Unusual perpendicular anisotropy in Co2TiSi films

Abstract

Thin films of Co2TiSi on MgO are investigated experimentally and theoretically. The films were produced by magnetron sputtering on MgO(0 0 1) and have a thickness of about 100 nm. As bulk Co2TiSi, they crystallize in the normal cubic Heusler (L21) structure, but the films are slightly distorted (c/a = 1.0014) and contain some antisite disorder. The films exhibit a robust perpendicular anisotropy of 0.5 MJ m-3. This result is surprising for several reasons. First, surface and interface anisotropies are too small to explain perpendicular anisotropy in such rather thick films. Second, Co2TiSi has a substantial magnetization and crystallizes in a cubic Heusler structure, so that conventional wisdom predicts a preferential magnetization direction in the film plane rather than perpendicular. Third, the lattice strain of 0.14% is unable to account for the perpendicular anisotropy. We explain the perpendicular anisotropy as a quasicubic symmetry breaking chemical-ordering effect promoted by the substrate.