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NiMnGa Heusler alloy plays a key role as reference system for ferromagnetic shape memory alloys (FeSMAs) and their peculiar functional properties including large magnetic-field-induced strain, magnetocaloric and elastocaloric effects. Moreover, the microstructure of polycrystalline NiMnGa alloys has been investigated and optimized to improve the mechanical properties and to reduce their typical brittleness. For this reason, increasing interest has been devoted to different kinds of fabrication routes for this alloy, such as powder metallurgy processes. In the present study, a polycrystalline Ni50Mn30Ga20 (atomic %) alloy is produced by means of an unconventional sintering method that involves the canning of powders and the subsequent processing by hot rolling. This process was implemented according to the results obtained by means of the open die pressing (ODP) sintering of NiMnGa which was investigated in a previous work. The present study is aimed at developing an alternative and cost-effective sintering method for the consolidation of fully-dense NiMnGa samples. The process allowed reducing the intrinsic brittleness of the alloy. A heat treatment at 925 °C for 6 h was applied and allowed achieving a maximum adiabatic ΔT of +6.3 °C and - 4.5 °C with a strain of 4 % and a strain rate of 400 %/min in compression. The novel method led to very promising elastocaloric properties, making NiMnGa a suitable candidate for solid-state cooling and heating applications.

In the present report, we have studied the structural, microstructural, magnetic, exchange blas (EB) properties and magnetoresistance (MR) in Mn rich (Mn at. -50%) Mn50Ni50-xSnx (x = 10) shape memory Heusler alloy ribbons, prepared using the melt spinning method. These ribbons were found to exhibit a first order structural (i.e., martensitic) transition at around 205 K from the high temperature austenite to the low temperature martensite phase. The martensitic transition occurs at comparatively lower temperatures in these ribbons than in the same bulk alloy. Curie temperature of the austenite phase (TCA) is found to be larger i.e., around room temperature than that of bulk alloy. A significant EB field (HEB) of around 960 Oe has been observed at 2 K for these ribbons, which is found to be comparable to that reported for other Heusler systems. The presence of the EB effect in these ribbons is attributed to the coexistence of FM/AFM exchange interactions in the martensite phase. They are also found to show a maximum negative MR of around 12% near the martensitic transition, for 50 kOe field change. Investigation of DC magnetization, AC susceptibility measurements and the observation of training effect (i.e., characteristic feature of EB) strongly corroborates with the coexistence of FM/AFM exchange interactions in the martensite phase of these ribbons, which results in the large EB. The effects of temperature and magnetic field on the EB properties and MR have also been studied here. [ABSTRACT FROM AUTHOR]

Featured Application: High-speed actuators and sensors. To meet the great requirements of future technologies, such as robots, single-crystal (SC) Ni-Mn-Ga alloys and their composites were designed and investigated in this study. Ferromagnetic shape memory alloys (FSMAs) are promising materials for applications in high-speed actuators, which are core components of robots; however, there are some issues of embrittlement and small deformation strain. Therefore, in this work, we first prepared SC Ni-Mn-Ga alloys for fundamental investigations of the shape deformations under the application of different fields (e.g., compressive and magnetic fields). Second, the SC Ni-Mn-Ga alloys were integrated with polymers of epoxy resin or silicone rubber to solve the embrittlement problem. The obvious two-stage yielding and sudden intensifying of the magnetization both suggest martensite variant reorientation (MVR) under the compressive and magnetic fields, respectively. Micro-computed tomography (μCT) and an X-ray diffractometer were utilized for the observations of shape deformation brought about by the MVR of the SC Ni-Mn-Ga particles in the polymer matrix. Clear MVR and shape deformation could be found in the compressed composites. [ABSTRACT FROM AUTHOR]

High-speed actuators are greatly required in this decade due to the fast development of future technologies, such as Internet-of-Things (IoT) and robots. The ferromagnetic shape memory alloys (FSMAs), whose shape change could be driven by applying an external magnetic field, possess a rapid response. Hence, these materials are considered promising candidates for the applications of future technologies. Among the FSMAs, the Ni-Mn-Ga-based materials were chosen for their large shape deformation strain and appropriate phase transformation temperatures for near-room temperature applications. Nevertheless, it is widely known that both the intrinsic brittleness of the Ni-Mn-Ga alloy and the constraint of shape deformation strain due to the existence of grain boundaries in the polycrystal inhibit the applications. Therefore, various Ni-Mn-Ga-based composite materials were designed in this study, and their shape deformation behaviors induced by compressive or magnetic fields were examined by the in situ micro CT observations. In addition, the dependence of the martensite variant reorientation (MVR) on the crystallographic directions was also investigated. It was found that most of the MVRs are active within the magnetic field range applied in the regime of the <100>p, <110>p, and <111>p of the single-crystal {100}p Ni-Mn-Ga cubes. [ABSTRACT FROM AUTHOR]

The martensitic phase transition and exchange bias effect of the Ni-Mn-based ferromagnetic shape memory alloys (FSMAs) Ni45Co5Mn37In13 (Ni-Co-Mn-In) films are investigated in this paper. The martensitic transformation properties of the Ni-Co-Mn-In alloy target material are manipulated by the process of electric arc melting, melt-fast quenching, and high-temperature thermal pressure. The Ni-Co-Mn-In alloy films with martensite phase transition characteristics are obtained by adjusting deposition parameters on the (001) MgO substrate, which shows a significant exchange bias (EB) effect at different temperatures. With increasing sputtering power and time, the film thickness increases, resulting in a gradual relaxation of the constraints at the interface between the film and the substrate (the interfacial strain decreases as the increase of thin film thickness), which promotes the martensite phase transition. Between zero-field cooling (ZFC) and field-cooled (FC) curve obvious division zone, the decrease of exchange bias field (HEB) and coercive force field (Hc) with an increase in test temperature is due to ferromagnetic (FM) interaction begins to dominate, resulting in a reduction of antiferromagnetic (AFM) anisotropy at the interface. The maximal HEB and Hc reach ~465.7 Oe and ~306.9 Oe at 5 K, respectively. The manipulation of the martensitic transformation and EB effect of the Ni-Co-Mn-In alloy films demonstrates potential application in the field of information and spintronics. [ABSTRACT FROM AUTHOR]

Studies on Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) have been quite active topics in recent years. Unlike previous reports where the amount of Co doping was generally less than 8 at%, in this work Ni55−xCoxMn20Ga25 (8.5 ≤ x ≤ 11.0) alloys were studied with high Co doping. Unusual effects of both composition and magnetic field on phase transitions were observed. With the increase in substitution of Co for Ni, the magnetic transition temperatures increase gradually but the martensitic transformation temperature decreases quite sharply. In particular, the average decrease in the martensitic transformation temperatures is up to 100 K which is nearly twice that in the case of Co content of less than 8 at%. Further, under an applied magnetic field ranging from 0.03 to 0.60 T, abnormal stabilization of a martensite phase with lower magnetization was monitored. Magnetic entropy change of 1.6 J·kg−1·K−1 was induced at the martensitic transformation of Ni46.5Co8.5Mn20Ga25 alloy by an applied field of 1 T. The magnetic contributions, including the magnetocrystalline anisotropy and the Zeeman energy, to the thermodynamics of the martensitic transformation are considered to understand the observed unusual phenomena. This work results in new insights into the understanding of Co-doped Ni–Mn-based ferromagnetic shape memory alloys. [ABSTRACT FROM AUTHOR]

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance. [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 present study investigates the influence of iron (Fe) doping on the martensitic transformation and magnetic properties of Ni50−xMn37Sn13Fex(x = 0.5, 1, 1.5) magnetic shape memory alloys in ribbon form. The ribbons were prepared using arc-melting followed by melt-spinning processes and were characterized using X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, and vibrating sample magnetometry. Our findings demonstrate that the addition of Fe shifts the martensitic transformation to lower temperatures and increases the Curie point of the austenitic phase, TcA\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$T_{c}^{\text A}$$\end{document}, leading to an enhanced magnetism in the austenitic phase. Moreover, a significant increase in the magnetization jump (ΔM)\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$(\Delta M)$$\end{document} is observed, from 3.3emu\ g-1\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$ 3.3\;{\text{emu\ g}}^{-1} $$\end{document} for x=1\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$x=1$$\end{document} to 17emu\ g-1\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$17 \, {\text{emu\ g}}^{-1} $$\end{document} for x=1.5\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$x=1.5$$\end{document} under a 50Oe\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$50 \, \text {Oe}$$\end{document} applied magnetic field. The structural transformations are also found to be sensitive to the external applied magnetic field. The isothermal magnetization curves exhibit the exchange bias effect, which confirms the coexistence of antiferromagnetic and ferromagnetic coupling in the samples. Furthermore, the exchange bias effect increases with the Fe content. [ABSTRACT FROM AUTHOR]

Polycrystalline Ni-Co-Mn-Sn based ferromagnetic shape memory alloys (FSMAs) show promise as actuator materials, but their practical application involving magnetic field induced strain (MFIS) is often limited by three factors: the requirement for high magnetic fields (> 5 T), martensitic transition temperature away from room temperature, and limited recovery of pre-strain applied to the martensite phase. Current work investigates the martensitic transition and shape memory effect under the application of magnetic field for bulk polycrystalline Ni 37 Co 11 Mn 43 Sn 9 alloy. The outcome of the study reveals a metamagnetic transition from the martensitic phase to the austenitic phase at a low field of 2.8 T at 300 K which results 0.25 % spontaneous MFIS. Interestingly, 1.3 % pre-strained specimen registers a 100 % recovery with the application of magnetic field of 4.5 T. Furthermore, the pre-strained specimen exhibited a two-way shape memory effect between a strain value of 1.0 % to 1.55 % during the field loading and unloading sequences. Notably, this study demonstrates, for the first time to the best of our knowledge that the spontaneous MFIS and pre-strain add together. This finding paves the way for achieving a giant MFIS by pre-straining a Ni-Mn-Sn/In class of FSMAs which shows large spontaneous MFIS. [Display omitted] [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) and ferromagnetic shape memory alloys (FeSMAs) have recently attracted interest for solid state refrigeration applications. Among NiMnGa-based quaternary systems, NiMnGaCu exhibits an interesting giant magnetocaloric effect thanks to the overlapping of the temperatures related to the magnetic transition and the thermoelastic martensitic transformation (TMT); in particular, for compositions with Cu content of approximately 6 at%. In the present work, we investigated the improvement effect of TMT on the total entropy change (ΔS) in the elastocaloric performances of polycrystalline Ni50Mn18.5Cu6.5Ga25 at% alloy samples, just above room temperature. We report an extensive calorimetric and thermomechanical characterization to explore correlations between microstructural properties induced by the selected thermal treatment and elastocaloric response, aiming at providing the basis to develop more efficient materials based on this quaternary system. Both ΔT and ΔS values obtained from mechanical curves at different temperatures and strain recovery tests under fixed load vs. T were considered. Maximum values of ΔS = 55.9 J/KgK and ΔT = 4.5 K were attained with, respectively, a stress of 65 MPa and strain of 4%. The evaluation of the coefficient of performance (COP) was carried out from a cyclic test. [ABSTRACT FROM AUTHOR]

Ni-Mn-Sn ferromagnetic shape memory alloys, which can be stimulated by an external magnetic field, exhibit a fast response and have aroused wide attention. However, the fixed and restricted working temperature range has become a challenge in practical application. Here, we introduced strain engineering, which is an effective strategy to dynamically tune the broad working temperature region of Ni-Co-Mn-Sn alloys. The influence of biaxial strain on the working temperature range of Ni-Co-Mn-Sn alloy was systematically investigated by the ab initio calculation. These calculation results show a wide working temperature range (200 K) in Ni14Co2Mn13Sn3 FSMAs can be achieved with a slight strain from 1.5% to −1.5%, and this wide working temperature range makes Ni14Co2Mn13Sn3 meet the application requirements for both low-temperature and high-temperature (151–356 K) simultaneously. Moreover, strain engineering is demonstrated to be an effective method of tuning martensitic transformation. The strain can enhance the stability of the Ni14Co2Mn13Sn3 martensitic phase. In addition, the effects of strain on the magnetic properties and the martensitic transformation are explained by the electronic structure in Ni14Co2Mn13Sn3 FSMAs. [ABSTRACT FROM AUTHOR]

High mobility of twin boundaries in modulated martensites of Ni-Mn-Ga-based ferromagnetic shape memory alloys holds a promise for unique magnetomechanical applications. This feature has not been fully understood so far, and in particular, it has yet not been unveiled what makes the lattice mechanics of modulated Ni-Mn-Ga specifically different from other martensitic alloys. Here, results of dedicated laser-ultrasonic measurements on hierarchically twinned five-layer modulated (10M) crystals fill this gap. Using a combination of transient grating spectroscopy and laser-based resonant ultrasound spectroscopy, it is confirmed that there is a shear elastic instability in the lattice, being significantly stronger than in any other martensitic material and also than what the first-principles calculations for Ni-Mn-Ga predict. The experimental results reveal that the instability is directly related to the lattice modulations. A lattice-scale mechanism of dynamic faulting of the modulation sequence that explains this behavior is proposed; this mechanism can explain the extraordinary mobility of twin boundaries in 10M., (© 2024 The Author(s). Advanced Materials published by Wiley‐VCH GmbH.)

To meet the great requirements of future technologies, such as robots, single-crystal (SC) Ni-Mn-Ga alloys and their composites were designed and investigated in this study. Ferromagnetic shape memory alloys (FSMAs) are promising materials for applications in high-speed actuators, which are core components of robots; however, there are some issues of embrittlement and small deformation strain. Therefore, in this work, we first prepared SC Ni-Mn-Ga alloys for fundamental investigations of the shape deformations under the application of different fields (e.g., compressive and magnetic fields). Second, the SC Ni-Mn-Ga alloys were integrated with polymers of epoxy resin or silicone rubber to solve the embrittlement problem. The obvious two-stage yielding and sudden intensifying of the magnetization both suggest martensite variant reorientation (MVR) under the compressive and magnetic fields, respectively. Micro-computed tomography (μCT) and an X-ray diffractometer were utilized for the observations of shape deformation brought about by the MVR of the SC Ni-Mn-Ga particles in the polymer matrix. Clear MVR and shape deformation could be found in the compressed composites.

High-speed actuators are greatly required in this decade due to the fast development of future technologies, such as Internet-of-Things (IoT) and robots. The ferromagnetic shape memory alloys (FSMAs), whose shape change could be driven by applying an external magnetic field, possess a rapid response. Hence, these materials are considered promising candidates for the applications of future technologies. Among the FSMAs, the Ni-Mn-Ga-based materials were chosen for their large shape deformation strain and appropriate phase transformation temperatures for near-room temperature applications. Nevertheless, it is widely known that both the intrinsic brittleness of the Ni-Mn-Ga alloy and the constraint of shape deformation strain due to the existence of grain boundaries in the polycrystal inhibit the applications. Therefore, various Ni-Mn-Ga-based composite materials were designed in this study, and their shape deformation behaviors induced by compressive or magnetic fields were examined by the in situ micro CT observations. In addition, the dependence of the martensite variant reorientation (MVR) on the crystallographic directions was also investigated. It was found that most of the MVRs are active within the magnetic field range applied in the regime of the p, p, and p of the single-crystal {100}p Ni-Mn-Ga cubes.

This review summarizes recent advances in first‐principles simulations of the structural, mechanical, and magnetomechanical properties of Ni–Mn–Ga‐based ferromagnetic shape memory alloys from the perspective of experimental characterization. It focuses on calculations that can capture the main features of the low‐temperature phases, particularly the appearance of spatial modulations, the occurrence of intermartensitic transitions, and the high mobility of the twin boundaries, which allows for magnetically induced reorientation. Although the calculations can only be used to interpret some of these features and are not yet ready to be used as a tool for designing new alloys with the required properties, the gap between experimental observations and simulations is narrowing. When experiments and theory are discussed together, new challenges arise for both. In addition, new results on first‐principles‐determined elastic constants of the 10M martensite of Ni–Mn–Ga are presented and discussed with respect to the high mobility of the twin boundaries. [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. [ABSTRACT FROM AUTHOR]

Melt-spun ribbons of nominal composition Ni50Mn36-xFexSn14 (x = 0, 2, and 3) were prepared by melt-spinning. The alloys undergo a martensitic transformation from L21 austenite to an orthorhombic 4O martensite on cooling, as determined by X-ray powder diffraction analysis. Replacement of Mn by Fe linearly reduces the characteristic temperatures of the martensitic transformation (the equilibrium temperature decreases from 328 to 285 K) and reduces the Curie temperature of the austenite phase (from 336 to 300 K), whereas the effect of the applied magnetic field on the martensite transition temperatures is negligible. Magnetic measurements (zero-field cooled, ZFC, and field cooled, FC, curves, AC susceptibility measurements) hint the coexistence of two different ferromagnetic martensitic magnetic phases. Moreover, the AC susceptibility measurements and the irreversibility of the ZFC and FC curves point towards the presence of antiferromagnetic and ferromagnetic interactions in the martensitic phase. All samples exhibit spontaneous exchange bias at 2 K, with double-shifted loops, whereas the evolution of the conventional exchange bias with the temperature agrees quite well with the behavior of ferromagnetic regions surrounded by spin-glass regions or with the coexistence of ferromagnetic–antiferromagnetic interactions. Ni50Mn36-xFexSn14 ribbons present a moderate inverse magnetocaloric effect (with a maximum of the magnetic entropy change of 5.7 Jkg−1K−1 for μ0H = 3 T for x = 3). It is worth to note that these materials feature a significant reservoir (up to 44 Jkg−1K−1 for x = 2) of magnetic entropy change, linked to the proximity of the austenitic ferromagnetic transition to the martensitic transformation.

The substitution of small amounts of Co and Fe into Ni-Mn-Ga-based ferromagnetic shape memory alloys (FSMAs) is widely recognized for its ability to finely tune their multifunctional properties. This study investigated the effects of the substitution of each of the parent elements, in amounts of Co = 1, 3, and 5 (at.%), on the crystalline structure transformation temperature, microstructure, and magnetic properties of the nominal alloy Ni 50 Mn 25 Ga 20 Fe 5. An induction melting technique was used to fabricate a series of 10 alloys with and without Co-substitution. In addition to determining the evolution of its magnetic and structural characteristics, Co-doping has been found to have an unexpected and beneficial effect on the grain microstructure. The findings of the study revealed that the Ni 49 Co 1 Mn 25 Ga 20 Fe 5 alloy exhibited significant grain growth while demonstrating the mixed 10 M + 14 M martensitic crystal structure at ambient temperature. Separating an oriented grain of 1 × 2 mm enabled the experimental demonstration of magnetic field-induced structural rearrangement through twin-domain motion. Varying the levels of Co-substitution and the specific element being substituted allows tuning between the stabilisation of cubic austenite, modulated martensites (10 M or 14 M), or a non-modulated martensite at ambient temperature. Using standard melting facilities to fabricate large grains of polycrystalline ingots with 10 M + 14 M Co-doped Ni-Mn-Ga-Fe actuating elements offers a practical advantage over growing single crystals. • Co-substitutions for Ni, Mn, and Ga in the parent alloy Ni50Mn25Ga20Fe5. • Ni49Co1Mn25Ga20Fe5 shows martensitic phases (10 M + 14 M) at room temperature. • Mixed (10 M + 14 M) martensite structure has potential for functional MSM devices. • Mixed (10 M + 14 M) martensite structure has lower twinning stress and larger MFIS. [ABSTRACT FROM AUTHOR]

The martensitic phase transition and exchange bias effect of the Ni-Mn-based ferromagnetic shape memory alloys (FSMAs) Ni45Co5Mn37In13 (Ni-Co-Mn-In) films are investigated in this paper. The martensitic transformation properties of the Ni-Co-Mn-In alloy target material are manipulated by the process of electric arc melting, melt-fast quenching, and high-temperature thermal pressure. The Ni-Co-Mn-In alloy films with martensite phase transition characteristics are obtained by adjusting deposition parameters on the (001) MgO substrate, which shows a significant exchange bias (EB) effect at different temperatures. With increasing sputtering power and time, the film thickness increases, resulting in a gradual relaxation of the constraints at the interface between the film and the substrate (the interfacial strain decreases as the increase of thin film thickness), which promotes the martensite phase transition. Between zero-field cooling (ZFC) and field-cooled (FC) curve obvious division zone, the decrease of exchange bias field (HEB) and coercive force field (Hc) with an increase in test temperature is due to ferromagnetic (FM) interaction begins to dominate, resulting in a reduction of antiferromagnetic (AFM) anisotropy at the interface. The maximal HEB and Hc reach ~465.7 Oe and ~306.9 Oe at 5 K, respectively. The manipulation of the martensitic transformation and EB effect of the Ni-Co-Mn-In alloy films demonstrates potential application in the field of information and spintronics.

This short communication is dedicated to memory of Rostislav Mikhailovich Grechishkin, an outstanding Soviet and Russian physicist, a specialist in the field of magnetic phenomena, Dr. of physical and mathematical Sciences, Professor of the Applied Physics Department at the Faculty of Physics and Technology of Tver State University, head of a scientific school in magnetoelectronics.

In this work, we have proposed a new method to remotely control and operate the working mechanism of a suitable Ferromagnetic Shape Memory Alloy (FSMA) microactuator system using only a focused low powered laser beam. Model of a simple microgripper system fabricated from a FSMA material and operated by applying the newly discovered Photo Induced Micro Actuation (PIMA) property of FSMAs has been proposed in this report. The design of the fabricated FSMA microgripper was developed using a standard Computer Aided Engineering (CAE) software. The operating mechanism of the newly modelled microgripper assembly was thoroughly characterized numerically with Finite Element Analysis (FEA) method using the CAE software. A prototype of the microgripper system, as designed in the CAE software numerically, was fabricated and its working mechanism controlled by the PIMA effect was systematically studied. The main advantage of using this laser actuated microgripper system is the reduction in weight of the gripper assembly due to the absence of either electrical motors along with other electrical accessories or complex hydraulics systems as used in conventional gripper systems to control the working mechanism of the gripper end effectors. The gripping mechanism of the prototype microgripper system can be controlled only by the low powered laser beams. This is the first system attempted in the world where the PIMA effect was employed in any kind of microactuator application. [ABSTRACT FROM AUTHOR]

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.

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.

Shape memory alloys (SMAs) and ferromagnetic shape memory alloys (FeSMAs) have recently attracted interest for solid state refrigeration applications. Among NiMnGa-based quaternary systems, NiMnGaCu exhibits an interesting giant magnetocaloric effect thanks to the overlapping of the temperatures related to the magnetic transition and the thermoelastic martensitic transformation (TMT); in particular, for compositions with Cu content of approximately 6 at%. In the present work, we investigated the improvement effect of TMT on the total entropy change (ΔS) in the elastocaloric performances of polycrystalline Ni50Mn18.5Cu6.5Ga25 at% alloy samples, just above room temperature. We report an extensive calorimetric and thermomechanical characterization to explore correlations between microstructural properties induced by the selected thermal treatment and elastocaloric response, aiming at providing the basis to develop more efficient materials based on this quaternary system. Both ΔT and ΔS values obtained from mechanical curves at different temperatures and strain recovery tests under fixed load vs. T were considered. Maximum values of ΔS = 55.9 J/KgK and ΔT = 4.5 K were attained with, respectively, a stress of 65 MPa and strain of 4%. The evaluation of the coefficient of performance (COP) was carried out from a cyclic test.

Ni-Mn-Sn ferromagnetic shape memory alloys, which can be stimulated by an external magnetic field, exhibit a fast response and have aroused wide attention. However, the fixed and restricted working temperature range has become a challenge in practical application. Here, we introduced strain engineering, which is an effective strategy to dynamically tune the broad working temperature region of Ni-Co-Mn-Sn alloys. The influence of biaxial strain on the working temperature range of Ni-Co-Mn-Sn alloy was systematically investigated by the ab initio calculation. These calculation results show a wide working temperature range (200 K) in Ni14Co2Mn13Sn3 FSMAs can be achieved with a slight strain from 1.5% to −1.5%, and this wide working temperature range makes Ni14Co2Mn13Sn3 meet the application requirements for both low-temperature and high-temperature (151–356 K) simultaneously. Moreover, strain engineering is demonstrated to be an effective method of tuning martensitic transformation. The strain can enhance the stability of the Ni14Co2Mn13Sn3 martensitic phase. In addition, the effects of strain on the magnetic properties and the martensitic transformation are explained by the electronic structure in Ni14Co2Mn13Sn3 FSMAs.

• Attain BCC type structure in melt-spun Fe-Mn-Ga alloys. • Uncover the instability in the martensitic transformation of Fe-Mn-Ga alloys. • Realize the magnetic-field-induced martensitic transformation. This study investigates the crystal structural, magnetic properties, and phase transformation of melt-spun Fe-Mn-Ga alloys. We found that melt-spun Fe 50 Mn 25 Ga 25 alloy crystallizes into a γ phase, which exhibits ferromagnetism at room temperature, and undergoes a magnetic transition at 257 K. On the other hand, melt-spun Fe 49 Mn 24 Ga 27 , Fe 43 Mn 28 Ga 29 and Fe 44 Mn 28 Ga 28 alloys exhibit a single body-centered cubic phase, with the latter two alloys undergo martensitic transformation during cooling. Additionally, a magnetic-field-induced martensitic transformation was observed within the temperature range of 180 to 160 K. Notably, the martensitic transformation temperatures of these two melt-spun Fe-Mn-Ga alloys are significantly lower compared to other fabrication methods, potentially attributed to the atomic disorder, microstructure and internal stresses that arise during the melt-spun fabrication process. These findings expand the available preparation methods for Fe-Mn-Ga alloys and offer new possibilities for future applications. [ABSTRACT FROM AUTHOR]

Investigations on the properties of polycrystalline bulk and melt-spun ribbons of composition Ni50Mn29Ga21 are contrasted. Both samples are subjected to quenching after melting and then were homogenized at the same temperature. Though the electron per atom ratio is unchanged, the quenched-in disorder has caused the structural and magnetic transformations to occur in close proximity in the ribbon giving rise to magnetic entropy change of –9.2 J/kg-K at the martensitic transformation. The bulk alloy showed significantly high magnetic field-induced strain of 0.7% at 5 kOe magnetic field and an apparent compressive strain of 5.3% at 40 MPa compression stress, without training the sample. Large grain size and micro-twinning are observed in the bulk, while nano-scale twinning is seen in the ribbon. The variations in the process altered the degree of ordering, which modified the transformation temperatures, the crystal structure, the microstructures of the ribbon and the mechanical properties of the bulk specimen significantly. [ABSTRACT FROM AUTHOR]

The influence of the partial substitution of Fe by Si and thermal treatments on the structural, magnetic and magnetostrictive properties of the Fe67.5Pd30.5Si2 rapidly solidified ribbons has been investigated. A remarkable decrease in the martensite transformation temperature, with ~ 65 K lower than that of the Fe–Pd archetype alloy, is observed in the as-prepared ribbons. The thermal treatments shift the martensite transformation temperatures upward, with approximately 13 K for the higher thermal treatment. Also, these induce an improvement in the crystallinity in these ribbons with high texture and an increase in the crystallite size as a result of reducing the internal defects and stress. The thermodynamic considerations discussed in the frame of the Clapeyron–Clausius relation by using the calorimetric and thermomagnetic measurements (up to 7 T) reveal a weak influence of the magnetic fields on the martensitic transformation temperatures (~ 0.5 K/T). The magnetostriction decrease with temperature under small magnetic fields was discussed, beside an unusual behaviour in the technically saturated domain. This behaviour is based on the coexistence of the ordinary and forced magnetostrictions, the last one increasing faster with the temperature decreasing. [ABSTRACT FROM AUTHOR]

Investigations on the properties of polycrystalline bulk and melt-spun ribbons of composition Ni50Mn29Ga21 are contrasted. Both samples are subjected to quenching after melting and then were homogenized at the same temperature. Though the electron per atom ratio is unchanged, the quenched-in disorder has caused the structural and magnetic transformations to occur in close proximity in the ribbon giving rise to magnetic entropy change of –9.2 J/kg-K at the martensitic transformation. The bulk alloy showed significantly high magnetic field-induced strain of 0.7% at 5 kOe magnetic field and an apparent compressive strain of 5.3% at 40 MPa compression stress, without training the sample. Large grain size and micro-twinning are observed in the bulk, while nano-scale twinning is seen in the ribbon. The variations in the process altered the degree of ordering, which modified the transformation temperatures, the crystal structure, the microstructures of the ribbon and the mechanical properties of the bulk specimen significantly. [ABSTRACT FROM AUTHOR]

Polycrystalline Ni50Mn29Ga21 thin ribbons were prepared by melt-spinning and the effect of annealing temperatures on the microstructural evolution, phase transformations, crystal structure, and the magnetic properties of the ribbons was systematically investigated. Different annealing temperatures from 473 to 1100 K were used to alter the degree of quenched-in disorder gradually, which led to evolution of microstructure from cellular to well-defined grain structure. The crystal structure was found to be 7 M monoclinic in ribbons annealed at 773 K or below, while a minority phase with 5 M tetragonal structure is found to co-exist on annealing at 1100 K. The as-spun ribbon exhibit superparamagnetic behavior whereas the sample annealed at 1100 K did not exhibit the same. The variations in the saturation magnetization with annealing temperatures were systematically correlated. The saturation magnetization and the martensitic transformation temperature show a correlation to the increase in the degree of Mn ordering. [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.

Epitaxial Ni-Mn-Ga thin films have been extensively investigated, due to their potential applications in magnetic micro-electro-mechanical systems. It has been proposed that the martensitic phase in the A-oriented film is much more stable than that in the A-oriented film. Nevertheless, the magnetic properties, microstructural features, and crystal structures of martensite in such films have not been fully revealed. In this work, the A-oriented Ni51.0Mn27.5Ga21.5 films with different thicknesses were prepared by epitaxially growing on Al2O3(1 1 2¯ 0) substrate by magnetron sputtering. The characterization by X-ray diffraction technique and transmission electron microscopy revealed that all the Ni51.0Mn27.5Ga21.5 films are of 7M martensite at the ambient temperature, with their Type-I and Type-II twinning interfaces nearly parallel to the substrate surface.

In the present report, we have studied the structural, microstructural, magnetic, exchange bias (EB) properties and magnetoresistance (MR) in Mn rich (Mn at. ∼50%) Mn50Ni50−xSnx (x = 10) shape memory Heusler alloy ribbons, prepared using the melt spinning method. These ribbons were found to exhibit a first order structural (i.e., martensitic) transition at around 205 K from the high temperature austenite to the low temperature martensite phase. The martensitic transition occurs at comparatively lower temperatures in these ribbons than in the same bulk alloy. Curie temperature of the austenite phase (TCA) is found to be larger i.e., around room temperature than that of bulk alloy. A significant EB field (HEB) of around 960 Oe has been observed at 2 K for these ribbons, which is found to be comparable to that reported for other Heusler systems. The presence of the EB effect in these ribbons is attributed to the coexistence of FM/AFM exchange interactions in the martensite phase. They are also found to show a maximum negative MR of around 12% near the martensitic transition, for 50 kOe field change. Investigation of DC magnetization, AC susceptibility measurements and the observation of training effect (i.e., characteristic feature of EB) strongly corroborates with the coexistence of FM/AFM exchange interactions in the martensite phase of these ribbons, which results in the large EB. The effects of temperature and magnetic field on the EB properties and MR have also been studied here.

The Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) are a group of active materials that undergo martensitic transformations (MTs) induced by temperature, stress and/or magnetic fields, resulting in large recoverable mechanical deformations. Their fast response and high energy density make them ideal candidates for implementation in sensors and actuators. Recently, the development of high-temperature FSMAs (HTFSMAs), working at temperatures over 373 K, has become an important task to meet the current requirements of modern technologies. The magnetic and magnetoelastic properties of FSMAs are very sensitive to the interactions between the magnetic moments of atoms that, in turn, depend on the atomic positions within the lattice. Here we investigate a series of the polycrystalline Ni 45 Mn 25-x Ga 20 Co 5 Cu 5 Fe x (x = 0, 1, 2 and 5 at%) HTFSMAs. Their MT and Curie temperatures, the crystal structures of the martensitic and austenitic phases, the temperature evolutions of the lattice parameters of both phases and the atomic site occupancies have been studied by means of powder neutron diffraction measurements, complemented by standard characterization techniques. Based on the atomic site occupancies and additional measurements of the saturation magnetization, the influence of the structure and atomic ordering on the magnetism in these materials is analysed. The most promising candidate for high temperature magnetic actuation is the alloy with Fe 5 at%, following a combination of the high MT and Curie temperatures, around 370 K and 440 K, respectively, with a low tetragonality ratio, c / a ≈ 1.16, in the proximity of MT. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Magnetic-shape-memory Heusler thin films have a great potential for new-concept integrated devices, such as microactuators, energy harvesters and solid-state microrefrigerators, thanks to the intimate coupling between structure and magnetism. The control of microstructure in their martensitic phase is crucial for their full exploitation. Here, by an accurate magnetic and structural investigation at different length-scales, we demonstrate how growth temperature and simple post-growth treatments, i.e. post-annealing at low T , magnetic field cooling and mechanical stress, are suitable to manipulate the twin variant configuration in epitaxial thin films. X-type variants with out-of-plane magnetic easy axis or Y-type variants with in-plane magnetic easy-axis can be selected, as well as their geometrical distribution in films with mixed X/Y-type microstructure. The mechanisms behind the overall and local manipulation of microstructure are discussed by taking into account the role of microstructural defects, disorder and external fields on the martensitic transformation path. Our findings provide a platform for a controlled manipulation of microstructure and magnetism in magnetic-shape-memory thin films, which opens up a window towards engineering smart magnetic materials for multiple purposes. This remarkable "magnetic flexibility" makes magnetic-shape-memory alloys a unique class, among magnetic materials, for the easy tuning of the magnetic configuration at different length scales. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The Co–Ni–Al new ferromagnetic shape memory alloys (FSMAs) with Mn replacing of minor Al were casted by a cylindrical chilled copper way, then the origin ingots were cut into small specimens and annealed by a technology process of 1473 K × 12 h, followed by quenching into the ice water. The result shows that Mn addition plays a positive role in adjusting the microstructure of γ-phase, promoting the self-synergistic change of β-phase to γ-phase, increasing the magnetization saturation and the magneto-crystalline anisotropy constant (K). The best suitable Mn content approximately x = 1.5 is beneficial for adjusting both martensitic and magnetic transitions; further, the Co38Ni34Al26.5Mn1.5 alloy has the biggest χmax (magnetic susceptibility) value (0.46%) and the broadest magnetic transition scale (near the room temperature) with the biggest △T value reaches to 27 K, respectively. Therefore, the addition of Mn with a minor content is very effective in optimizing the Co–Ni–Al FSMAs for obtaining a good magnetic-field-induced strain, indicating a potential and prospect application. [ABSTRACT FROM AUTHOR]

The present work investigates the effects of Mn substitution with additions of Fe (0.5, 1, 2, and 3 at%) on the crystal structure, phase transformation temperatures, and magnetic properties of casted and heat-treated Ni 49.8 Mn 28.5−x Ga 21.7 Fe x magnetic shape memory (MSM) alloys. The alloys doped with 0.5 and 1 at% Fe exhibited single-phase seven-layered modulated (14 M) and five-layered modulated (10 M) martensite structures at ambient temperature, whereas the alloys doped with 2 and 3 at% Fe exhibited L2 1 Heusler austenite structures at ambient temperature. The Ni 49.8 Mn 27.5 Ga 21.7 Fe 1 alloy exhibiting the 10 M crystal structure at ambient temperature was selected for making ribbons using the melt-spinning process. The effects of heat treatment on grain growth, crystal structure, and other functional properties of the melt-spun ribbons were studied. The ribbons heat-treated at 1273 K exhibited a 10 M martensite structure and an average grain size of 220 µm along the ribbon plane. The results show that Ni 49.8 Mn 27.5 Ga 21.7 Fe 1 is a particularly promising composition for designing and manufacturing functional MSM micro-actuators. • Compositions of Ni49.8Mn28-xGa21.7Fex (x = 0.5, 1, 2, 3 at%) was casted. • Ni49.8Mn27.5Ga21.7Fe1 alloy show (10 M) martensite structures at room temperature. • Heat-treated Ni49.8Mn27.5Ga21.7Fe1 melt-spun ribbons at 1273 K show 10 M structure. • An average grain size of 220 µm along the ribbon plane was observed. • For designing functional MSM micro-devices, Ni49.8Mn27.5Ga21.7Fe1 is promising. [ABSTRACT FROM AUTHOR]

The strange Photo induced micro actuation (PIMA) properties associated with Cu doped Co-Ni-Al Ferromagnetic Shape Memory Alloys (FSMA) has showed enhancement of mechanical properties in our early studies. This enhancement and optimized PIMA properties have opened a scope of various laser-controlled real engineering applications. In this study, we have explored the microstructural scaling and determined the evolved force due to the laser action in optimized Cu doped Co-Ni-Al alloy system with a detailed insight. This unique PIMA property is rare in literature and actual reason are still not explored. EBSD, TEM, AFM/MFM based techniques were further adopted to understand the role of microstructure in more detail. The evolved magnetic domain patterns for actuating and non-actuating alloys have been analyzed in details. Atomic Force Microscopy was adopted to estimate the constitutive mechanical properties of the thin alloy samples by mechanical spectrometry. This estimation was directly used to predict a general trend in force distribution pattern of the Cu-Co-Ni-Al alloys strips through FEM analysis. The crux of FEM based model and its simulated results can be applied for real time PIMA oriented engineering application and device manufacturing. The detail microstructural exploration and its effect in the PIMA response along with generalized deflection-force model have been presented in the current study. [Display omitted] • Probing unexplored photo induced micro actuation effect in Cu doped Co-Ni-Al based ferromagnetic shape memory alloy. • STEM-EELS and MFM based investigation to pin the role of actuation active phase. • Finite Element Model to recognize the force pattern due to the laser action on alloy sample. [ABSTRACT FROM AUTHOR]

This study investigated the effect of rapid solidification and post-processing on microstructure, composition and magneto-structural properties of Ni 50 Mn 29 Ga 21 (at.%) magnetic shape memory alloy (MSMA). Samples with varying cooling rates (1.1 ∙ 102 to 3.2 ∙ 103K/s) were prepared using arc melting for a comparative study in as-solidified condition and after two different heat treatments. A notable change from columnar-dendritic to cellular-dendritic solidification and a decrease in solidification length scale was observed at more rapid solidification. All samples showed Mn enrichment and corresponding Ga depletion at the sub-grain boundaries. The fine-scale solidification structure, chemical heterogeneity and lack of ordering resulted in broad martensitic transformations. At more rapid solidification, a decrease in saturation magnetization and abnormal slope changes in the hysteresis loop were observed. Heat-treatment resulted in grain growth, significant reduction in transformation breadth, and increase in magnetization saturation. Rapidly solidified samples with an estimated characteristic cooling rate of 1.1 ∙ 103K/s most closely resembled microstructures and magneto-structural properties from laser directed energy deposition (L-DED) processing. The results of this study help to understand cooling rate effects in L-DED additive manufactured Ni-Mn-Ga and provide post-processing routes that will result in microstructures and properties that resemble those of functional MSMA. [Display omitted] [ABSTRACT FROM AUTHOR]

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K—which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.

The outstanding role of internal stress in the silicone matrix driving a magnetostrain reversal of embedded particles of the Ni–Mn-Ga ferromagnetic shape memory alloy after removal of magnetic field, has been disclosed by finite element (FE) simulations and validated by experiment. For simulations, the three case studies have been considered: an isolated particle, and the particles pairs aligned parallel or perpendicularly to the applied magnetic field. The isolated particle provided a 0.4 ​MPa of the reverse stress accumulated in a matrix that was insufficient to recover the composite shape after the removal of magnetic field. Simulations revealed that the strong elastic inter-particle interactions are needed to enhance both the local effective stiffness of composite and the reverse stress. The case where the particles pairs with the optimized inter-particle distance are aligned perpendicularly to the applied magnetic field, is more favorable to obtain the largest magnetostrain recovery. Simulation results demonstrate a 1.8% of the compressive magnetostrain of the 30 ​vol%Ni–Mn-Ga/silicone composite under the field applied parallel to the particles chains, which is in agreement with the experiment. The criteria for a selection of matrix, facilitating a large reversible magnetostrain of composite, have been determined.

The relevance of the entropy and in particular the outstanding role of the magnetic contribution is analyzed in a non-equilibrium arrested austenite phase in a Ni 45 Mn 36.7 In 13.3 Co 5 metamagnetic shape memory alloy. The Debye and Bragg-Williams approximations have been used to analyze and quantify the vibrational and magnetic contributions respectively, to the total entropy change linked to the martensitic transformation. The sign on the entropy change associated to the forward austenite to martensite transformation is different depending on whether it occurs at low or at high temperature as a consequence of the different vibrational and magnetic contributions. Unlabelled Image [ABSTRACT FROM AUTHOR]

The nonvolatile electric field control of the magnetocaloric effect (MCE) can be achieved in the Ni-Co-Mn-Sn/(011)0.7Pb(Mg 1/3 Nb 2/3)O 3 –0.3PbTiO 3 composite with the application of asymmetric bipolar electric fields. By controlling the electric fields, different nonvolatile strain states can be obtained and transmit to Ni-Co-Mn-Sn ribbon, which leads to an increase of phase transition temperature. As a result, the effective magnetic refrigeration temperature region can be extended by applying the asymmetric bipolar electric fields. This nonvolatile control of the MCE would have promising applications in designing more efficient magnetocaloric devices. Unlabelled Image [ABSTRACT FROM AUTHOR]

We provide in-depth physical insight into the enhancement of the magnetic properties of metamagnetic shape memory alloys produced by thermal treatment and cobalt doping. We use neutron scattering to study the atomic order and magnetic structures in the austenitic phases of Ni 50 Mn 34 In 16 and Ni 45 Co 5 Mn 37 In 13 alloys in two different states induced by thermal treatments. The increase of the magnetization in the austenite phase, particularly by cobalt doping, is explained by the enhanced ferromagnetic coupling between the magnetic moments located in octahedral sites. The spin density maps obtained from polarized neutron diffraction reveal the magnetic interaction pathways responsible for this coupling scheme. Unlabelled Image [ABSTRACT FROM AUTHOR]

• Ferromagnetic shape memory alloys, as a new type smart material, used in phononic crystals firstly. • This article is the first time to control negative refraction and focusing by using magnetic field. • It provides a novel strategy for the tunable PCs with negative refraction and focusing behaviors. The refraction and focusing behaviors of two-dimensional phononic crystals (PCs) tuned by external magnetic fields are investigated systematically. The PCs are constructed by inserting ferromagnetic shape memory alloys (FSMAs) between tungsten cylinders and water matrix. Under the magnetic field, material parameters of FSMAs will vary, which furthermore changes the frequency spectrum and equi-frequency surfaces of acoustic waves in sonic crystals. It is revealed that the refraction angle can be tuned from a positive value to a negative one when the PC slab suffers from continuous magnetic fields. Moreover, the focusing behavior can be achieved and controlled artificially by applying a proper magnetic field. These interesting phenomena of the tunable sonic crystals with FSMAs can provide us guidance for the design and application of novel acoustic devices, as well as the active control of acoustic waves. [ABSTRACT FROM AUTHOR]

• A crystal-plasticity based constitutive model is established in the framework of finite strain and irreversible thermodynamics. • Three types of martensite phases, 5M, 7M and NM are introduced and five different martensite transformation processes, the transformations between A and 5M, A and 7M, A and NM, 5M and 7M, 7M and NM phases are considered. • The proposed model can reasonably describe the deformation behaviors of single crystal NiMnGa ferromagnetic shape memory alloy at various temperatures. Existing experimental results have shown that multi-step martensite transformation can be induced by the applied stress in single crystal NiMnGa ferromagnetic shape memory alloys (FSMAs) and the transformation property depends on the ambient temperature strongly. In this work, a crystal-plasticity based constitutive model is established to quantify such a phenomenon. Based on the crystallographic symmetry, the lattice structures and eigenstrains of austenite (A) and three different martensite phases, i.e., the modulated martensite phases with a periodicity of 5 lattice cells (5M) and a periodicity of 7 lattice cells (7M), and a non-modulated (NM) martensite phase are analyzed. A three-dimensional (3D) single crystal constitutive model is constructed in the framework of finite strain by considering five different martensite transformation processes, i.e., the transformations between A and 5M, A and 7M, A and NM, 5M and 7M, and 7M and NM phases. Thermodynamically consistent kinetic equations for the five different martensite transformation processes are proposed based on the constructed Helmholtz free energy and Clausius's dissipative inequality. The capability of the proposed model to describe the tensile stress-induced multi-step martensite transformation of single crystal NiMnGa FSMA in [100] crystallographic orientation is verified by comparing the predictions with corresponding experimental results. Further, the proposed model is used to predict the deformations of the single crystal under the tension and compression in various crystallographic orientations. [ABSTRACT FROM AUTHOR]