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Powder bed fusion of metals using a laser beam (PBF-LB/M) is an established additive manufacturing (AM) method that can be used to fabricate geometrically complex NdFe-B magnets. However, the magnetic properties of Nd-Fe-B magnets manufactured by PBF-LB/M are typically inferior to conventionally produced magnets. To overcome this drawback, we modified the surface of the permanent magnet feedstock powder with 1 wt.% surfactant-free Ag nanoparticles (NPs) supporting the formation of relevant phases required for permanent magnetic performance to achieve a suitable micro- and nanostructure after AM. Our study is accompanied by finite element simulations, revealing the impact and dependency of process parameters during PBF-LB/M: a wide temperature field with a high-gradient profile in the front and on the bottom of an overheated region, implying a vast local heating/cooling rate and in-process high thermal stress. We found experimentally that the as-built part density can be affected by both the laser power and scan speed, causing a reduction in density as both parameters increase. The functionality and microstructural properties are also investigated via VSM, HR-SEM, EDX, EBSD, and exemplarily with HR-TEM-EDX and APT. Our study found that modifying MQP-S with Ag NPs increases the coercivity by approximately 20%, which we correlate to a decreased grain size. Additionally, we identified three distinct phases in the modified and unmodified samples, where Ag is primarily found in the intergranular and Nd-rich phases of the as-built parts. Overall, the study's findings contribute to the understanding of the factors that affect the quality and magnetic properties of Nd-Fe-B magnets fabricated through PBF-LB/M and provide valuable insights for further research in this area.

A multi-stimuli cooling cycle can be used to increase the cyclic caloric performance of multicaloric materials like Ni-Mn-In Heusler alloys. However, the use of a uniaxial compressive stress as an additional external stimulus to a magnetic field requires good mechanical stability. Improvement of mechanical stability and strength by doping has been shown in several studies. However, doping is always accompanied by grain refinement and a change in transition temperature. This raises the question of the extent to which mechanical strength is related to grain refinement, transition temperature, or precipitates. This study shows a direct comparison between a single-phase Ni-Mn-Sn and a two-phase Gd-doped Ni-Mn-In alloy with the same transition temperature and grain size. It is shown that the excellent magnetocaloric properties of the Ni-Mn-In matrix are maintained with doping. The isothermal entropy change and adiabatic temperature change are reduced by only 15% in the two-phase Ni-Mn-In-Heusler alloy compared to the single-phase alloy, which is resulting from a slight increase in thermal hysteresis and the width of the transition. Due to the same grain size and transition temperature, this effect can be directly related to the precipitates. The introduction of Gd precipitates leads to a 100% improvement in mechanical strength, which is significantly lower than the improvement observed for Ni-Mn-In alloys with grain refinement and Gd precipitates. This reveals that a significant contribution to the improved mechanical stability in Gd-doped Heusler alloys is related to grain refinement., Comment: Keywords: metal matrix composites, magnetocaloric, magneto-structural phase transitions, microstructure, mechanical properties, Heusler alloy

Ni-Mn-based Heusler alloys like Ni-Mn-Sn show an elastocaloric as well as magnetocaloric effect during the magneto-structural phase transition, making this material interesting for solid-state cooling application. Material processing by additive manufacturing can overcome difficulties related to machinability of the alloys, caused by their intrinsic brittleness. Since the magnetic properties and transition temperature are highly sensitive to the chemical composition, it is essential to understand and monitoring these properties over the entire processing chain. In the present work the microstructural and magnetic properties from gas-atomized powder to post-processed Ni-Mn-Sn alloy are investigated. Direct energy deposition was used for processing, promoting the evolution of a polycrystalline microstructure being characterized by elongated grains along the building direction. A complete and sharp martensitic transformation can be achieved after applying a subsequent heat treatment at 1173 K for 24 h. The Mn-evaporation of 1.3 at. % and the formation of Mn-oxide during DED-processing lead to an increase of the transition temperature of 45 K and a decrease of magnetization, clearly pointing at the necessity of controlling the composition, oxygen partial pressure and magnetic properties over the entire processing chain.

We are witnessing a great transition towards a society powered by renewable energies to meet the ever-stringent climate target. Hydrogen, as an energy carrier, will play a key role in building a climate-neutral society. Although liquid hydrogen is essential for hydrogen storage and transportation, liquefying hydrogen is costly with the conventional methods based on Joule-Thomas effect. As an emerging technology which is potentially more efficient, magnetocaloric hydrogen liquefaction can be a “game-changer”. In this work, we have investigated the rare-earth-based Laves phases RAl and RNi2 for magnetocaloric hydrogen liquefaction. We have noticed an unaddressed feature that the magnetocaloric effect of second-order magnetocaloric materials can become “giant” near the hydrogen boiling point. This feature indicates strong correlations, down to the boiling point of hydrogen, among the three important quantities of the magnetocaloric effect: the maximum magnetic entropy change ΔSmaxm, the maximum adiabatic temperature change ΔTmaxad , and the Curie temperature TC. Via a comprehensive literature review, we interpret the correlations for a rare-earth intermetallic series as two trends: (1) ΔSmaxm increases with decreasing TC; (2) ΔTmaxad decreases near room temperature with decreasing TC but increases at cryogenic temperatures. Moreover, we have developed a mean-field approach to describe these two trends theoretically. The dependence of ΔSmaxm and ΔTmaxad on TC revealed in this work helps researchers quickly anticipate the magnetocaloric performance of rare-earth-based compounds, guiding material design and accelerating the discoveries of magnetocaloric materials for hydrogen liquefaction.

Hydrogen has the largest gravimetric energy density among all chemical fuels. At the same time, the density of gaseous ${\mathrm{H}}_{2}$ is extremely low, which makes its compression to high pressures, liquefaction, or solid-state storage necessary for transport purposes. Liquid hydrogen ($\mathrm{L}{\mathrm{H}}_{2}$) can be transported in a dewar under atmospheric pressure, but this requires energy-intensive cooling down to $20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Magnetocaloric materials have great potential to revolutionize gas liquefaction to make $\mathrm{L}{\mathrm{H}}_{2}$ more competitive as fuel. In this paper, we investigate a series of Laves-phase materials regarding their structural, magnetic, and magnetocaloric properties in high magnetic fields. The three compounds $\mathrm{Ho}{\mathrm{Al}}_{2}$, ${\mathrm{Ho}}_{0.5}{\mathrm{Dy}}_{0.5}{\mathrm{Al}}_{2}$, and $\mathrm{Dy}{\mathrm{Al}}_{2}$ are suited for building a stack for cooling from liquid-nitrogen temperature ($77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) down to the boiling point of hydrogen at $20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This is evident from our direct measurements of the adiabatic temperature change in pulsed magnetic fields, which we compare with calorimetric data measured in a static field. With this methodology, we are now able to study the suitability of magnetocaloric materials down to low temperatures up to the highest magnetic fields of 50 T.

The adiabatic temperature change ΔTad of a Mn1:3Fe0:7P0:5Si0:55 Fe2P-type alloy was measured under different magnetic field-sweep rates from 0:93 Ts-1 to 2870 Ts-1. We find a field-sweep-rate independent magnetocaloric effect due a partial alignment of magnetic moments in the paramagnetic region overlapping with the magnetocaloric effect of the first-order phase transition. Additionally, the first-order phase transition is not completed even in fields up to 20T leading to a non-saturating behavior of ΔTad. Measurements in different pulsed fields reveal that the first-order phase transition cannot follow the fast field changes as previously assumed, resulting in a distinct field-dependent hysteresis in ΔTad.

Measurements of the magnetization in quasistatic and pulsed magnetic fields with different sweep rates, measurements of the specific heat in various magnetic fields, and direct measurements of the adiabatic temperature change have been employed to study the metamagnetic phase transition from an antiferromagnetic (AF) to the ferromagnetic (FM) state in an ${({\mathrm{Fe}}_{0.98}{\mathrm{Ni}}_{0.02})}_{49}{\mathrm{Rh}}_{51}$ alloy with a critical AF-FM transition temperature, ${T}_{\mathrm{tr}}$, reduced to 266 K. Based on the obtained results, a magnetic phase diagram for this alloy has been constructed. The AF-FM transition induced by the magnetic field below 10 K is found to occur in a steplike fashion in contrast to smooth behavior at $10\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl{T}_{\mathrm{tr}}$. The adiabatic temperature change $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ in the magnetic field of 2 T exceeds 6.5 K in pulsed fields (\ensuremath{\sim}100 T/s) and in the Halbach setup (\ensuremath{\sim}0.5 T/s), which is in agreement with the estimation from the S-T diagram constructed based on the specific heat measurements. The reversible $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ reaches \ensuremath{-}4.6 K under cyclic conditions in the Halbach setup (2 T). A complete transformation to the FM state in the whole temperature range requires a magnetic field of 14 T. Direct measurements of $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ in pulsed fields of 14 T revealed an irreversible part of the magnetocaloric effect associated with the presence of magnetic hysteresis and respective losses during the magnetization process.

Novel multicaloric cooling utilizing the giant caloric response of Ni-Mn-based metamagnetic shape-memory alloys to different external stimuli such as magnetic field, uniaxial load and hydrostatic pressure is a promising candidate for energy-efficient and environmentally-friendly refrigeration. However, the role of microstructure when several external fields are applied simultaneously or sequentially has been scarcely discussed in literature. Here, we synthesized ternary Ni-Mn-In alloys by suction casting and analyzed the microstructural influence on the response to magnetic fields and uniaxial load. SEM-EBSD reveals a distinct core-shell microstructure with a radially symmetric solidification texture resulting in a two-step martensitic transformation. In correlation with temperature-dependent XRD a significant effect of grain orientation on the stress-induced martensitic transformation is demonstrated. The influence of microstructure on the magnetic-field-induced transformation dynamics is studied by strain measurements in static and pulsed fields. Temperature-stress and temperature-magnetic field phase diagrams are established and single caloric performances are characterized in terms of ${\Delta}s_{T}$ and ${\Delta}T_{ad}$. The cyclic ${\Delta}T_{ad}$ values are compared to the ones achieved in the multicaloric exploiting-hysteresis cycle. It turns out that a tailored microstructure and the combination of both stimuli enable outstanding caloric effects in moderate external fields which can significantly exceed the single caloric performances. In particular for Ni-Mn-In, the maximum cyclic effect in magnetic fields of 1.9 T is increased by more than 200 % to -4.1 K when a moderate sequential stress of 59 MPa is applied. Our results illustrate the crucial role of microstructure for multicaloric cooling using Ni-Mn-based metamagnetic shape-memory alloys.

Ni-Mn-based metamagnetic shape-memory alloys exhibit a giant thermal response to magnetic fields and uniaxial stress which can be utilized in single caloric or multicaloric cooling concepts for energy-efficient and sustainable refrigeration. However, during cyclic operation these alloys suffer from structural and functional fatigue as a result of their high intrinsic brittleness. Here, we present based on Fe-doping of Ni-Mn-In a microstructure design strategy which simultaneously improves cyclic stability and maintains the excellent magnetocaloric and elastocaloric properties. Our results reveal that precipitation of a strongly Fe-enriched and In-depleted coherent secondary gamma-phase at grain boundaries can ensure excellent mechanical stability by hindering intergranular fracture during cyclic loading. In this way, a large elastocaloric effect of -4.5 K was achieved for more than 16000 cycles without structural or functional degradation, which corresponds to an increase of the cyclic stability by more than three orders of magnitude as compared to single-phase Ni-Mn-In-(Fe). In addition, we demonstrate that the large magnetocaloric effect of single-phase Ni-Mn-In-(Fe) can be preserved in the dual-phase material when the secondary gamma-phase is exclusively formed at grain boundaries as the martensitic transformation within the Heusler matrix is barely affected. This way, an adiabatic temperature change of -3 K and an isothermal entropy change of 15 $Jkg^{-1}K^{-1}$ was obtained in 2 T for dual-phase Ni-Mn-In-Fe. We expect that this concept can be applied to other single caloric and mutlicaloric materials, therewith paving the way for solid-state caloric cooling applications.

The inverse magnetocaloric effect in Ni-Mn based Heusler compounds occurs during the magnetostructural transition between low-temperature, low-magnetization martensite and high-temperature, high-magnetization austenite. In this study, we analyze the metamagnetic transformation of a Ni49.8Mn35In15.2 compound by simultaneous adiabatic temperature change ΔTad and strain Δl/l0 measurements in pulsed magnetic fields up to 10 T. We observe a ΔTad of −10 K and a Δl/l0 of −0.22% when the reverse martensitic transition is fully induced at a starting temperature of 285 K. By a variation of the magnetic field-sweep rates between 316, 865, and 1850 T s−1, the transitional dynamics of the reverse martensitic transformation have been investigated. Our experiments reveal an apparent delay upon the end of the reverse martensitic transformation at field rates exceeding 865 T s−1 which is related to the annihilation of retained martensite. As a consequence, the field hysteresis increases and higher fields are required to saturate the transition. In contrast, no time-dependent effects on the onset of the reverse martensitic transformation were observed in the studied field-sweep range. Our results demonstrate that kinetic effects in Heusler compounds strongly affect the magnetic cooling cycle, especially when utilizing a multicaloric “exploiting-hysteresis cycle” where high magnetic field-sweep rates are employed.

Microstructure design allows to prevent intergranular cracking and premature failure in Co–Ni–Ga shape memory alloys. Favorable grain boundary configurations are established using additive manufacturing techniques, namely, direct energy deposition (DED) and laser powder bed fusion (L-PBF). L-PBF allows to establish a columnar grain structure. In the Co–Ni–Ga alloy processed by DED, a microstructure with strong ⟨001⟩ texture is obtained. In line with optimized microstructures, the general transformation behavior is essential for performance. Transition parameters such as transition temperature and thermal hysteresis depend on chemical composition, homogeneity, and presence of precipitates. However, these parameters are highly dependent on the processing method used. Herein, the first-order magnetostructural transformation and magnetization properties of Co–Ni–Ga processed by DED and L-PBF are compared with single-crystalline and as-cast material. In the alloy processed by L-PBF, Ga evaporation and extensive formation of the ferromagnetic Co-rich γ'-phase are observed, promoting a very wide transformation range and large thermal hysteresis. In comparison, following DED, the material is characterized by minor chemical inhomogeneity and transition and magnetization behavior being similar to that of a single crystal. This clearly renders DED-processed Co–Ni–Ga to become a promising candidate material for future shape memory applications. © 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.

Novel Ni-Co-Mn-Ti all- d -metal Heusler alloys are exciting due to large multicaloric effects combined with enhanced mechanical properties. An optimized heat treatment for a series of these compounds leads to very sharp phase transitions in bulk alloys with isothermal entropy changes of up to 38 J kg−1 K−1 for a magnetic field change of 2 T. The differences of as-cast and annealed samples are analyzed by investigating microstructure and phase transitions in detail by optical microscopy. We identify different grain structures as well as stoichiometric (in)homogenieties as reasons for differently sharp martensitic transitions after ideal and non-ideal annealing. We develop alloy design rules for tuning the magnetostructural phase transition and evaluate specifically the sensitivity of the transition temperature towards the externally applied magnetic fields (dTt/μ0dH) by analyzing the different stoichiometries. We then set up a phase diagram including martensitic transition temperatures and austenite Curie temperatures depending on the e/a ratio for varying Co and Ti content. The evolution of the Curie temperature with changing stoi- chiometry is compared to other Heusler systems. Density Functional Theory calculations reveal a correlation of TC with the stoichiometry as well as with the order state of the austenite. This combined approach of experiment and theory allows for an efficient development of new systems towards promising magnetocaloric properties. Direct adiabatic temperature change measurements show here the largest value of -4 K in a magnetic field change of 1.93 T for Ni35Co15Mn37Ti13.

Natural gas is useful for the transition from traditional fossil fuels to renewable energies. The consumption of liquid natural gas has been rising, and the demand is predicted to double by 2040. In this context, magnetocaloric gas liquefaction, as an emerging and energy-saving technology, could be an alternative to the traditional gas-compression refrigeration. In this work, we report a large magnetic entropy change of 7.42 J/kg K under a magnetic field change of 2 T in Nd2In at 109 K, which is near the boiling temperature of natural gas of 112 K. The maximum adiabatic temperature change reaches 1.13 K under a magnetic field change of 1.95 T and is fully reversible. The magnetic phase transition is confirmed to be of the first-order type with the negligible thermal hysteresis. Further investigations on the thermal expansion and the magnetostriction reveal that the magnetic transition undergoes two stages with a negligible volume change. The longitudinal strain increases with magnetic fields and then decreases. These interesting properties are useful for the practical design of a magnetocaloric natural gas liquefaction system and for the fundamental understanding of the phase transitions in other RE2In intermetallics.

WOS: 000457409600012 Off stoichiometric Heuslers in the form Ni(50)Mn(50-x)Z(x), where Z can be a group 13-15 element of the periodic system, decompose at about 650 K into a ferromagnetic full Heusler Ni(50)Mn(25)Z(25) and an antiferromagnetic Ni50Mn50 component. We study here the case for Z as Sb and report on shell-ferromagnetic properties as well as thermal instabilities. Unlike the case for other Z-elements, in Ni50Mn50-xSbx, the minimum decomposition temperature corresponds to a temperature lying within the austenite state so that it is possible to observe the change in the martensitic transition temperature while annealing, thus providing further information on the change of composition during annealing. Scherrer analysis performed on emerging peaks related to the cubic full-Heusler shows that the precipitate size for shell-FM properties to become observable is around 5-10 nm. Other than vertical shifts in the field-dependence of the magnetization, which are also observed in compounds with Z other than Sb, concurrent exchange-bias effects are observed in the case with Z as Sb. (C) Author(s). Deutsche Forschungsgemeinschaft (DFG)German Research Foundation (DFG) [SPP 1599] This work was supported by the Deutsche Forschungsgemeinschaft (DFG) (No. SPP 1599). The DAAD is gratefully acknowledged.

Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first-order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First-order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change DTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first-order magnetocaloric materials such as Ni–Mn-based Heusler alloys, FeRh, La(FeSi)13-based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size- and time-dependence of the MCE at magnetostructural transitions.

61st Annual Conference on Magnetism and Magnetic Materials (MMM) - OCT 31-NOV 04, 2016 - New Orleans, LA WOS: 000402797100292 Next to the multifunctional properties of Ni-Mn-based Heusler alloys new functionalities related to shell-ferromagnetism are emerging. To understand in more detail the properties of shell-ferromagnetism we examine a decomposed Ni50.0Mn45.1In4.9 off-stoichiometric compound using magnetic resonance techniques which provides details on magnetic interactions. We find that the ferromagnetic resonance profile of the shell-ferromagnetic state is symmetric for positive and negative fields and is independent of the direction of the field-sweep except for the hysteresis observed at small fields. (C) 2017 Author(s). AIP Publishing, IEEE Magnet soc Deutsche ForschungsgemeinschaftGerman Research Foundation (DFG) [SPP 1599] Work supported by the Deutsche Forschungsgemeinschaft (SPP 1599).

The phase‐down scenario of conventional refrigerants used in gas–vapor compressors and the demand for environmentally friendly and efficient cooling make the search for alternative technologies more important than ever. Magnetic refrigeration utilizing the magnetocaloric effect of magnetic materials could be that alternative. However, there are still several challenges to be overcome before having devices that are competitive with those based on the conventional gas–vapor technology. In this paper a rigorous assessment of the most relevant examples of 14 different magnetocaloric material families is presented and those are compared in terms of their adiabatic temperature and isothermal entropy change under cycling in magnetic‐field changes of 1 and 2 T, criticality aspects, and the amount of heat that they can transfer per cycle. The work is based on magnetic, direct thermometric, and calorimetric measurements made under similar conditions and in the same devices. Such a wide‐ranging study has not been carried out before. This data sets the basis for more advanced modeling and machine learning approaches in the near future.

Mn1.96−x Cr x Co0.04Sb with x = 0.05 and 0.09 has been investigated for its magnetocaloric properties using structural, magnetization and calorimetric methods. These compounds exhibit an antiferromagnetic-ferrimagnetic transition of which the temperature can be adjusted by the Cr concentration. The transition temperature is at room temperature for x = 0.09. Magnetocaloric properties are presented as entropy-change and direct temperature-change. We show that these compounds have a narrow transitional hysteresis and exhibit the inverse magnetocaloric effect. We find a nearly hysteresis-free transition with a 2.6 K temperature-change around 287 K making it attractive for magnetic-cooling.

Ni2MnX-based Heusler (X: main group element), when enriched with Mn, will decompose into ferromagnetic Ni2MnX and antiferromagnetic NiMn when temper-annealed around 650 K. When the starting material is chosen such that the X-composition is about 5 at. % and the annealing takes place in the presence of a magnetic field of about 1 T, the resulting material is a composite of nanoprecipitate strongly pinned shell-ferromagnets with a soft ferromagnetic core embedded in the antiferromagnetic matrix. We show that the shells of the precipitates are so strongly pinned that the estimated field required to fully reorient the spins is in the order of 20 T. We examine in a Ni50.0Mn45.1In4.9 sample the pinning and the magnetic interactions of the precipitate and the matrix with magnetization and ferromagnetic resonance studies carried out in fields ranging up to 14 and 12 T, respectively. © 2017 Author(s). Deutsche Forschungsgemeinschaft: SPP 1599 This work was supported by the Deutsche Forschungsgemeinschaft (SPP 1599).

Solid-state magnetic refrigeration could disrupt conventional technology based on gas compressors, but affordable materials with large magnetocaloric response are needed. Using direct measurements of adiabatic temperature change in three different regimes of magnetic-field sweep rate, the authors investigate the dynamics of the martensitic transition in a class of Heusler alloys. Here phase-boundary motion is not an issue, but the speed of nucleation can be a limiting factor at high sweep rates. For everyday devices working at around 10 Hz, though, these compounds are promising.

The present study on magnetic and structural properties of Ni50Mn45Ga5 confirms that structural metastability is an inherent property of Ni50Mn50–xXx Heusler alloys with X as In, Ga, and Sn. The ternary alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic L21, Ni50Mn25Ga25, and tetragonal L10 Ni50Mn50. Depending on the annealing temperature, the magnetic-proximity effect giving rise to shell-ferromagnetism has been observed when annealing is carried out under an external magnetic field. The upper and lower remanence values MR+ and MR− have the same sign even at high temperatures. Such alloys can be promising candidates for heat- and magnetic-field-resistant magnetic recording media.

We study the technologically important (0 0 1)-texture formation in 10 nm thick (Fe0.9Cu0.1)52Pt48 and, as reference, Fe49Pt51 alloy films. The samples are grown on SiO2(200 nm)/Si(0 0 1) substrates at ambient temperature by pulsed laser deposition. Subsequent rapid thermal processing (RTP) at 650 °C for various time steps drives the initially nanocrystalline and chemically disordered films into the tetragonal L10 phase accompanied by a strong (0 0 1)-texture leading to perpendicular magnetic anisotropy. The fundamental role of the chemical order during short-time annealing as an additional source of strain in the films is experimentally addressed. The structural and magnetic results indicate selective grain growth leading to the (0 0 1)-texture. Strongly prolonged annealing, however, leads to a reorientation of grains towards the (1 1 1)-texture pointing to the increasing importance of surface energies when the initial strain has released.

The irreversibility of the martensite transition in magnetic shape memory alloys (MSMAs) with respect to the external magnetic field is one of the biggest challenges that limits their application as giant caloric materials. This transition is a magnetostructural transition that is accompanied with a steep drop in magnetization (i.e.,$\mathrm{\ensuremath{\Delta}}M$) around the martensite start temperature $({\mathrm{M}}_{\mathrm{s}})$ due to the lower magnetization of the martensite phase. In this Rapid Communication, we show that $\mathrm{\ensuremath{\Delta}}M$ around ${\mathrm{M}}_{\mathrm{s}}$ in Mn-rich Ni-Mn-based MSMAs gets suppressed by two orders of magnitude in crushed powders due to the stabilization of the martensite phase at temperatures well above ${\mathrm{M}}_{\mathrm{s}}$ and the austenite finish $({\mathrm{A}}_{\mathrm{f}})$ temperatures due to residual stresses. Analysis of the intensities and the FWHM of the x-ray powder-diffraction patterns reveals stabilized martensite phase fractions as $97%, 75%$, and $90%$ with corresponding residual microstrains as $5.4%, 5.6%$, and $3%$ in crushed powders of the three different Mn-rich Ni-Mn alloys, namely, $\mathrm{M}{\mathrm{n}}_{1.8}\mathrm{N}{\mathrm{i}}_{1.8}\mathrm{I}{\mathrm{n}}_{0.4}, \mathrm{M}{\mathrm{n}}_{1.75}\mathrm{N}{\mathrm{i}}_{1.25}\mathrm{Ga}$, and $\mathrm{M}{\mathrm{n}}_{1.9}\mathrm{N}{\mathrm{i}}_{1.1}\mathrm{Ga}$, respectively. Even after annealing at 773 K, the residual stress stabilized martensite phase does not fully revert to the equilibrium cubic austenite phase as the magnetostructural transition is only partially restored with a reduced value of $\mathrm{\ensuremath{\Delta}}M$. Our results have a very significant bearing on the application of such alloys as inverse magnetocaloric and barocaloric materials.

We study the dependence of the magnetocaloric effect on the magnetic field-change-rate the first order magnetostructural transition in Mn3GaC by measuring the adiabatic temperature change ΔT at three different time scales: 11 mT s−1, 700 mT s−1, and ∼1000 T s−1. We find that the maximum adiabatic temperature-change of about 5 K is reached in the 11 mT s−1 and 700 mT s−1 rates, whereas for the ∼1000 T s−1-rate the transition lags the change in the magnetic field so that the maximum adiabatic temperature-change is not attained.