Magnetic-field-induced shape recovery by reverse phase transformation

Crystallographic insights into Ni–Co–Mn–In metamagnetic shape memory alloys

In the present work, the morphological and crystallographic features of the 6M modulated martensite in the Ni45Co5Mn37In13 alloy were investigated by electron backscatter diffraction and high-resolution transmission electron microscopy (HRTEM) at room temperature. The 6M modulated martensite is in plate form and organized in colonies within which the plates stretch roughly in the same direction. Each colony has four types of orientation variants that are related to three kinds of twin relations, i.e. \{ 1 \bar 2\bar 3\} _{\rm M} type I, \langle \bar 3\bar 3{{1}} \rangle _{\rm M} type II and {{\{ 103\} }}_{\rm M} compound twins. The twinning shears of type I and type II twins are the same and equal to 0.2681, being about one order of magnitude higher than that of the compound twin (0.0330). Variant interfaces are microscopically defined by their corresponding twinning plane K 1. The HRTEM investigations show that the interfaces of the type I twin are straight and coherent at atomic scale, whereas those of the type II and compound twins are `stepped'. The step height of the compound twin interfaces is much larger than that of the type II twin interfaces. In view of variant organization, there is only one oriented type I interface and one compound twin interface, but there are two oriented type II interfaces which have an angular deviation of ±5.32° with respect to the type I twin interface. The results of the present work provide comprehensive information on morphological and crystallographic features of Ni–Co–Mn–In metamagnetic shape memory alloys.

Mechanocaloric effects in shape memory alloys

Shape memory alloys (SMA) are a class of ferroic materials which undergo a structural (martensitic) transition where the associated ferroic property is a lattice distortion (strain). The sensitiveness of the transition to the conjugated external field (stress), together with the latent heat of the transition, gives rise to giant mechanocaloric effects. In non-magnetic SMA, the lattice distortion is mostly described by a pure shear and the martensitic transition in this family of alloys is strongly affected by uniaxial stress, whereas it is basically insensitive to hydrostatic pressure. As a result, non-magnetic alloys exhibit giant elastocaloric effects but negligible barocaloric effects. By contrast, in a number of magnetic SMA, the lattice distortion at the martensitic transition involves a volume change in addition to the shear strain. Those alloys are affected by both uniaxial stress and hydrostatic pressure and they exhibit giant elastocaloric and barocaloric effects. The paper aims at providing a critical survey of available experimental data on elastocaloric and barocaloric effects in magnetic and non-magnetic SMA.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Effect of grain constraint on the field requirements for magnetocaloric effect in Ni45Co5Mn40Sn10 melt-spun ribbons

The influence of grain constraint on the magnetic field levels required to complete the isothermal martensitic transformation in magnetic shape memory alloys has been demonstrated for a NiCoMnSn alloy, and the magnetocaloric performance of an optimally heat treated alloy was quantified. Ni45CoxMn45-xSn10 melt spun ribbons with x = 2, 4, 5, and 6 were characterized. The x = 5 sample was determined to exhibit the lowest transformation thermal hysteresis (7 K) and transformation temperature range during transformation from paramagnetic austenite to nonmagnetic martensite, as well as a large latent heat of transformation (45 J kg-1 K-1). For this composition, it was found that increasing the grain size to thickness ratio of the ribbons from 0.2 to 1.2, through select heat treatments, resulted in a decrease in the magnetic field required to induce the martensitic transformation by about 3 T due to the corresponding reduction in the martensitic transformation temperature range. This decrease in the field requirement ultimately led to a larger magnetocaloric entropy change achieved under relatively smaller magnetic field levels. The giant inverse magnetocaloric effect of the optimized alloy was measured and showed that up to 25 J kg-1 K-1 was generated by driving the martensitic transition with magnetic fields up to 7 T.

Shape-morphing composites with designed micro-architectures

Shape memory polymers (SMPs) are attractive materials due to their unique mechanical properties, including high deformation capacity and shape recovery. SMPs are easier to process, lightweight, and inexpensive compared to their metallic counterparts, shape memory alloys. However, SMPs are limited to relatively small form factors due to their low recovery stresses. Lightweight, micro-architected composite SMPs may overcome these size limitations and offer the ability to combine functional properties (e.g., electrical conductivity) with shape memory behavior. Fabrication of 3D SMP thermoset structures via traditional manufacturing methods is challenging, especially for designs that are composed of multiple materials within porous microarchitectures designed for specific shape change strategies, e.g. sequential shape recovery. We report thermoset SMP composite inks containing some materials from renewable resources that can be 3D printed into complex, multi-material architectures that exhibit programmable shape changes with temperature and time. Through addition of fiber-based fillers, we demonstrate printing of electrically conductive SMPs where multiple shape states may induce functional changes in a device and that shape changes can be actuated via heating of printed composites. The ability of SMPs to recover their original shapes will be advantageous for a broad range of applications, including medical, aerospace, and robotic devices.

Determination of the vibrational contribution to the entropy change at the martensitic transformation in Ni-Mn-Sn metamagnetic shape memory alloys: a combined approach of time-of-flight neutron spectroscopy and ab initio calculations

The different contributions to the entropy change linked to the austenite-martensitic transition in a Ni-Mn-Sn metamagnetic shape memory alloy have been determined by combining different experimental techniques. The vibrational contribution has been inferred from the vibrational density of states of both the martensitic and austenite phases. This has been accomplished by combining time-of-flight neutron scattering measurements and ab initio calculations. Further, the electronic part of the entropy change has also been calculated. Since the martensitic transformation takes place between two paramagnetic phases, the magnetic contribution can be neglected and the entropy change can be reduced to the sum of two terms: vibrational and electronic. The obtained value of the vibrational contribution ([Formula: see text]) nearly provides the total entropy change measured by calorimetry ([Formula: see text]), the difference being the electronic contribution within the experimental error.

Premartensitic transition and relevant magnetic effects in Ni50Mn34In15.5Al0.5 alloy

Resistance measurement, in situ optical microscopic observation, thermal and magnetic measurements have been carried out on Ni₅₀Mn₃₄In_15.5Al_0.5 alloy. The existence of a pronounced premartensitic transition prior to martensitic transition can be characterized by microstructure evolution as well as exothermic peak and smooth decrease of resistance and magnetization with obvious hysteresis over a wide temperature range upon cooling. Consequently, the alloy undergoes two successive magneto-structural transitions consisting of premartensitic and martensitic transitions. Magnetoelastic coupling between magnetic and structural degrees of freedom would be responsible for the appearance of premartensitic transition, as evinced by the distinct shift of transitions temperatures to lower temperature with external applied field of 50 kOe. The inverse premartensitic transition induced by magnetic field results in large magnetoresistance, and contributes to the enhanced inverse magnetocaloric effect through enlarging the peak value and temperature interval of magnetic entropy change ΔS_m.

Intermartensitic Transformation and Enhanced Exchange Bias in Pd (Pt) -doped Ni-Mn-Sn alloys

In this work, we studied the phase transitions and exchange bias of Ni_50−xMn₃₆Sn₁₄T_x (T = Pd, Pt; x = 0, 1, 2, 3) alloys. An intermartensitic transition (IMT), not observed in Ni₅₀Mn₃₆Sn₁₄ alloy, was induced by the proper application of negative chemical pressure by Pd(Pt) doping in Ni_50−xMn₃₆Sn₁₄T_x (T = Pd, Pt) alloys. IMT weakened and was suppressed with the increase of applied field; it also disappeared with further increase of Pd(Pt) content (x = 3 for Pd and x = 2 for Pt). Another striking result is that exchange bias effect, ascribed to the percolating ferromagnetic domains coexisting with spin glass phase, is notably enhanced by nonmagnetic Pd(Pt) addition. The increase of unidirectional anisotropy by the addition of Pd(Pt) impurities with strong spin-orbit coupling was explained by Dzyaloshinsky-Moriya interactions in spin glass phase.

Realization of magnetostructural coupling by modifying structural transitions in MnNiSi-CoNiGe system with a wide Curie-temperature window

The magnetostructural coupling between structural and magnetic transitions leads to magneto-multifunctionalities of phase-transition alloys. Due to the increasing demands of multifunctional applications, to search for the new materials with tunable magnetostructural transformations in a large operating temperature range is important. In this work, we demonstrate that by chemically alloying MnNiSi with CoNiGe, the structural transformation temperature of MnNiSi (1200 K) is remarkably decreased by almost 1000 K. A tunable magnetostructural transformation between the paramagnetic hexagonal and ferromagnetic orthorhombic phase over a wide temperature window from 425 to 125 K is realized in (MnNiSi)1-x(CoNiGe)x system. The magnetic-field-induced magnetostructural transformation is accompanied by the high-performance magnetocaloric effect, proving that MnNiSi-CoNiGe system is a promising candidate for magnetic cooling refrigerant.

Crystal structure and crystallographic characteristics of martensite in Ni50Mn38Sb12 alloys

For Ni–Mn–Sb ferromagnetic multifunctional alloys, the crystal structures of martensite variants and the orientation relationships between them are decisive factors for their magnetic field-induced behaviours and are hence of importance. Such information has rarely been reported in the literature. In the present work, the crystal structure, microstructure and orientation relationships of Ni–Mn–Sb martensite were thoroughly investigated by X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Through XRD analyses, the crystal structure of the martensite, including the crystal system, the space group, the lattice parameters and the atomic coordinates, was fully resolved. The structure is orthorhombic and can be represented with a 4O superstructure. EBSD analyses show that the Ni–Mn–Sb martensite has a lamellar form. One martensite lamella corresponds to one orientation variant. The lamellae are organized in long plate-shaped colonies. Within each colony, four distinct orientation variants (A, B, C and D) appear repeatedly and extend in roughly the same direction. The four variants are twin related to one another, with variants A and C (or variants B and D) forming a type I twin, variants A and B (or C and D) a type II twin, and variants A and D (or B and C) a compound twin. The complete twinning elements for each twin relation were thus fully determined. The interfaces between the variants were identified to be their corresponding twinning planes. All these results provide fundamental information for Ni–Mn–Sb alloys that is useful for interpreting their magnetic and mechanical characteristics.

Critical dependence of magnetostructural coupling and magnetocaloric effect on particle size in Mn-Fe-Ni-Ge compounds

Magnetostructural coupling, which is the coincidence of crystallographic and magnetic transition, has obtained intense attention for its abundant magnetoresponse effects and promising technological applications, such as solid-state refrigeration, magnetic actuators and sensors. The hexagonal Ni₂In-type compounds have attracted much attraction due to the strong magnetostructural coupling and the resulted giant negative thermal expansion and magnetocaloric effect. However, the as-prepared samples are quite brittle and naturally collapse into powders. Here, we report the effect of particle size on the magnetostructural coupling and magnetocaloric effect in the Ni₂In-type Mn-Fe-Ni-Ge compound, which undergoes a large lattice change across the transformation from paramagnetic austenite to ferromagnetic martensite. The disappearance of martensitic transformation in a large amount of austenitic phase with reducing particle size, to our best knowledge, has not been reported up to now. The ratio can be as high as 40.6% when the MnNi_0.8Fe_0.2Ge bulk was broken into particles in the size range of 5~15 μm. Meanwhile, the remained magnetostructural transition gets wider and the magnetic hysteresis becomes smaller. As a result, the entropy change drops, but the effective cooling power RC_effe increases and attains to the maximum at particles in the range of 20~40 μm. These observations provide constructive information and highly benefit practical applications for this class of novel magnetoresponse materials.

A jumping shape memory alloy under heat

Shape memory alloys are typical temperature-sensitive metallic functional materials due to superelasticity and shape recovery characteristics. The conventional shape memory effect involves the formation and deformation of thermally induced martensite and its reverse transformation. The shape recovery process usually takes place over a temperature range, showing relatively low temperature-sensitivity. Here we report novel Cu-Al-Fe-Mn shape memory alloys. Their stress-strain and shape recovery behaviors are clearly different from the conventional shape memory alloys. In this study, although the Cu-12.2Al-4.3Fe-6.6Mn and Cu-12.9Al-3.8Fe-5.6Mn alloys possess predominantly L2₁ parent before deformation, the 2H martensite stress-induced from L2₁ parent could be retained after unloading. Furthermore, their shape recovery response is extremely temperature-sensitive, in which a giant residual strain of about 9% recovers instantly and completely during heating. At the same time, the phenomenon of the jumping of the sample occurs. It is originated from the instantaneous completion of the reverse transformation of the stabilized 2H martensite. This novel Cu-Al-Fe-Mn shape memory alloys have great potentials as new temperature-sensitive functional materials.

Application of strain to orbital-spin-coupled system MnV2O4 at cryogenic temperatures within a transmission electron microscope

The impact of mechanical stress on the morphology of crystallographic and magnetic domains in shape-controlled specimens of an orbital-spin-coupled system, MnV2O4, was examined by cryogenic Lorentz microscopy. Because of the difference in thermal expansion coefficients of MnV2O4 and the supporting Mo mesh, compression on the order of 0.01% was applied to the thin-foil specimens near the structural/magnetic phase transformation temperatures. The extent of compression was comparable to the lattice striction associated with the cubic-to-tetragonal phase transformation in MnV2O4 The applied strain thus clearly influenced the morphology of crystallographic domains (i.e. twinning configuration in the tetragonal phase) produced during cooling. The magnetic domain structure was entirely dependent on the configuration of twinning in the tetragonal phase. The observations in this study provided useful information for understanding the relationship between the crystallographic domains and the magnetic domains in MnV2O4.

P. S, Thomas S, Suresh KG, Varma MR. Martensitic transition, spin glass behavior and enhanced exchange bias in Si substituted Ni50Mn36Sn14 Heusler alloys

The martensitic transition and exchange bias properties of the Heusler alloy system Ni₅₀Mn₃₆Sn_14−xSi_x (x = 0, 1, 2 and 3) have been investigated.

High-field magneto-thermo-mechanical testing system for characterizing multiferroic bulk alloys

Multiferroic meta-magnetic shape memory alloys are well known for exhibiting large magnetic field induced actuation strains, giant magnetocaloric effects, magneto-resistance, and structural and magnetic glassy behaviors. Thus, they are candidates for improving modern day sensing, actuation, magneto-resistance, and solid-state refrigeration processes. Until now, however, experimental apparatuses have typically been able to probe a limited ferroic parameter space in these materials, i.e., only concurrent thermal and mechanical responses, or magnetic and thermal responses. To overcome this barrier and better understand the coupling of multiple fields on materials behavior, a magneto-thermo-mechanical characterization device has been designed and implemented. This device is capable of compressing a specimen at load levels up to 5300 N collinearly with applied fields up to 9 T between temperatures of -100 °C and 120 °C. Uniaxial stress, strain, temperature, magnetic field, and the volumetric average magnetization have been simultaneously measured under mixed loading conditions on a NiCoMnIn meta-magnetic shape memory alloy and a few selected results are presented here.

Strain behavior and lattice dynamics in Ni50Mn35In15

The lattice dynamics in the polycrystalline shape-memory Heusler alloy Ni50Mn35In15 have been studied by means of resonant ultrasound spectroscopy (RUS). RUS spectra were collected in a frequency range 100-1200 kHz between 10 and 350 K. Ni50Mn35In15 exhibits a ferromagnetic transition at 313 K in the austenite phase and a martensitic transition at 248 K accompanied by a change of the magnetic state. Furthermore it displays a paramagnetic to ferrimagnetic transition within the martensitic phase. We determined the temperature dependence of the shear modulus and the acoustic attenuation of Ni50Mn35In15 and compared it with magnetization data. Following the structural softening, which accompanies the martensitic transition as a pretransitional phenomenon, a strong stiffening of the lattice is observed at the martensitic magneto-structural transition. Only a weak magnetoelastic coupling is evidenced at the Curie temperatures both in austenite and martensite phases. The large acoustic damping in the martensitic phase compared with the austenitic phase reflects the motion of the twin walls, which freezes out in the low temperature region.

Patterning Oxide Nanopillars at the Atomic Scale by Phase Transformation

Phase transformations in crystalline materials are common in nature and often modify dramatically properties of materials. The ability to precisely control them with a high spatial precision represents a significant step forward in realizing new functionalities in confined dimensions. However, such control is extremely challenging particularly at the atomic scale due to the intricacies in governing thermodynamic conditions with a high spatial accuracy. Here, we apply focused electron beam of a scanning transmission electron microscope to irradiate SrNbO3.4 crystals and demonstrate a precise control of a phase transformation from layered SrNbO3.4 to perovskite SrNbO3 at the atomic scale. By purposely squeezing O atoms out of the vertex-sharing NbO6 octahedral slabs, their neighboring slabs zip together, resulting in a patterning of SrNbO3 nanopillars in SrNbO3.4 matrix. Such phase transformations can be spatially manipulated with an atomic precision, opening up a novel avenue for materials design and processing and also for advanced nanodevice fabrication.

Two successive magneto-structural transformations and their relation to enhanced magnetocaloric effect for Ni55.8Mn18.1Ga26.1 Heusler alloy

In the present work, two successive magneto-structural transformations (MSTs) consisting of martensitic and intermartensitic transitions have been observed in polycrystalline Ni55.8Mn18.1Ga26.1 Heusler alloy. Benefiting from the additional latent heat contributed from intermediate phase, this alloy exhibits a large transition entropy change ΔStr with the value of ~27 J/kg K. Moreover, the magnetocaloric effect (MCE) has been also evaluated in terms of Maxwell relation. For a magnetic field change of 30 kOe, it was found that the calculated value of refrigeration capacity in Ni55.8Mn18.1Ga26.1 attains to ~72 J/kg around room temperature, which significantly surpasses those obtained for many Ni-Mn based Heusler alloys in the same condition. Such an enhanced MCE can be ascribed to the fact that the isothermal entropy change ΔST is spread over a relatively wide temperature interval owing to existence of two successive MSTs for studied sample.

Drastic change in density of states upon martensitic phase transition for metamagnetic shape memory alloy Ni2Mn(1+x)In(1-x)

We have unravelled the electronic structure of a class of metamagnetic shape memory alloy Ni2Mn1+x In1-x by combining bulk-sensitive hard x-ray photoelectron spectroscopy and first-principles density-functional calculations. A sharp drop in the Ni 3d e(g) density of states forming a pseudogap in the martensitic phase transition (MPT) for x   =   0.36 has been observed near the Fermi level. As a feature of MPT, hysteretic behaviour of this drop has been confirmed in both cooling and warming. This pseudogap is responsible for the giant negative magnetoresistance. The experimental result is well reproduced by the first principle calculation. We have also clarified theoretically that the MPT is linked to a competition of ferromagnetic and anti-ferromagnetic coupling between ordinary and anti-site Mn atoms.

Achieving giant magnetically induced reorientation of martensitic variants in magnetic shape-memory Ni-Mn-Ga Films by microstructure engineering

Giant magnetically induced twin variant reorientation, comparable in intensity with bulk single crystals, is obtained in epitaxial magnetic shape-memory thin films. It is found to be tunable in intensity and spatial response by the fine control of microstructural patterns at the nanoscopic and microscopic scales. A thorough experimental study (including electron holography) allows a multiscale comprehension of the phenomenon.

Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias

Rational material design can accelerate the discovery of materials with improved functionalities. This approach can be implemented in Heusler compounds with tunable magnetic sublattices to demonstrate unprecedented magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. Our design approach is also demonstrated on a second material with a magnetic transition above room temperature, Mn-Fe-Ga, exemplifying the universality of the concept and the feasibility of room-temperature applications. These findings may lead to the development of magneto-electronic devices and rare-earth-free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.

Phase transformations in binary colloidal monolayers

Phase transformations can be difficult to characterize at the microscopic level due to the inability to directly observe individual atomic motions. Model colloidal systems, by contrast, permit the direct observation of individual particle dynamics and of collective rearrangements, which allows for real-space characterization of phase transitions. Here, we study a quasi-two-dimensional, binary colloidal alloy that exhibits liquid-solid and solid-solid phase transitions, focusing on the kinetics of a diffusionless transformation between two crystal phases. Experiments are conducted on a monolayer of magnetic and nonmagnetic spheres suspended in a thin layer of ferrofluid and exposed to a tunable magnetic field. A theoretical model of hard spheres with point dipoles at their centers is used to guide the choice of experimental parameters and characterize the underlying materials physics. When the applied field is normal to the fluid layer, a checkerboard crystal forms; when the angle between the field and the normal is sufficiently large, a striped crystal assembles. As the field is slowly tilted away from the normal, we find that the transformation pathway between the two phases depends strongly on crystal orientation, field strength, and degree of confinement of the monolayer. In some cases, the pathway occurs by smooth magnetostrictive shear, while in others it involves the sudden formation of martensitic plates.

Tuning structural instability toward enhanced magnetocaloric effect around room temperature in MnCo(1-x)Zn(x)Ge

Magnetocaloric effect is the phenomenon that temperature change of a magnetic material is induced by application of a magnetic field. This effect can be applied to environmentally-benign magnetic refrigeration technology. Here we show a key role of magnetic-field-induced structural instability in enhancing the magnetocaloric effect for MnCo_1−xZn_xGe alloys (x = 0–0.05). The increase in x rapidly reduces the martensitic transition temperature while keeping the ferromagnetic transition around room temperature. Fine tuning of x around x = 0.03 leads to the concomitant structural and ferromagnetic transition in a cooling process, giving rise to enhanced magnetocaloric effect as well as magnetic-field-induced structural transition. Analyses of the structural phase diagrams in the T-H plane in terms of Landau free-energy phenomenology accounts for the characteristic x-dependence of the observed magnetocaloric effect, pointing to the importance of the magnetostructural coupling for the design of high-performance magnetocalorics.

Constitutive modelling of magnetic shape memory alloys with discrete and continuous symmetries

A free energy-based constitutive formulation is considered for magnetic shape memory alloys. Internal state variables are introduced whose evolution describes the transition from reference state to the deformed and transformed one. We impose material symmetry restrictions on the Gibbs free energy and on the evolution equations of the internal state variables. Discrete symmetry is considered for single crystals, whereas continuous symmetry is considered for polycrystalline materials.

Large noncollinearity and spin reorientation in the novel Mn2RhSn Heusler magnet

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn(2)RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn(2)RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

A biomechanical research of growth control of spine by shape memory alloy staples

Shape memory alloy (SMA) staples in nickel titanium with shape memory effect are effective for spinal growth control. This study was designed to evaluate the biomechanical properties of the staples and observe the stability of the fixed segments spine after the staples were implanted. According to the vertical distance of the vertebrae, SMA staples of 5, 6.5, and 8 mm were designed. The recovery stress of 24 SMA staples in three groups was measured. The pullout strength of SMA staples and stainless steel staples in each functional spinal unit was measured. Each of the six fresh specimens was divided into three conditions: normal, single staple, and double staples. Under each condition, the angle and torque of spinal movements in six directions were tested. Results show that the differences in recovery stress and maximum pullout strength between groups were statistically significant. In left and right bending, flextion, and extention, the stability of spine was decreased in conditions of single staple and double staples. Biomechanical function of SMA staples was superior to stainless steel staple. SMA staples have the function of hemiepiphyseal compression and kyphosis and scoliosis model of thoracic vertebrae in goat could be successfully created by the fusionless technique.

Applications of memory alloy stent in vertebral fractures

Background

The aim of this study was to evaluate the feasibility of treating vertebral compression fractures using an autonomously developed nitinol memory alloy vertebral stent.

Material/Methods

Thoracolumbar vertebral specimens from adult human cadavers were made into models of compression fractures. The models were divided into group A, which received percutaneous kyphoplasty (PKP), balloon dilation, and nitinol memory alloy vertebral stent implantation (PKP + nitinol stent group); group B, which received percutaneous vertebroplasty (PVP) and direct implantation of a nitinol memory alloy vertebral stent (PVP + nitinol stent group); and group C, which received PKP, balloon dilation, and bone cement vertebroplasty (PKP + polymethylmethacrylate (PMMA) group). Vertebral heights were measured before and after the surgery and the water bath incubation to compare the impact of the 3 different surgical approaches on reducing vertebral compression.

Results

The 3 surgical groups could all significantly restore the heights of compressed vertebral bodies. The vertebral heights of the PKP + nitinol stent group, PVP + nitinol stent group, and PKP + PMMA group were changed from the preoperative levels of (1.59±0.08) cm, (1.68±0.08) cm, and (1.66±0.11) cm to the postoperative levels of (2.00±0.09) cm, (1.87±0.04) cm, and (1.99±0.09) cm, respectively. After the water bath, the vertebral heights of each group were changed to (2.10±0.07) cm, (1.98±0.09) cm, and (2.00±0.10) cm, respectively. Pairwise comparison of the differences between the preoperative and postoperative vertebral heights showed that group A and group B differed significantly (P=0.000); group B and group C differed significantly (P=0.003); and group A and group C had no significant difference (P=0.172). Pairwise comparison of the differences in the vertebral heights before and after the water bath showed that group A and group C differed significantly (P=0.000); group B and group C differed significantly (P=0.000); and group A and group B had no significant difference (P=0.157).

Conclusions

The nitinol memory alloy stents can effectively support and reduce the compression of vertebral endplates and can be used to treat vertebral compression fractures without neurological symptoms.

Magnetic Properties of the Ferromagnetic Shape Memory Alloys Ni50+xMn27-xGa23 in Magnetic Fields

Thermal strain, permeability, and magnetization measurements of the ferromagnetic shape memory alloys Ni_50+xMn_27−xGa₂₃ (x = 2.0, 2.5, 2.7) were performed. For x = 2.7, in which the martensite transition and the ferromagnetic transition occur at the same temperature, the martensite transition starting temperature T_Ms shift in magnetic fields around a zero magnetic field was estimated to be dT_Ms/dB = 1.1 ± 0.2 K/T, thus indicating that magnetic fields influences martensite transition. We discussed the itinerant electron magnetism of x = 2.0 and 2.5. As for x = 2.5, the M⁴vs. B/M plot crosses the origin of the coordinate axis at the Curie temperature, and the plot indicates a good linear relation behavior around the Curie temperature. The result is in agreement with the theory by Takahashi, concerning itinerant electron ferromagnets.

Taming the first-order transition in giant magnetocaloric materials

Large magnetically driven temperature changes are observed in MnFe(P,Si,B) materials simultaneously with large entropy changes, limited (thermal or magnetic) hysteresis, and good mechanical stability. The partial substitution of B for P in MnFe(P,Si) compounds is found to be an ideal parameter to control the latent heat observed at the Curie point without deteriorating the magnetic properties, which results in promising magnetocaloric properties suitable for magnetic refrigeration.

Magnetic field and atomic order effect on the martensitic transformation of a metamagnetic alloy

The martensitic transformation (MT) of metamagnetic shape memory alloys is very sensitive to the applied magnetic field and atomic order. We analyze the alloy Ni50Mn34.5In15.5 in magnetic fields up to 13 T. The alloy has been prepared both in an ordered state by slow cooling, and in a disordered state by rapid quenching. In both cases the dependence of the martensitic transition temperature on the field is highly nonlinear. Such departure from linearity is due to a decrease of the entropy change at the transition, ΔS, with the applied field. This can be explained by the ordering effect of the magnetic field on the frustrated magnetic structure of the alloy in the martensitic phase. Compliance with a recent model, relying on the strong magnetoelastic interactions in these compounds, is very satisfactory.

First order magneto-structural phase transition and associated multi-functional properties in magnetic solids

We show that the first order magneto-structural phase transitions observed in various classes of magnetic solids are often accompanied by useful multi-functional properties, namely giant magneto-resistance, magneto-caloric effect and magneto-striction. We highlight various characteristic features associated with a disorder influenced first order phase transition namely supercooling, superheating, phase-coexistence and metastability, in several magnetic materials and discuss how a proper understanding of the transition process can help in fine tuning of the accompanied functional properties. Magneto-elastic coupling is a key element in this first order phase transition, and methods need to be explored for maximizing the contributions from both the lattice and the magnetic degree of freedom while simultaneously minimizing the thermomagnetic hysteresis loss. An analogy is also drawn with the first order phase transition observed in dielectric materials and vortex matter of type-II superconductors.

Large zero-field cooled exchange-bias in bulk Mn2PtGa

We report a large exchange-bias effect after zero-field cooling the new tetragonal Heusler compound Mn(2)PtGa from the paramagnetic state. The first-principles calculation and the magnetic measurements reveal that Mn(2)PtGa orders ferrimagnetically with some ferromagnetic inclusions. We show that ferrimagnetic ordering is essential to isothermally induce the exchange anisotropy needed for the zero-field cooled exchange bias during the virgin magnetization process. The complex magnetic behavior at low temperatures is characterized by the coexistence of a field-induced irreversible magnetic behavior and a spin-glass-like phase. The field-induced irreversibility originates from an unusual first-order ferrimagnetic to antiferromagnetic transition, whereas the spin-glass-like state forms due to the existence of antisite disorder intrinsic to the material.