Magnetic anisotropy of mesoscale-twinned Ni-Mn-Ga thin films

The ferromagnetic resonance data obtained for the twinned orthorhombic martensitic phase of Ni-Mn-Ga film epitaxially grown on MgO(100) substrate are presented. The reported data prove that the mesoscale twinning reduces the value of in-plane magnetic anisotropy field of the Ni-Mn-Ga film by an order of magnitude. The reduced magnetic anisotropy field corresponds to the tetragonal symmetry, while the unit cells of the film are orthorhombic. The experimentally observed change of the in-plane magnetic anisotropy is explained in the framework of the magnetoelastic model of martensite, and the second-order and fourth-order magnetic anisotropy constants are evaluated. The perpendicular magnetic anisotropy constant proved to be negative and small in the absolute value. Therefore the estimated value of the magnetic domain wall width is comparable with the widths of mesoscale twins observed in the investigated film. This confirms an idea that the magnetic vectors of twin components are strongly coupled by the exchange interaction.