Experimentally correlating thermal hysteresis and phase compatibility in multifunctional Heusler alloys

Abstract

Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first-order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural, and magnetic properties of the ${\mathrm{Ni}}_{2}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Ga}}_{0.84}{\mathrm{Al}}_{0.16}$ Heusler alloys. The alloys present a thermal hysteresis reduction of \ensuremath{\sim}60% when the Cu content in the compound varies from $x=0.10$ to $x=0.25$, with a minimum hysteresis width of 6 K being achieved. We applied the geometric nonlinear theory of martensite to address the phase compatibility, quantified by the parameter ${\ensuremath{\lambda}}_{2}$, the middle eigenvalue of the transformation stretch tensor, and found that the minimum of hysteresis is associated with a better crystallographic compatibility (${\ensuremath{\lambda}}_{2}$ closer to 1) between the austenite and martensite phases. In addition, we show that the valleylike properties of hysteresis found in the ${\mathrm{Ni}}_{2}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Ga}}_{0.84}{\mathrm{Al}}_{0.16}$ compounds is present in several other alloys in the literature. These results provide pathways to understand as well as to master the phase compatibility and ultimately achieve a low thermal hysteresis in multifunctional Heusler alloys.