Magnetocaloric Effect and Critical Behavior of <inline-formula> <tex-math notation="TeX">${\rm Ni}_{42}{\rm Ag}_{8}{\rm Mn}_{37}{\rm Sn}_{13}$ </tex-math></inline-formula> Alloys

Abstract

This paper presents the magnetocaloric effect and critical behavior of alloy ingot and ribbon samples of ${\rm Ni}_{50}{\rm Mn}_{37}{\rm Sn}_{13}$ doped with 8% Ag, which were prepared by an arc-melting and rapidly quenched melt-spinning methods, respectively. Experimental results reveal that a partial replacement of Ag for Ni leads to stamping out the antiferromagnetic martensitic phase. This means that there is only the austenitic phase with a ferromagnetic–paramagnetic (FM–PM) phase-transition temperature of $T_{\rm C}\approx 295~{\rm K}$ . Detailed studies and analyses around the phase transition region prove both samples undergoing a second-order magnetic phase transition. Basing on magnetic field dependences of magnetization, we have determined the magnetic-entropy change $(\Delta S_{m})$ of the samples. Under a field change of 10 kOe, the maximum magnetic-entropy change $(\vert \Delta S_{\rm max}\vert)$ reaches values 0.54 and 0.69 ${\rm J}\cdot~{\rm kg}^{-1}\cdot{\rm K}^{-1}$ for the alloy ingot and ribbon, respectively. Using Landau's phase-transition theory, and careful analyses of the magnetic data around the FM–PM transition region, we have determined the critical parameters ( $T_{\rm C}$ , $\beta$ , $\gamma$ , and $\delta$ ) in the low field range (below 10 kOe) with $T_{\rm C}=294.8~{\rm K}$ , $\beta=0.469\pm 0.011$ , $\gamma=1.149\pm 0.060$ , and $\delta=3.4\pm 0.1$ for the alloy ingot, and with $T_{\rm C}=294.4~{\rm K}$ , $\beta=0.449\pm 0.005$ , $\gamma=1.319\pm 0.040$ , and $\delta=3.9\pm 0.1$ for the alloy ribbon. One can see that $\beta$ values fall in between those expected for the 3-D Heisenberg model $(\beta=0.365)$ and mean-field theory $(\beta=0.5)$ . This indicates a coexistence of short-range and long-range FM interactions in both the samples. The nature of changes in value related to the critical parameters and maximum $\Delta S_{m}$ is thoroughly discussed by means of structural analyses.