Enhanced caloric effect induced by magnetoelastic coupling in NiMnGaCu Heusler alloys: Experimental study and theoretical analysis

On the basis of a phenomenological Landau model combined with comprehensive experimental studies, the magnetostructural transition behavior and field induced caloric effects for NiMnGaCu Heusler alloys have been investigated. In ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{25\ensuremath{-}x}{\mathrm{Ga}}_{25}{\mathrm{Cu}}_{x}$ alloys with $x=5.5$, 6, and 6.5, both magnetocaloric entropy change ($\mathrm{\ensuremath{\Delta}}S$) and elastocaloric temperature change ($\mathrm{\ensuremath{\Delta}}T$) increase with the increment of Cu content. The maximum $\mathrm{\ensuremath{\Delta}}S$ of $1.01\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and $\mathrm{\ensuremath{\Delta}}T$ of 8.1 K are obtained for the alloy with $x=6.5$. In order to explore the physical origin behind the large caloric effect, here we quantitatively propose a crucial coefficient of magnetoelastic coupling $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\kappa}}$ by utilizing a thermodynamic formalism within the framework of the Landau approach. It has been verified that the enhancement of the strength of magnetoelastic coupling between lattice and magnetic freedoms results in the increased caloric response for NiMnGaCu alloys. Thus, the strengthened coupling of the magnetoelastic effect can be considered as an effective way to improve the caloric performance for these alloys having the same sign of magnetic and elastic entropy changes contributed to the total caloric effect.