Effect of crystallographic alignment on the magnetocaloric effect in alloys near the Ni2MnGa stoichiometry

Abstract

Prior to the development of commercial applications of magnetic refrigerator technology, a large magnetocaloric effect (MCE) in polycrystalline materials must be realized for relatively low magnetic field changes. To increase the MCE, a crystallographic alignment technique, consisting of thermal cycling about the martensite phase transition temperature under a compressive stress, was applied to Heusler alloys with nominal composition Ni2+xMn1−xGa (x = 0.14, 0.16). Magnetic measurements prior to grain alignment show that the maximum entropy changes of −16 J kg−1K−1 and −24 J kg−1K−1 for samples with x = 0.14 and 0.16, respectively, occurred for a magnetic field change of 7 T. After grain alignment, there was a 56%–79% enhancement of the maximum magnetic entropy change for the same magnetic field change of 7 T. This suggests that thermal cycling under compressive stress may either increase grain alignment (e.g., texture) along the magnetic easy (001) axis, and/or enhance the ease with which a magnetic field is later able to grow favorably oriented twin variants that manifests as an increase of magnetization of the material. Therefore, such an alignment technique may be utilized to enhance the MCE of similar Heusler alloys.