A magnetocaloric study of the magnetostructural transitions in NiCr${}_2$O${}_4$

Abstract

The spinel NiCr${}_2$O${}_4$ is known to show a ferrimagnetic transition at $T_c=70$~K, and magneto-structural transitions at $T_s=30$~K and $T_o=20$~K\@. We present a detailed magnetic and magnetocaloric effect (MCE $=-\Delta S_M(T)$) study across these transitions. The $-\Delta S_M(T)$ shows a positive anomaly at $T_c$, $T_s$, and $T_o$. In addition to these anomalies, we report a new unreported feature at $T\approx 8.5$~K where $-\Delta S_M(T)$ shows a negative anomaly or the inverse MCE. An Arrot plot of the isothermal magnetization data reveals important information about the nature of the possible phases revealed in $-\Delta S_M(T)$. We have also made a scaling analysis of the $-\Delta S_M(T)$ data around these transitions. This analysis suggests that the transition at $T_c$ is a second-order Mean field like transition, the transition at $T_s$ is not second order and is non-mean field like, while the new transition at $T=8.5$~K is non-mean field like but is second order in nature. Our study demonstrates that magnetocaloric effect is sensitive to magneto-structural changes in materials and can be used for the identification of new phases and transitions.

7 pages, 9 figures