Inapplicability of the Maxwell relation for the quantification of caloric effects in anisotropic ferroic materials

• Multiferroic materials are promising candidates for magnetocaloric applications.
• A Maxwell relation is often used for quantification of the entropy change.
• Using a Ni-Mn-Ga single crystal, we demonstrate the failure of this indirect method.
• We discuss an advanced method to avoid apparent effects.

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. Additionally to the well-known spurious “colossal” results near first-order phase transitions and limitations in the presence of magnetic domains, we report the fundamental failure of this approach for materials that show structural reorientations in external fields. We analyze exemplarily the magnetic shape memory alloy Ni-Mn-Ga. Application of a magnetic field causes magnetically induced reorientation of the crystal structure in the martensite phase. This results in a spurious inverse magnetocaloric effect that disappears when the measurement procedure is repeated. This failure is universal as the vector character of the applied field is not considered in the common scalar evaluation of a Maxwell relation.