Magnetocaloric effect in Ni-Mn-Ga thin films under concurrent magnetostructural and Curie transitions

An investigation of the magnetocaloric effect for Ni-Mn-Ga films with a composition chosen to yield the highly advantageous magnetostructural phase transition between the paramagnetic austenitic and the ferromagnetic martensitic phases is presented. The observed effect is particularly strong at low magnetic fields, yielding a maximum negative entropy change of −1.4 J/kg K for a field change of only 0.5 T. It is also observed that the cooling process yields a 40% larger entropy change compared to the heating process. Temperature dependent magnetic, structural, and transport measurements indicate that the entropy peak difference between cooling and heating cycles is associated with a stronger overlap of the Curie transition of the austenitic phase with the magnetostructural phase transition upon cooling. The observed behavior is significant to micro-length-scale spot cooling applications utilizing thin films and large-scale magnetic refrigeration applications where low magnetic fields are favorable.