Transformation behaviors,structural and magnetic characteristics of Ni-Mn-Ga films on MgO （001）

Ferromagnetic Ni-Mn-Ga films were fabricated by depositing on MgO （001） substrates at temperatures from 673 K to 923 K. Microstructure, crystal structure, martensitic transformation behavior, and magnetic properties of the films were studied. With increasing deposition temperature, the surface morphology of the films transforms from granular to continu- ous. The martensitic transformation temperature is not dependent on deposition temperature; while transformation behavior is affected substantially by deposition temperature. X-ray analysis reveals that the film deposited at 873 K has a 7M marten- site phase, and its magnetization curve provides a typical step-increase, indicating the occurrence of magnetically induced reorientation （MIR）. In situ magnetic domain structure observation on the film deposited at 873 K reflects that the marten- sitic transformation could be divided into two periods： nucleation and growth, in the form of stripe domains. The MIR occurs at the temperature at which martensitic transformation starts, and the switching field increases with the decrease of temperature due to damped thermal activation. The magnetically induced martensitic transformation is related to the difference of magnetization between martensite and austenite. A shift of martensite temperature of dT/dH = 0.43 K/T is observed, consistent with the theoretical value, 0.41 K/T.     

Evolution of magnetic domain structure of martensite in Ni–Mn–Ga films under the interplay of the temperature and magnetic field

Ferromagnetic shape memory Ni-Mn-Ga films with 7M modulated structure were prepared on MgO （001） substrates by magnetron sputtering. Magnetization process with a typical two-hysteresis loop indicates the occurrence of the reversible magnetic field-induced reorientation. Magnetic domain structure and twin structure of the film were controlled by the in- terplay of the magnetic and temperature field. With cooling under an out-of-plane magnetic field, the evolution of magnetic domain structure reveals that martensitic transformation could be divided into two periods： nucleation and growth. With an in-plane magnetic field applied to a thermomagnetic-treated film, the evolution of magnetic domain structure gives evidence of a reorientation of twin variants of martensite. A microstructural model is described to define the twin structure and to produce the magnetic domain structure at the beginning of martensitic transformation; based on this model, the relationship between the twin structure and the magnetic domain structure for the treated film under an in-plane field is also described.     

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.     

Combined experimental and theoretical study on the effect of Nb content on martensitic transformation of NbRu shape memory alloys

The effect of Nb content on the martensitic transformation of NbRu high-temperature shape memory alloys is investigated by experiments and first-principles calculations. We calculate the lattice parameters, density of states, charge density, and heats of formation of Nb_50+xRu_50-x β phase. The results show that an increase in Nb content increases the stability of Nb_50+xRu_50-x β phase, leading to a significant decrease of the β to β' martensitic transformation temperature. In addition, the mechanism of the effects of Nb content on phase stability and martensitic transformation temperature is studied on the basis of electronic structure.     

Microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated by spark plasma sintering

The microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated using the spark plasma sintering method have been investigated. The results show that both the as-sintered and annealed sintered specimens exhibit typical martensitic transformation behaviours. The martensite of the sintered specimen after annealing exhibits a ferromagnetic nature. Moreover,study of the fracture surface indicates that the transgranular fracture contributes to the higher ductility of sintered Ni-Mn-Ga alloy. In addition,the transformation strain in sintered Ni-Mn-Ga alloy is studied for the first time.     

A macroscopic constitutive model of shape memory alloy considering cyclic effects

This paper presents a macroscopic constitutive model reproducing the hysteretic behaviors of the superelastic shape memory alloy (SMA) under cyclic loading. The progressive increase of residual strain with the increased cycle number in such materials is assumed to be a consequence of the progressive increase of residual stress-induced martensitic volume fraction upon the cyclic effects. The progressive decrease of phase transformation critical stresses with the increased cycle number in such materials is assumed to be a result from the progressive increase of phase transformation critical temperatures upon the cyclic effects. A cyclic evolution equation is supposed to describe the influences of cycle effects on the material properties of the SMA under cyclic loading. A phase transformation equation expressing the phase transformation behaviors of the SMA under cyclic loading is established based on the differential relationship between martensitic volume fraction and the free energy increment of phase transformation. A mechanical constitutive equation predicting the mechanical characteristics of the SMA under cyclic loading is developed on the basis of thermodynamics and continuum mechanics. The cyclic evolution equation, phase transformation equation, and mechanical constitutive equation together compose the presented macroscopic constitutive model considering cyclic effects. Results of the numerical simulations illustrate that it can well reproduce the superelastic hysteretic behaviors of the SMA under cyclic loading.     

Effects of Cu on the martensitic transformation and magnetic properties of Mn_(50)Ni_(40)In_(10) alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40-xCuxIn10, Mn50-xCuxNi40In10, and Mn50Ni40In10-xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.     

The effect of Si content on the martensitic transformation temperature of Ni55.5Fe18Ga26.5-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 Ni55.5Fe18Ga26.5-xSix 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 Ni55.5Fe18Ga26.5-xSix 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.     

Room temperature damping correlated to the microstructures in Cu–20.4Al–8.7Mn

The damping capacity of the shape memory alloy Cu–20.4Al–8.7Mn(at.%) at room temperature is investigated by an internal friction technique.Results indicate that the alloy exhibits higher damping capacity in the Martensitic condition than that in the austenitic condition due to the latter having lower intrinsic damping capacity and pinning effect coming from the precipitate particles.The maximum damping capacity is obtained in the coexistence condition of Martensite and austenite.The condition can be achieved when processing an isothermal ageing for the as-cast sample at temperatures of 100°C–150°C.Three possible mechanisms are considered to account for the maximum damping capacity.They are listed as much increased interfaces between twin boundaries,owing to the thinning of martensitic plates,martensitic transformation induced by the applied stress during internal friction measurements,phase transformation itself based on the coexistence of martensitic and austenitic phases with a macroscopic amount.However,the contribution of the first mechanism is predominant.     

Large Barocaloric Effect with High Pressure-Driving Efficiency in a Hexagonal MnNi_(0.77)Fe_(0.23)Ge Alloy

The hydrostatic pressure is expected to be an effective knob to tune the magnetostructural phase transitions of hexagonal MM'X alloys(M and M' denote transition metals and X represents main group elements).We perform magnetization measurements under hydrostatic pressure on an MM'X martensitic MnNi_(0.77)Fe_(0.23)Ge alloy.The magnetostructural transition temperature can be efficiently tuned to lower temperatures by applying moderate pressures,with a giant shift rate of-151 K/GPa.A temperature span of 30 K is obtained under the pressure,within which a large magnetic entropy change of-23 J·kg~(-1)K~(-1) in a field change of 5 T is induced by the mechanical energy gain due to the large volume change.Meanwhile,a decoupling of structural and magnetic transitions is observed at low temperatures when the martensitic transition temperature is lower than the Curie temperature.These results show a multi-parameter tunable caloric effect that benefits the solid-state cooling.     

Martensitic transformation and giant magnetic entropy change in Ni_(42.8)Mn_(40.3)Co_(5.7)Sn_(11.2) alloy

The crystal structure, phase transition, and magnetocaloric effect in Ni42.8Mn40.3Co5.7Sn11.2 alloy are investigated by structure analysis and magnetic measurements. A large magnetic entropy change of 45.6 J/kg.K is obtained at 215 K under a magnetic field of 30 kOe （1 Oe = 79.5775 A.m-1）. The effective refrigerant capacity of Ni42.8Mn40.3Co5.7Sn11.2 alloy reaches 72.1 J/kg under an applied field changing from 0 to 30 kOe. The external magnetic field shifts the martensitic transition temperature about 3-4 K/10 kOe towards low temperature, indicating that magnetic field can retard the phase transition to a certain extent. The origin of large magnetic entropy change is discussed in the paper.     

Perpendicular magnetic tunnel junction and its application in magnetic random access memory

Recent progresses in magnetic tunnel junctions with perpendicular magnetic anisotropy(PMA) are reviewed and summarized. At first, the concept and source of perpendicular magnetic anisotropy(PMA) are introduced. Next, a historical overview of PMA materials as magnetic electrodes, such as the RE–TM alloys TbFeCo and GdFeCo, novel tetragonal manganese alloys Mn–Ga, L10-ordered(Co, Fe)/Pt alloy, multilayer film [Co, Fe, CoFe/Pt, Pd, Ni, Au]N, and ultra-thin magnetic metal/oxidized barrier is offered. The other part of the article focuses on the optimization and fabrication of CoFeB/MgO/CoFeB p-MTJs, which is thought to have high potential to meet the main demands for non-volatile magnetic random access memory.     

Martensitic transformation and related magnetic effects in Ni-Mn-based ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys, which undergo the martensitic transformation, are famous multifunctional materials. They exhibit many interesting magnetic properties around the martensitic transformation temperature due to the strong coupling between magnetism and structure. Tuning magnetic phase transition and optimizing the magnetic effects in these alloys are of great importance. In this paper, the regulation of martensitic transformation and the investigation of some related magnetic effects in Ni-Mn-based alloys are reviewed based on our recent research results.     

Ni-X-In(X=Mn,Fe和Co)合金的缺陷稳定性和磁性能的第一性原理研究

Ni-Mn-In是一种新型的磁控形状记忆合金, 它通过磁场诱导逆马氏体相变实现形状记忆效应. 实验中常围绕化学计量比Ni₂MnIn合金进行成分调整, 以获得适宜的马氏体相变温度与居里温度, 在这个过程中必然会产生多种点缺陷. 本文使用量子力学计算软件包VASP, 在密度泛函理论的框架下通过第一原理计算, 系统地研究了非化学计量比Ni-X-In(X=Mn, Fe 和Co)合金的缺陷形成能和磁性能. 反位缺陷中, In和Ni在X亚晶格的反位缺陷(In_X和Ni_X)的形成能最低, Ni和X反位于Y的亚晶格(Ni_Y和X_Y)得到较高的形成能. 因此, In原子可以稳定立方母相的结构, 而X原子对母相结构稳定性的影响则相反; 空位缺陷中最高的形成能出现在In空位缺陷, 再次肯定了In原子对稳定母相结构的作用. 此外, 详细研究了点缺陷周围原子的磁性能以及电荷分布. 本文的计算结果在指导实验中的成分设计和开发新型磁控形状记忆合金方面具有重要意义.     

Modeling of the magnetic shape memory effect in Ni<Subscript>2</Subscript>MnGa films

The magnetic shape memory effect in Ni₂MnGa films is discussed and simulated in terms of the theory of diffuse martensitic transformations. Theoretical temperature and field dependences of the film deformation are obtained. Changes in the film shape under the action of the magnetic field and temperature are calculated.     

Nonlinear dynamic characteristics and optimal control of a giant magnetostrictive film-shaped memory alloy composite plate subjected to in-plane stochastic excitation

The nonlinear dynamic characteristics and optimal control of a giant magnetostrictive film(GMF)-shaped memory alloy(SMA) composite plate subjected to in-plane stochastic excitation are studied. GMF is prepared based on an SMA plate, and combined into a GMF–SMA composite plate. The Van der Pol item is improved to explain the hysteretic phenomena of GMF and SMA, and the nonlinear dynamics model of a GMF–SMA composite cantilever plate subjected to in-plane stochastic excitation is developed. The stochastic stability of the system is analyzed, and the steady-state probability density function of the dynamic response of the system is obtained. The condition of stochastic Hopf bifurcation is discussed, the reliability function of the system is provided, and then the probability density of the first-passage time is given. Finally, the stochastic optimal control strategy is proposed by the stochastic dynamic programming method.Numerical simulation shows that the stability of the trivial solution varies with bifurcation parameters, and stochastic Hopf bifurcation appears in the process; the system’s reliability is improved through stochastic optimal control, and the firstpassage time is delayed. A GMF–SMA composite plate combines the advantages of GMF and SMA, and can reduce vibration through passive control and active control effectively. The results are helpful for the engineering applications of GMF–SMA composite plates.     

Ni-Fe-Ga磁致形状记忆合金中间马氏体相变

采用差示扫描量热和x射线衍射技术研究Ni-Fe-Ga磁致形状记忆合金的马氏体相变行为.结 果发现,在多晶Ni565_56.5Fe190_19.0Ga245_{2 4.5}和Ni563_56.3Fe170_17.0 Ga267_26.7合金中除马氏体相变外,还观察到一次完整的、正相变和逆相 变对应出现、单 纯由温度诱发的中间马氏体相变.该中间马氏体相变与马氏体相变均为热弹性相变. 关键词： Ni-Fe-Ga 中间马氏体相变 磁致形状记忆合金