In this study, we investigate the entropy change of magnetostructural transformation and associated magnetocaloric properties in a Ni50Mn18.5Ga25Cu6.5 Heusler alloy. In this alloy, the structural and magnetic components (i.e., lattice vibration and magnetic entropy changes) synergistically contribute to total entropy change during the magnetostructural transformation. The unique synergistic feature of the alloy results in a large phase transformation entropy change of -1.05 J/molK, in which the structural component accounts for -63% and the magnetic component primarily contributes the remaining -37%. Within the framework of a Landau model, simulative magnetocaloric entropy change increases with magnetic field due to the synergistic feature of the structural and magnetic components. In addition, an adiabatic temperature change up to +6.5 K is obtained in the Ni50Mn18.5Ga25Cu6.5 alloy under a magnetic field of 8 T.

The relaxation processes of the adiabatic temperature changes (Delta T-ad) at the phase transitions in Ni45Mn43CoSn11, Ni50Mn36.5In13.5, and Ni50Mn35In14.25B0.75 Heusler alloys with different magnetic structures have been studied using a direct extraction method in magnetic fields up to 14 T. It has been found that Delta T-ad exhibits short relaxation times (less than 10(-1) (s)) in the vicinity of the second order phase transitions at the Curie temperatures. The relaxation times of the first order martensitic transitions strongly depend on the latent heat of the transition and can be characterized by a logarithmic law. (C) 2019 Elsevier B.V. All rights reserved.

The adiabatic temperature change ( $\Delta T_{\mathrm {ad}}$ ) in Ni45Mn43CoSn11 has been measured by a direct method in magnetic field changes up to 14 T. Large reversible magnetocaloric effects resulting in $\Delta T_{\mathrm {ad}}$ of about = −11 and 5 K have been observed for magnetic field changes of 14 T at the magnetostructural ( $T_{A} \sim 260$ K) and magnetic transitions ( $T_{C} \sim 320$ K), respectively. The impact of the thermomagnetic history on $\Delta T_{\mathrm {ad}}$ at high magnetic fields has been reported. The significant observed changes in the relaxation time of $\Delta T_{\mathrm {ad}}$ , depending on the type of the phase transitions, magnetization, and demagnetization cycle are discussed.

•Direct measurements of ΔTad for ΔH up to 14 T of Ni50Mn35In14.25B0.75 Heusler alloy.•The magnitude and sing of ΔTad dependency on the heating/cooling rate.•Magnetic fields larger than 12 T (on cooling) results in the kinetic arrest.•The effects of the temperature and magnetic history on MCE.

Magnetocaloric effects (MCE) in the Ni50Mn35In15, Ni50.2Mn34.85In14.95, and Ni50Mn35In14.25B0.75 Heusler alloys have been studied employing direct measurements of the adiabatic temperature changes (Delta T-ad) using the extraction method with magnetic field changes (Delta H) up to 14 T, differential scanning calorimetry, and magnetization measurements in magnetic fields up to 14 T. Both the Delta T-ad and the entropy changes (Delta S) values increase as the martensitic transition approaches the Curie temperature of the austenitic phase. The Delta T-ad as a function of magnetic field increases up to a maximum value, which depends on the current temperature, and shows a tendency to saturate in high fields. The influence of the rate of magnetic field change and the rate of heating on the Delta T-ad of Ni50Mn35In14.25B0.75 has been studied. It has been shown that an increase in the heating rate from 7 to 23 K/ min results in an increase of Delta T-ad by about 40% for Delta H = 10 T. The effect is discussed in the terms of the influence the heating rate on the nucleation of the austenitic phase. The results on critical magnetic fields for MCE saturation and kinetic effects are applicable to any other system displaying magnetostructural, first-order phase transition. (C) 2016 Elsevier B. V. All rights reserved.

The magnetic properties and reversibility of the magnetocaloric effect of Ni50Mn35In15 have been studied in the vicinity of the phase transition using magnetization and direct adiabatic temperature change (Delta T-ad) measurements in magnetic fields up to 14 T. The magnetostructural phase transitions (MSTs) between a martensitic phase (MP) with low magnetization (paramagnetic or antiferromagnetic) and a nearly ferromagnetic austenitic phase were detected from thermomagnetic curves, M(T,H), at the applied magnetic fields up to 5 T. The MST temperature was found to be nearly independent of magnetic field for H < 5 T, and shifted to lower temperature with the further increase of magnetic field to 14 T. A large and nearly reversible inverse magnetocaloric effect (MCE) with Delta T-ad similar to -11 kappa for a magnetic field change of Delta H = 14 T was observed in the vicinity of the MST. The irreversibility of Delta T-ad was found to be 1 K. A direct Delta T-ad of +7 K for Delta H = 14 T was detected at the second order ferromagnetic-paramagnetic phase transition. The obtained results have been discussed in terms of the suppression of antiferromagnetic correlations with the application of a strong magnetic field, and a reversibility of the initial magnetic state of the MP with applied magnetic field when the MST coincides with T-C. (C) 2016 Elsevier B.V. All rights reserved.