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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 (7K) 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. [ABSTRACT FROM AUTHOR]

The influence of Fe doping for Mn in CoMn1-xFexGe alloys on their structural and magnetic phase transitions was investigated. CoMn1-xFexGe alloys display a first-order magneto-structural transition from a high temperature paramagnetic hexagonal Ni2In-type crystal structure to a low temperature ferromagnetic orthorhombic TiNiSi-type structure. An entropy change of ΔSM=-32.8 J/kgK was measured and adiabatic temperature change of -8.0K was computed for the x=0.085 alloy under a magnetic field of 7 T in the vicinity of the phase transition at 317 K. With increasing Fe content, transition temperatures and magnetic entropy changes decrease. In addition to investigating the effect of replacing Mn with Fe, the accessibility of the giant measured entropy change was quantified by means of the minimum magnetic field required to start and to complete the magnetostructural phase transition. [ABSTRACT FROM AUTHOR]

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The giant magnetocaloric effect was quantified in CoMn1-xFexGe (x = 0.085–0.12) nom. at. % polycrystals across the high temperature hexagonal (P63/mmc) to low temperature orthorhombic (Pnma) phase transition via differential scanning calorimetry (DSC) and multiple (thermo) magnetization measurements. It was found that increasing Fe content led to the decrease of both the martensitic transformation temperature and entropy change ( Δ S ) at the point of the phase transition. Moreover, first-time magnetocaloric measurements resulted in irreproducible entropy change versus temperature diagrams, which was attributed to the release of internal pressure in bulk samples that disintegrated into powder upon transformation. CoMn1-xFexGe demonstrated larger magnetic field-induced entropy changes and giant magnetocaloric effect (MCE) compared to other CoMnGe alloys doped with Si, Sn, Ti, and Ga. However, the observed brittleness and apparent change in volume at the magnetic transition was posited to influence the material's potential for regenerative applications. [ABSTRACT FROM AUTHOR]

In this study, we examine Ni 49 Nb 1 Mn 36 In 14 (nom. at.%) magnetic shape memory alloy (MSMA) to illustrate the inverse magnetocaloric effect (MCE) using the fraction method. The magnetic entropy change,ΔSmag, was calculated with both, the fraction method and the thermomagnetic Maxwell relation. Our results demonstrate that there exists a large magnetization difference between field-cooling and field-heating histories in Ni 49 Nb 1 Mn 36 In 14 (nom. at.%) MSMA, which can be attributed to the pinning of lattice entropy and magnetic entropy, as it is well-known that the temperature and applied magnetic fields have an opposing effect on the total entropy change. In addition, we describe the inverse MCE and the contradictory roles on the total entropy change between the two stimuli via the fraction method., (Creative Commons Attribution license.)

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. Ni 45 Co x Mn 45-x Sn 10 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.