Tailoring of thermomagnetic properties in Ni-Mn-Ga films through Cu addition

Abstract

Thermomagnetic generators enable the conversion of low-grade waste heat into electric energy. The performance of a generator is intimately connected with the active thermomagnetic material used. Heusler alloys had been proposed as ideal systems for thermomagnetic microsystems, as they comprise a tuneable transition temperature just above room temperature, a steep change of magnetization within a narrow temperature change, a low heat capacity, and are easily processable by common deposition techniques. In this work, we present a path to optimize Heusler films for thermomagnetic applications, which need different properties compared to magnetocaloric applications. We focus on the key thermomagnetic properties like 1) the thermomagnetic working temperature T* and 2) the change of magnetization with the change of temperature ΔMΔT, and correlate them with common properties like 3) crystal structure, 4) martensitic transition temperature, 5) Curie temperature, and 6) spontaneous magnetization Ms. We systematically examine all these properties on polycrystalline Ni-Mn-Ga-Cu films prepared by combinatorial sputter deposition and subjected to a heat treatment. Our analysis allows disentangling the effects in changing the number of valence electrons trough the addition of Cu, and the alteration of chemical order before and after heat treatment.