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Magnetic shape memory alloys exhibit various multifunctional properties, which range from high stroke actuation and magnetocaloric refrigeration to thermomagnetic energy harvesting. Most of these applications benefit from miniaturization and a single crystalline state. Epitaxial film growth is so far only possible on some oxidic substrates, but they are expensive and incompatible with standard microsystem technologies. Here, epitaxial growth of Ni–Mn–based Heusler alloys with single crystal‐like properties on silicon substrates is demonstrated by using a SrTiO3 buffer. It is shown that this allows using standard microfabrication technologies to prepare partly freestanding patterns. This approach is versatile, as its applicability for the NiTi shape memory alloy is demonstrated and spintronic and thermoelectric Heusler alloys are discussed. This paves the way for integrating additional multifunctional effects into state‐of‐the‐art microelectronic and micromechanical technology, which is based on silicon. [ABSTRACT FROM AUTHOR]

The magnetic domain structures and martensite microstructures of porous Ni2MnGa Heusler alloys with various circle-shaped and ellipse-shaped pores were systematically studied by the phase field method. The magnetization curves and magnetic field-induced strains (MFIS) at the external field were determined. A mesoscopic mechanism was proposed for simulation to reveal the influence of the pores on the microstructures and the MFIS of porous magnetic shape memory alloy. The stress concentration effect and the recovery strain of the porous alloy are studied. The results indicate the MFIS value increases when ellipse-shaped pores elongate along the twin boundary. The effects of porosity and pore size on MFIS for porous Ni-Mn-Ga alloys with randomly distributed pores were also explored. The present study is of guiding significance for understanding the role played by pores on the MFIS and may provide a possible way to adjust the functional properties of ferromagnetic shape memory alloys. [ABSTRACT FROM AUTHOR]

In this study, magnetostriction measurements were performed on the ferromagnetic Heusler alloy, Ni2MnGa0.88Cu0.12, which is characterized by the occurrence of the martensitic phase and ferromagnetic transitions at the same temperature. In the austenite and martensite phases, the alloy crystallizes in the L21 and D022-like crystal structure, respectively. As the crystal structure changes at the martensitic transition temperature (TM), a large magnetostriction due to the martensitic and ferromagnetic transitions induced by magnetic fields is expected to occur. First, magnetization (M-H) measurements are performed, and metamagnetic transitions are observed in the magnetic field of μ0H = 4 T at 344 K. This result shows that the phase transition was induced by the magnetic field under a constant temperature. Forced magnetostriction measurements (ΔL/L) are then performed under a constant temperature and atmospheric pressure (P = 0.1 MPa). Magnetostriction up to 1300 ppm is observed around TM. The magnetization results and magnetostriction measurements showed the occurrence of the magnetic-field-induced strain from the paramagnetic austenite phase to the ferromagnetic martensite phase. As a reference sample, we measure the magnetostriction of the Ni2MnGa-type (Ni50Mn30Ga20) alloy, which causes the martensite phase transition at TM = 315 K. The measurement of magnetostriction at room temperature (298 K) showed a magnetostriction of 3300 ppm. The magnetostriction of Ni2MnGa0.88Cu0.12 is observed to be one-third that of Ni50Mn30Ga20 but larger than that of Terfenol-D (800 ppm), which is renowned as the giant magnetostriction alloy. [ABSTRACT FROM AUTHOR]

Post-quench atomic reordering processes undergone by Ni2Mn1−xCuxGa alloys have been characterized in detail. The obtained results corroborate the hypothesis that proposes an atomic ordering process additional to the B2↔L21 one, consisting of the relocation in the Mn sublattice of Cu atoms misplaced by quench in the Ni sublattice. In addition, the results suggest that the ordering of the Cu atoms and the L21 ordering can occur in different sequences depending on the starting state of order. The analysis of the saturation magnetization validates the occurrence of two types of atomic movements; the values corresponding to different post-quench stages have been compared with those calculated for different atomic configurations, supporting the relocation mechanism of Cu atoms as the most plausible mechanism. The effect of the quenching temperature on the reordering processes has been also studied, and an assessment of the degree of quenched disorder is provided, suggesting the existence of an order–disorder transition associated with Cu atoms ordering. Finally, the effect of the Cu amount has been analyzed, confirming that a greater amount of Cu intensifies the process associated to ordering of Cu atoms, which takes place even in martensite. [ABSTRACT FROM AUTHOR]

The magnetic domain structures and martensite microstructures of porous Ni2MnGa Heusler alloys with various circle-shaped and ellipse-shaped pores were systematically studied by the phase field method. The magnetization curves and magnetic field-induced strains (MFIS) at the external field were determined. A mesoscopic mechanism was proposed for simulation to reveal the influence of the pores on the microstructures and the MFIS of porous magnetic shape memory alloy. The stress concentration effect and the recovery strain of the porous alloy are studied. The results indicate the MFIS value increases when ellipse-shaped pores elongate along the twin boundary. The effects of porosity and pore size on MFIS for porous Ni-Mn-Ga alloys with randomly distributed pores were also explored. The present study is of guiding significance for understanding the role played by pores on the MFIS and may provide a possible way to adjust the functional properties of ferromagnetic shape memory alloys.

In this study, magnetostriction measurements were performed on the ferromagnetic Heusler alloy, Ni2MnGa0.88Cu0.12, which is characterized by the occurrence of the martensitic phase and ferromagnetic transitions at the same temperature. In the austenite and martensite phases, the alloy crystallizes in the L21 and D022-like crystal structure, respectively. As the crystal structure changes at the martensitic transition temperature (TM), a large magnetostriction due to the martensitic and ferromagnetic transitions induced by magnetic fields is expected to occur. First, magnetization (M-H) measurements are performed, and metamagnetic transitions are observed in the magnetic field of μ0H = 4 T at 344 K. This result shows that the phase transition was induced by the magnetic field under a constant temperature. Forced magnetostriction measurements (ΔL/L) are then performed under a constant temperature and atmospheric pressure (P = 0.1 MPa). Magnetostriction up to 1300 ppm is observed around TM. The magnetization results and magnetostriction measurements showed the occurrence of the magnetic-field-induced strain from the paramagnetic austenite phase to the ferromagnetic martensite phase. As a reference sample, we measure the magnetostriction of the Ni2MnGa-type (Ni50Mn30Ga20) alloy, which causes the martensite phase transition at TM = 315 K. The measurement of magnetostriction at room temperature (298 K) showed a magnetostriction of 3300 ppm. The magnetostriction of Ni2MnGa0.88Cu0.12 is observed to be one-third that of Ni50Mn30Ga20 but larger than that of Terfenol-D (800 ppm), which is renowned as the giant magnetostriction alloy.

Quenched Ni2Mn1−xCuxGa ferromagnetic shape memory alloys undergo two consecutive post-quench ordering processes. The kinetics of order recovery has been analysed in detail for Ni2Mn0.8Cu0.2Ga, based on the calorimetric curves obtained during post-quench heating at constant rates. Isoconversional methods have been used to determine the activation energy, the pre-exponential factor, and the reaction model that best fits the two reordering processes. The kinetic analysis has been extended to samples quenched from different temperatures. The kinetic study shows that order improvement processes in quenched Ni2Mn0.8Cu0.2Ga alloys can be described by a first order reaction model, consistent with site-saturation nucleation and homogeneous diffusion-controlled growth, with apparent activation energies around 1.1 eV. The pre-exponential factors, especially those obtained for samples quenched from different temperatures, highlight the crucial role of the vacancies retained by high temperature quenching on the atomic reordering underlying the observed processes. [ABSTRACT FROM AUTHOR]

Reversible diffusionless martensitic transformation is the physical basis for the excellent properties of shape memory alloys. Precise control of transition behavior is generally regarded as an effective approach to improve the performance of alloys. Here, we propose a novel strategy to site-specifically control the transition behavior of Ni 2 MnGa shape memory alloy at the microscale. The nanoscale omega-like structure was found to appear and distribute uniformly in the parent matrix after high-energy ion beam irradiation, which resulted in the anisotropy of the {110} plane shuffling along the <110> direction. Consequently, the low-temperature phase of the sample changed from multi-variant five-layer modulated martensite to single-variant seven-layer modulated martensite. We further show that the specific phase transformation in Ni 2 MnGa shape memory alloys can be precisely controlled by the high-energy ion beam irradiation. This work should not only point out the potential risk of performance for the shape memory alloys serving in irradiation environments, but also contribute a novel way to fabricate more intelligent and specific-functional micro-/nano-devices of shape memory alloys. [Display omitted] • The nanoscale omega-like structure can be introduced into the Ni 2 MnGa alloy via high-energy ion beam irradiation. • The martensitic transformation path of the Ni 2 MnGa alloy can be controlled through the formation of omega-like phase. • Site-specifically controlling phase transformation path in Ni 2 MnGa alloy can be achieved by the focused ion beam (FIB) system. [ABSTRACT FROM AUTHOR]

Quenched Ni2Mn1−xCuxGa ferromagnetic shape memory alloys undergo two consecutive post-quench ordering processes. The kinetics of order recovery has been analysed in detail for Ni2Mn0.8Cu0.2Ga, based on the calorimetric curves obtained during post-quench heating at constant rates. Isoconversional methods have been used to determine the activation energy, the pre-exponential factor, and the reaction model that best fits the two reordering processes. The kinetic analysis has been extended to samples quenched from different temperatures. The kinetic study shows that order improvement processes in quenched Ni2Mn0.8Cu0.2Ga alloys can be described by a first order reaction model, consistent with site-saturation nucleation and homogeneous diffusion-controlled growth, with apparent activation energies around 1.1 eV. The pre-exponential factors, especially those obtained for samples quenched from different temperatures, highlight the crucial role of the vacancies retained by high temperature quenching on the atomic reordering underlying the observed processes.

• A systematic study of Ni–Mn–Ga substituted by elements from Cr to Cu is presented. • Transformation temperatures vary non–trivially with valence electron number. • Saturation magnetization at 10 K seems to follow Slater–Pauling curve. • Complex behavior of magnetocrystalline anisotropy is observed. Heusler Ni–Mn–Ga alloys with martensitic transformation being multiferroic materials exhibit magnetic shape memory (MSM) phenomena and moreover, these are touted for magneto– and multi–caloric effect (MCE). MCE benefits from high saturation magnetization and also large variability and sensitivity of transformation temperatures on alloying. In contrast, MSM phenomena are limited by low temperatures of both ferromagnetic and ferroelastic transformations. In an attempt to increase or modify the transformation temperatures and to understand the physical fundamentals beyond transformation variations and magnetic ordering, we experimentally investigate the magnetic and transformation properties in Ni 2 MnGa alloyed with transitional elements spanning from Cr to Cu. These elements are consecutively substituted on Ni, Mn, and Ga sites, to establish the effect of chemical substitution and elemental evolution. The elemental dependence of saturation magnetization, magnetocrystalline anisotropy, Curie temperature, and martensitic transformation temperatures are compared with (non–)stoichiometric Ni–Mn–Ga Heusler compounds. The observed behavior is complex and there are no clear trends of transformation temperatures as a function of the number of valence electrons or of non–bonding electrons per atom. The saturation magnetization at 10 K seems to follow the Slater–Pauling curve particularly for Ni substitution. Obtained results can serve as useful rectification for the theoretical prediction of new Ni–Mn–Ga–X Heusler alloys. [ABSTRACT FROM AUTHOR]

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The magnetization measurements at 5 K were carried out for Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) and Ni2MnGa1 − yCuy (0 ≤ y ≤ 0.25) alloys. All of the magnetization curves are characteristic of ferromagnetism or ferrimagnetism. By using Arrott plot analysis the spontaneous magnetization of all samples was determined from the magnetization curves. The magnetic moment per formula unit, μs, at 5 K was estimated from the spontaneous magnetization. For Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) alloys μs at 5 K decreases linearly with increasing x. On the other hand, the μs at 5 K for Ni2MnGa1 − yCuy (0 ≤ y ≤ 0.25) alloys decreases more steeply with increasing x compared to the μs for Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) alloys. On the basis of the experimental results, the site-occupation configurations of Ni2Mn1 − xCuxGa (0 ≤ x ≤ 0.40) and Ni2MnGa1 − yCuy (0 ≤ y ≤ 0.25) alloys are proposed.

Reviewing the results of recent first-principles calculations, we work out a close analogy between the two paradigmatic classes of magnetic shape memory materials, the ordered Ni2MnGa Heusler compound and the disordered Fe70Pd30 alloy. Despite fundamental differences between both systems, we can demonstrate that in both cases the very low formation energy for tetragonal twins on the smallest length scale opens an alternative transformation path into an adaptive hierarchical microstructure which is important for the functional behavior. The low energy of the (101) twin boundary corresponds to a shear instability which is associated to the soft transversal acoustic phonon in both systems. In turn, changes of the energy landscape upon magnetic disorder are responsible for the stability of austenite. This points out the strong influence of magnetoelastic coupling on the transformation process. [ABSTRACT FROM AUTHOR]

Abstract: We use X-ray diffraction to confirm that the 5M phase of the stoichiometric Heusler alloy Ni2MnGa has an incommensurate modulation with wavevector . We find the crystal to be macroscopically twinned with a structure well described by the theory of martensitic phase transitions, though theoretical calculations of the X-ray diffraction rule out the existence of an adaptive phase. The premartensite phase is found to be slightly incommensurate with . [Copyright &y& Elsevier]

Abstract: The novel mechanism of temperature-gradient-induced single phase boundary actuation is presented for a single crystalline ferromagnetic shape memory alloy (FSMA) foil. It is shown that applying a temperature gradient along the FSMA foil specimen results in the formation and propagation of a martensite–austenite phase boundary from the hot to the cold side, allowing for reproducible strain–temperature characteristics. The selection of martensite variants upon phase transformation is controlled by simultaneously applying a bias magnetic field, which determines the maximum strain response. Single phase boundary actuation is demonstrated for a Ni–Mn–Ga foil of 100μm thickness with 10M martensite structure at room temperature. A small temperature gradient of 5Kmm–1 and a bias field along the temperature gradient of 120mT are sufficient to achieve the maximum possible strain of 4.1%, corresponding to the length difference of the short c-axis of tetragonal martensite and the axis of cubic austenite. For a bias magnetic field in the perpendicular direction, the maximum strain change is −1.9%. [Copyright &y& Elsevier]

Abstract: Magnetic shape memory alloys have great promise as magneto-caloric effect refrigerant materials due to their combined magnetic and structural transitions. Computational and experimental research is reported on the Ni2MnGa material system. The magnetic states of this system are explored using the Wang-Landau statistical approach in conjunction with the Locally Self-consistent Multiple-Scattering method. The effects of alloying agents on the transition temperatures of the Ni2MnGa alloy are investigated using differential scanning calorimetry and superconducting quantum interference device. Experiments are performed at the Spallation Neutron Source at Oak Ridge National Laboratory to observe the structural and magnetic phase transformations. [Copyright &y& Elsevier]

Abstract: We present the magnetic properties and the magnetic phase diagram of Ni50Mn50−x Ga x ferromagnetic shape memory alloys across a wide concentration range. Martensitic transformation, intermediate transformation, B2–L21 order–disorder transformation, Néel and Curie temperatures are determined for the prepared samples. The martensitic transformation temperature decreases with increasing Ga concentration and bends two times when crossing the Curie temperature and the intermediate-phase transformation temperature. Spontaneous magnetization and its composition dependence were also investigated. Composition dependence of the transformation temperatures and the spontaneous magnetization in the martensite phase of Ni50Mn50−x Ga x are compared with those of Ni50Mn50−x In x and Ni50Mn50−x Sn x , revealing a similarity in the NiMn-based alloy systems. [Copyright &y& Elsevier]

Abstract: Magnetic shape memory alloys display magnetic-field-induced strain (MFIS) of up to 10% as single crystals. Polycrystalline materials are much easier to create but display a near-zero MFIS because twinning of neighboring grains introduces strain incompatibility, leading to high internal stresses. Pores reduce these incompatibilities between grains and thus increase the MFIS of polycrystalline Ni–Mn–Ga, which after training (thermo-magneto-mechanical cycling) exhibits MFIS as high as 8.7%. Here, we show that this training effect results from a decoupling of struts surrounding pores in polycrystalline Ni–Mn–Ga during the martensitic transformation. To show this effect in highly textured porous samples, neutron diffraction measurements were performed as a function of temperature for phase characterization and a method for structure analysis was developed. Texture measurements were conducted with a magnetic field applied at various orientations to the porous sample, demonstrating that selection of martensite variants takes place during cooling. [Copyright &y& Elsevier]

The magnetization measurements at 5 K were carried out for Ni2Mn1 - xCuxGa (0 ≤ x ≤ 0.40) and Ni2MnGa1 - yCuy (0 ≤ y ≤ 0.25) alloys. All of the magnetization curves are characteristic of ferromagnetism or ferrimagnetism. By using Arrott plot analysis the spontaneous magnetization of all samples was determined from the magnetization curves. The magnetic moment per formula unit, μs, at 5 K was estimated from the spontaneous magnetization. For Ni2Mn1 - xCuxGa (0 ≤ x ≤ 0.40) alloys μs at 5 K decreases linearly with increasing x. On the other hand, the μs at 5 K for Ni2MnGa1 - yCuy (0 ≤ y ≤ 0.25) alloys decreases more steeply with increasing x compared to the μs for Ni2Mn1 - xCuxGa (0 ≤ x ≤ 0.40) alloys. On the basis of the experimental results, the site-occupation configurations of Ni2Mn1 - xCuxGa (0 ≤ x ≤ 0.40) and Ni2MnGa1 - yCuy (0 ≤ y ≤ 0.25) alloys are proposed. [ABSTRACT FROM AUTHOR]

Abstract: The complex crystal structure and variant distribution of single-crystalline freestanding Ni–Mn–Ga films was studied in detail using X-ray diffraction in two- and four-circle geometry. Selective chemical etching of a chromium buffer layer facilitates the release of the rigid MgO substrate that would inhibit magnetically induced reorientation (MIR) of variants. The substrate-constrained as well as the freestanding films possess identical crystal structure featuring a seven-layered (pseudo-)orthorhombic modulation (7M/14M). Of the 12 different variants observed before and after releasing the film from the substrate, four are predominantly represented. These have the short c-axis and the long a-axis aligned in the film plane. This allows for a reduction of the stray field energy, since the c-axis is the magnetic easy axis. The variant distribution induces a distinct mesoscopic surface pattern that can be observed by light microscopy. On a freestanding microbridge, prepared by photolithography in combination with selective chemical etching, the phase transformation can thus be traced directly. After several heating cycles the surface pattern remains basically unchanged. Application of a magnetic field of 0.5T while the film transforms from austenite to martensite leads to a considerable change of the structure. [Copyright &y& Elsevier]

The magnetization and magnetic field-induced strain behavior of the ferromagnetic shape memory alloy, Ni2MnGa, under constant compressive stress were studied using the phase-field method. Based on the evolving magnetic domain and martensitic structures, we analyzed the cycling effect, magnetization hysteresis, strain recoveries, and coupling between the domain wall and martensite twin boundaries. We compared the switching behavior of single variant and multivariant martensite structures. We observed three types of magnetic field-induced strain mechanisms, depending on the magnitude of the applied compressive stress. The study revealed that the martensite microstructure of the magnetic shape memory alloy plays an important role in magnetization and strain evolution during loading and unloading of an external magnetic field under different stress conditions. The results are compared with existing experimental observations. [ABSTRACT FROM AUTHOR]

Abstract: Monocrystalline Ni–Mn–Ga alloys show magnetic-field-induced strains (MFIS) of up to 10% as a result of reversible twinning; by contrast, polycrystalline Ni–Mn–Ga shows near-zero MFIS due to strain incompatibilities at grain boundaries inhibiting twinning. Recently, we showed that porous polycrystalline Ni–Mn–Ga exhibits a small, but non-zero, MFIS value of 0.12% due to reduction of these incompatibilities by the porosity. Here, we study the effect of pore architecture on MFIS for polycrystalline Ni–Mn–Ga foams. Foams with a combination of large (∼550μm) and small (∼80μm) pores are fabricated by the replication method and exhibit thinner nodes and struts compared to foam containing only large (∼430μm) pores. When magnetically cycled, both types of foams exhibit repeatable MFIS of 0.24–0.28% without bias stress. As the cycle number increases from a few tens to a few thousands, the MFIS drops due to damage accumulation. The rate of MFIS decrease is lower in the dual-pore foam, as expected from reduced constraints on the twin boundary motion, since twins span the whole width of the thinner nodes and struts. [Copyright &y& Elsevier]

Abstract: (Ni52.5Mn23.5Ga24)100−x Co x (x =2, 4, 6, 8) alloys were prepared for the determination of their characteristic phase transformation temperatures (M s, M f, A s, A f) and high temperature yield strength of the parent phase. The driving force for the martensitic phase transformation in these alloys was estimated from thermodynamic point of view by taking into account the contributions of the shape deformation and the twin formation accompanying the martensitic phase transformation to the energy required to initiate the transformation; and a positive linear relationship between the driving force and the yield strength of the parent phase at M s temperature was established. [Copyright &y& Elsevier]

We studied the effect of selective substitution in Fe and Co 3d elements on the structural and magnetic phase transitions in the prototypical magnetic shape memory alloy Ni-Mn-Ga. We determined the phase diagram for each elemental doping substitution by means of calorimetry and ac susceptibility measurements. An effort was made to substitute each constituent element by the same amounts of doping and to study the role of parameters other than electronic concentration in controlling phase stability. Specifically, selective doping with atoms of similar atomic radii but different magnetic properties allowed us to investigate the role of magnetic interactions on the relative phase stability of the ternary compound. We determined the entropy change associated with the martensitic transition for each quaternary alloy to obtain further information on the effect of magnetism on the relative stability of the involved phases exhibited by these compounds. [ABSTRACT FROM AUTHOR]

Abstract: Since the discovery of NiMnGa shape memory alloys, including the ferromagnetic shape memory alloys and high-temperature shape memory alloys, the severe brittleness, especially the grain-boundary brittleness, has been a crucial bottleneck hindering their development. Here, high plasticity is reported in a high-temperature shape memory alloy, Ni50Mn25Cu18Ga7, with the martensitic transformation at 615°C and face-centered tetragonal martensite. The tensile stress and strain are 488.7MPa and 11.7%, respectively. A compressive true strain of 70% is monitored without fracture. [Copyright &y& Elsevier]

Abstract: For the first time, the 7M modulated structure, frequently observed in ferromagnetic shape memory Ni–Mn–Ga martensitic phases, is solved by powder diffraction analysis. Two polycrystalline samples with composition Ni2Mn1.2Ga0.8 and Ni2.15Mn0.85Ga, respectively, showing a 7M martensitic state stable at room temperature, were studied. The determination of the modulated crystal structure of Ni2Mn1.2Ga0.8 martensite was achieved by refining the X-ray powder diffraction pattern by the Rietveld method. The basic structure belongs to monoclinic symmetry. The crystal structure, solved within the superspace approach, is found to show an incommensurate 7M modulation with q =0.308c ∗. The Rietveld refinement for Ni2.15Mn0.85Ga martensite on the basis of neutron powder data surprisingly provides a very similar incommensurate 7M structure with the same periodicity and analogous modulation function. The incommensurate structure presents typical displacive modulation with several analogies with the Zhdanov (5, )2 stacking sequence. [Copyright &y& Elsevier]

Abstract: The motion of twin boundaries in a Ni48Mn27Ga25 (at.%) single crystal has been investigated by a low-frequency mechanical relaxation spectrum analyser. A distinct internal friction peak was observed when cooling from room temperature to −70°C at cooling rate of about 1K/min in this single crystal with M s temperature above 35°C. This internal friction has been measured as a function of temperature at four frequencies: 0.1, 0.5, 1, and 5Hz. The internal friction peaks were found to shift to high temperature with increasing frequency demonstrating a relaxation process. The activation enthalpy of the process was estimated as 0.6eV according to the Arrhenius equation. Associated with the literature results, the underlying dynamic process was attributed to the movement of partial twin dislocations within the {112} twinning planes. Interaction between twin boundaries and defects during the movement in this magnetically induced shape memory alloy was briefly discussed. [Copyright &y& Elsevier]

Abstract: Lorentz microscopy is employed to study the evolution of magnetic domain structures across ferromagnetic and ferroelastic martensitic transformations in near stoichiometric Ni2MnGa. In situ transmission electron microscopy studies have revealed a new phenomenon of magnetic domain memory of the austenite across both ferroic transformations. The reversible magnetic domain structure is attributed to pinning of the magnetic domain walls by antiphase boundaries in the Heusler phase. Two types of martensite domain structures, herringbone and stripe domain, have been observed. The type of domain arrangement is found to be dependent on the relative orientation of the easy axis of magnetization with respect to the thin foil normal. [Copyright &y& Elsevier]

Abstract: It is well known that the composition of ferromagnetic shape memory Ni–Mn–Ga Heusler alloys determines both temperature of martensitic transformations and the structure type of the product phase. In the present work we focused our attention on the structural study of the so-called “5M” modulated structure. In particular, the structure of Ni1.95Mn1.19Ga0.86 martensitic phase is analysed by powder X-ray diffraction (PXRD) and compared with that of the stoichiometric Ni2MnGa martensite. The study of the diffraction data reveals the occurrence of commensurate (C) structural modulation in Ni1.95Mn1.19Ga0.86; this contrasts with Ni2MnGa, where an incommensurate (IC) structural modulation was evident. The two phases also differ in the symmetry of the fundamental martensitic lattice. In fact, the incommensurate modulation is related to an orthorhombic basic structure, while the commensurate variant presents a monoclinic symmetry. The commensurate modulated structure has been investigated by using the superspace approach already adopted to solve the structure of Ni2MnGa martensite. The structure has been determined by Rietveld refinement of PXRD data. [Copyright &y& Elsevier]

Abstract: The microstructure and magnetic domain structure of austenitic Heusler Ni2MnGa are investigated as a function of heat treatment to study the interplay of anti-phase boundaries and magnetic domain walls. Conventional electron microscopy observations on arc-melted polycrystalline samples show that anti-phase boundaries in this system are invisible for standard two-beam imaging conditions, due to the large extinction distance of the Heusler superlattice reflections. Lorentz Fresnel and Foucault observations on quenched samples reveal a wavy magnetic domain morphology, reminiscent of curved anti-phase boundaries. A close inspection of the domain images indicates that the anti-phase boundaries have a magnetization state different from that of the matrix. Fresnel image simulations for a simple magnetization model are in good agreement with the observations. Magnetic coercivity measurements show a decrease in coercivity with annealing, which correlates with the microscopy observations of reduced anti-phase boundary density for annealed samples. [Copyright &y& Elsevier]

Abstract: The evidence for the preferential orientation of Ni–Mn–Ga martensitic variants by compression or magnetic field is presented. The evolution of martensitic twinning configuration in Ni50Mn27.5Ga22.5 single crystal was optically observed initially under the applied compressive stress and then followed by the magnetic field. The magnetic field induced strain (MFIS) was measured at different conditions. The MFIS was not observed in the original three-variant self-accommodated state. One percent MFIS was obtained after compression under 5MPa, attributed to a disappearance of the self-accommodated structure replaced by a two-variant structure. During magnetization, the favorable variants expand at the expense of others. After several cycles of compression and successive magnetization, a near single variant state and MFIS of 5.5% were achieved in the rectangular single crystal. Forty-five degree macroscopic twin bands were observed during the MFIS testing on (010) surface of an oriented rectangular sample with {100} faces. [Copyright &y& Elsevier]

Abstract: Neutron spectroscopy is an ideal technique to study the structure and dynamics of crystals. For the ferromagnetic shape memory alloy Ni2MnGa, all previously obtained information from inelastic neutron scattering experiments is restricted to the phonon dispersion in the austenitic (fcc) phase of alloys with different compositions. For the (tetragonally distorted) martensitic phase recent inelastic neutron scattering data are presented. These new data were taken on a single crystal with stoichiometric composition. A single-variant martensitic phase of the sample has been obtained by the application of magnetic fields in horizontal or vertical direction with respect to the scattering plane used in the experiments. The measured phonon-dispersion curves are compared with recently published ab initio (zero-temperature) phonon-dispersion calculations. The anomalous phonon behavior observed in both, the austenitic and martensitic phase is discussed. [Copyright &y& Elsevier]

Abstract: The ferromagnetic shape memory (MSM) alloy Ni2MnGa undergoes a martensitic transformation (MT) at T=220K on cooling. The structure of this phase is studied by powder X-ray diffraction experiment. The analysis of the experimental data combined with the huge information reported in literature allowed to conclude that the Ni2MnGa martensite shows an incommensurate modulated structure closely related to a five-fold layered superstructure. The symmetry of the basic structure is found to be orthorhombic. The structure is refined by Rietveld method with superspace group Immm(00γ)s00 having a=4.2187(1)Å, b=5.5534(1)Å and c=4.1899(1)Å and modulation vector q=0.4248(3)c*. The results show that the modulation is mainly related to the periodic shuffling of the atomic layers perpendicular to the c-axis of the orthorhombic basic structure. [Copyright &y& Elsevier]

Abstract: The existence of a magnetic moment at the nickel sites in ferromagnetic shape-memory alloys (FSMA) has long been speculated. Using a state-of-the-art X-ray magnetic circular dichroism (XMCD) technique we found that Ni 3d moments indeed exist in Ni–Mn–Ga-based FSMA. Band-structure calculations were used to simulate the XMCD spectra and estimate this moment to be . Such a Ni moment plays a critical role in the dispersions of non-collinear spin structures, which determine the finite-temperature magnetic properties. [Copyright &y& Elsevier]

We report results of ab initio calculations of the ferromagnetic Heusler alloy Ni2MnGa. Particular emphasis is placed on the stability of the low temperature tetragonal structure with c/a=0.94. This structure cannot be derived from the parent L21 structure by a simple homogeneous strain associated with the soft elastic constant C′. In order to stabilize the tetragonal phase, one has to take into account shuffles of atoms, which form a wave-like pattern of atomic displacements with a well defined period (modulation). While the modulation is related to the soft acoustic [1 1 0]-TA2 phonon mode observed in Ni2MnGa, we obtain additional atomic shuffles, which are related to acoustic-optical coupling of the phonons in Ni2MnGa. In addition, we have simulated an off-stoichiometric system, in which 25% of Mn atoms are replaced by Ni. The energy of this structure also exhibits a local minimum at c/a=0.94. This allows us to conclude that both shuffles and atomic disorder stabilize the c/a=0.94 structure. In both cases the stability seems to be associated with a dip in the minority-spin density of states (DOS) at the Fermi level, being related to the formation of hybrid states of Ni d and Ga p minority-spin orbitals. [Copyright &y& Elsevier]

Compositional dependence of magnetic entropy change in Ni2+xMn1-xGa Heusler alloys was investigated experimentally. The largest entropy change ΔS=20.7±1.5 J/K kg in the magnetic field H=1.8 T was revealed in the Ni2.18Mn0.82Ga alloy at a magneto-structural phase transition temperature of Ts=333.2 K. Magnetic-field-induced structural phase transition was observed. [Copyright &y& Elsevier]

Shape change (strain) was observed for a polycrystalline thermoelastic ferromagnetic shape memory alloy Ni–9.5at.% Mn–27at.% Ga upon cooling and healing with and without magnetic field. The shape change occurred at martensite start temperature, MS, and it was increased with increasing magnetic field. A large strain of 0.82% was observed when a magnetic field of 10 kOe was applied under a compressive stress of 126 MPa at 73 °C, several degrees above austenite finish temperature, Af. This magnetic field effect on changes of MS can be explained by a modified Clausius–Clapeyron relationship. [Copyright &y& Elsevier]

In single crystal Ni2MnGa that exhibits 4.6% magnetic field induced strain (MFIS) at 291 K, it is found that the effect lacks thermal stability. As the temperature rises, the MFIS decreases concomitantly with the lattice distortion. At the same time, the critical magnetic field necessary to activate twin boundary motion falls. A new intermediate martensitic phase is observed. [Copyright &y& Elsevier]

• In Ni 2 Mn 1−x Cu x Ga alloys post-quench atomic reordering show two separated processes. • This unusual behaviour has not been reported so far for Heusler–type alloys. • One of the processes is attributed to Mn diffusion giving rise to L2 1 reordering. • An additional process is exclusive to Ni 2 Mn 1− x Cu x Ga alloys. • The latter is attributed to diffusion of Cu atoms towards their most favourable sites. The effect of atomic order on the martensitic and magnetic transformations undergone by Ni 2 Mn 1− x Cu x Ga ferromagnetic shape memory alloys has been investigated. Different degrees of order have been induced by performing selected thermal treatments, and it has been found that, although both the structural (martensitic) and magnetic transformation temperatures increase with the improvement of the atomic order, each of these temperatures evolve with different kinetics. Reinforcing this result, during post–quench heating of Ni 2 Mn 1− x Cu x Ga alloys two consecutive DSC exothermic peaks are observed, while the ternary Ni–Mn–Ga, Co–doped Ni–Mn–Ga and even Cu–doped Ni–Mn–Ga alloy when Cu replaces Ni show a single peak. The kinetics of the post–quench ordering processes have been studied using the Kissinger's method, and two different activation energies have been obtained: 1.16 eV for the process that is common to all alloys and 1.35 eV for the one that is only observed in Ni 2 Mn 1− x Cu x Ga alloys. The former, responsible for the change in Curie temperature, is attributed to the improvement of L2 1 order mainly due to Mn diffusion; for the second, which underlies the rise of martensitic transformation temperatures, diffusion of Cu atoms, misplaced in the Ni sublattice after quench, towards their most favourable sites in the Mn sublattice is proposed as the responsible mechanism. [ABSTRACT FROM AUTHOR]