitions from the ferromagnetic austenite to the martesite state in off-stoichiometric Ni-Mn-(In/Sb) Heusler alloys. The phase transition temperatures and respective magnetic entropy changes (ΔS) depend on composition in these systems and have been determined from magnetization measurements in the temperature interval 5-400 K, and in magnetic fields up to 5 T. It is shown that, depending on the composition and doping scheme the “giant” ΔS=40-60 J/(kgK) (for a field change of 5 T) can be observed in the temperature range (300-360 K) for the Ga-based alloys. The interplay between or coupling of the various transitions in Ni₂Mn(Mn,X) systems with X=Sb and In leads to exchange bias effects, giant magnetoresistance, and both inverse and “normal” magnetocaloric effects.     

Annealing effect on phase transformation in nano structured Ni–Mn–Ga ferromagnetic shape memory alloy

Nano structured Ni_52.6Mn_23.7Ga_24.3 alloy was prepared using the ball milling technique. High martensitic transition temperatures are observed in the range between 336 and 367 K. The X-ray diffraction profile revealed that annealed Ni–Mn–Ga powder at 1073 K displays mixture phases of austenite and martensite. Annealing at 1173 K induces phase transformation from mixture phase to Heusler L2₁ structure, which confirms the high-temperature shape memory effect. On the contrary, the milled sample shows no evidence of shape memory effect. Furthermore, annealing at higher temperature (1273 K) shows the accumulation of oxidation, which leads to the loss of shape memory effect. The grain size increases with increasing annealing temperature and causes deterioration in the soft magnetic properties.     

Martensitic transformation in Cu-doped NiMnGa magnetic shape memory alloys

This paper studies the martensitic transformation in the Cu-doped NiMnGa alloys. The orthorhombic martensite transforms to L2₁ cubic austenite by Cu substituting for Ni in the Ni_50-xCu_xMn₃₁Ga₁₉ (x=2--10) alloys, the martensitic transformation temperature decreases significantly with the rate of 40 K per Cu atom addition. The variation of the Fermi sphere radius (k_F) is applied to evaluate the change of the martensitic transformation temperature. The increase of k_F leads to the increase of the martensitic transformation temperature.     

Successive occurrence of ferromagnetic and shape memory properties during crystallization of NiMnGa freestanding films

Ni₅₀Mn₃₀Ga₂₀ films of 13 μm thickness were fabricated by DC magnetron sputtering on unheated glass substrates. The As-deposited films are partially crystalline and crystallize during rapid annealing. The successive appearance of ferromagnetic and shape memory properties was observed as the annealing temperature was increased. Ferromagnetic properties evolved after annealing at 400 °C for 0.5 h, while thermal annealing of at least 600 °C for 0.5 h led to polycrystalline films that transformed reversibly and martensitically as shown by structural analysis and differential scanning calorimetry and confirmed by mechanical spectroscopy data. Magnetic measurements also revealed the influence of the post deposition annealing on the ferromagnetic hysteresis. Transition temperatures and reaction enthalpies of the martensitic phase transformation were strongly influenced by the temperature of the rapid annealing process. X-ray absorption fine-structure (XAFS) spectroscopy proved these changes to be related to the change in the chemical order. It is proposed that the annealing data reflect the evolution of the crystalline state. Ferromagnetic order is established already in nano-grained samples whereas the shape memory effect is only observed above a critical grain size.     

Phase transitions and thermal expansion in Ni<Subscript>51–<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>36 + <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>13</Subscript> alloys

Thermal expansion and structural and magnetic phase transitions in alloys of the Ni–Mn–Sn system have been investigated. The spontaneous martensitic transformation in Ni_51–xMn_{36 + x}Sn₁₃ (0 ≤ x ≤ 3) alloys is found to be accompanied by high jumps in the temperature dependences of the linear thermal expansion. The relative change in the linear sizes of these alloys at the martensitic transformation is ~1.5 × 10^–3. There are no anomalies in the magnetic-ordering temperature range in the temperature dependences of the coefficient of linear thermal expansion. The differences in the behavior of linear thermal expansion at the martensitic transformation in Ni_51–xMn_{36 + x}Sn₁₃ (0 ≤ x ≤ 3) and Ni₄₇Mn₄₀Sn₁₃(x = 4) alloys have been established.     

Low-temperature properties of magnetic Ni50 + x Mn25 ? x + y Ga25 ? y shape memory alloys

The effect of atomic disordering and deviation from the Ni₂MnGa stoichiometric composition on the low-temperature properties of alloys with magnetically controlled shape memory effect is studied. The specific features of the magnetic, galvanomagnetic, and electrical properties of alloys with magnetically controlled shape memory effect are discussed. The specific features of the magnetic, galvanomagnetic, and electrical properties of alloys Ni₅₀Mn₂₅Ga₂₅, Ni₅₄Mn₂₁Ga₂₅, and Ni₅₀Mn_28.5Ga_21.5 in the temperature range 2 ≤ T ≤ 80 K under magnetic fields H ≤ 12 MA/m are studied. Original Russian Text ? N.I. Kourov, V.V. Marchenkov, V.G. Pushin, A.V. Korolev, E.B. Marchenkova, H.W. Weber, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 11, pp. 2037–2042.     

Si掺杂的铁磁形状记忆合金Co50Ni21Ga29Six的物性研究

成功生长了Co₅₀Ni₂₁Ga₂₉:Si(x=1,2)单晶样品,对其磁性,马氏体相变及其相关性质进行了细致的测量.发现掺Si成分的单晶具有非常迅速的马氏体相变行为、2.5％的大相变应变、大于100 ppm的磁感生应变和4.5％的相变电阻.进一步研究指出,在CoNiGa合金中掺入适量Si元素,能够降低材料的马氏体相变温度,减小相变热滞后,提高材料的居里温度,并使得磁性原子的磁矩有所降低.尤其重要的是Si元素的添加能够增大材料马氏体的磁晶各向异性能,改善马氏体变体的迁移特性,从而获得更大的磁感生应变. 关键词： 铁磁形状记忆合金 Heusler合金 50Ni₂₁Ga₂₉Si_x')" href="#">Co₅₀Ni₂₁Ga₂₉Si_x     

Ni2MnGa(100) ferromagnetic shape memory alloy: A surface study

Ni₂MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.     

The effect of Si content on the martensitic transfor-mation temperature of Ni55.5e18Ga26.5-xSix alloys

This paper investigates the effects of substitution of Si for Ga on the martensitic transformation behaviours in Ni-Fe-Ga alloys by using optical metallographic microscope and differential scanning calorimetry (DSC) methods. The structure type of Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys is determined by x-ray diffraction (XRD), and the XRD patterns show the microstructure of Ni-Fe-Ga-Si alloys transformed from body-centred tetragonal martensite (with Si content x = 0) to body-centred cubic austenite (with x = 2) at room temperature. The martensitic transformation temperatures of the Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys decrease almost linearly with increasing Si content in the Si content range of x ≤ 3. Thermal treatment also plays an important role on martensitic transformation temperatures in the Ni-Fe-Ga-Si alloy. The valence electronic concentrations, size factor, L2₁ degree of order and strength of parent phase influence the martensitic transformation temperatures of the Ni-Fe-Ga-Si alloys. An understanding of the relationship between martensitic transformation temperatures and Si content will be significant for designing an appropriate Ni-Fe-Ga-Si alloy for a specific application at a given temperature.     

Ni2MnGa单晶马氏体相变过程摩擦耗能的热动力学计算

根据相界面摩擦原理，在推导出计算Ni₂MnGa系统热动力学参量的一般表示式的基础上，结合马氏体相变温度分别在室温以下、室温附近、室温以上三种非正配分比Ni₂MnGa单晶自发相变应变和交流磁化率随温度变化的测量结果，计算了三种样品马氏体相变过程中界面摩擦所消耗的能量.结果进一步表明正是相变过程中的界面摩擦导致了相变的热滞后，而三种样品马氏体相变过程的摩擦耗能和相变热滞后存在较大差别的原因在于三种样品马氏体相变生成物具有不同的结构. 关键词： 马氏体相变 应变 界面摩擦     

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni Mn Ga Heusler alloy

We have studied the isothermal entropy change around a first-order structural transformation and in correspondence to the second-order Curie transition in the ferromagnetic Heusler alloy Ni_2.15Mn_0.85Ga. The results have been compared with those obtained for the composition Ni_2.19Mn_0.81Ga, in which the martensitic structural transformation and the magnetic transition occur simultaneously. With a magnetic field span from 0 to 1.6 T, the magnetic entropy change reaches the value of 20 J/kg K when transitions are co-occurring, while 5 J/kg K is found when the only structural transition occurs. Received 27 September 2002 / Received in final form 17 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: solzi@fis.unipr.it