Self-activated reversibility in the magnetically induced reorientation of martensitic variants in ferromagnetic Ni-Mn-Ga films

Magnetically induced reorientation (MIR) of twin variants for [101]-oriented martensite has been studied for Ni-Mn-Ga thin films deposited on (001) YSZ substrates. The MIR effect displayed by these films is self-reversible, is characterized by a small increase in the magnetization, and shows a temperature-dependent activation field. These properties can be described in terms of a magnetic microstructure comprised of MIR-active and MIR-inactive twin variants, where the degree of activity depends on the relative orientations of the externally applied magnetic field and the easy axis of the magnetization of the variants. The MIR-active variants are responsible for the detwinning, while the MIR-inactive variants exert a restoring force on the MIR-active variants as they reorient. This interplay between the active and inactive variants is highly advantageous as it allows for the complete self-activation of the reversible MIR effect, a response which is desired for actuation applications. The results further demonstrate that the magnetic domain structure formed in the films is inherited from a substrate-induced crystallographic texture and microstructure. It is also suggested that a tailored MIR response could prove possible through an optimization of the film texture and microstructure.