Failure of the Maxwell relation for the quantification of caloric effects in ferroic materials

TL;DR

This paper demonstrates that the Maxwell relation often used to quantify caloric effects in ferroic materials can fail due to the vector nature of applied fields, leading to spurious results especially in magnetic shape memory alloys.

Contribution

It reveals the fundamental limitations of the Maxwell relation in ferroic caloric effect measurements, emphasizing the need for careful analysis of vector field effects.

Findings

Maxwell relation can produce spurious caloric effect signals.

Failure is demonstrated in Ni-Mn-Ga magnetic shape memory alloy.

Repeated measurements can eliminate the spurious effect.

Abstract

Giant caloric effects were reported in elasto-, electro- and magnetocaloric materials near phase transformations. Commonly, their entropy change is indirectly evaluated by a Maxwell relation. We report the fundamental failure of this approach. We analyze exemplarily the Ni-Mn-Ga magnetic shape memory alloy. An applied field results in magnetically induced reorientation of martensitic variants, which form during the phase transformation. This results in a spurious magnetocaloric effect, which only disappears when repeating the measurement a second time. This failure is universal as the vector character of the applied field is not considered in the common scalar evaluation of a Maxwell relation.