Planar Hall effect and magnetic anisotropy in epitaxially strained chromium dioxide thin films

We have measured the in-plane anisotropic magnetoresistance of 100 nm thick CrO2 thin films at liquid He temperatures. In low magnetic fields H, both the longitudinal and the transverse (planar Hall) resistance show abrupt switches, which characteristically depend on the orientation of H. All the experimental findings consistently demonstrate that the magnetic anisotropy in these CrO2 thin films is biaxial. We show that the biaxial magnetic anisotropy is due to epitaxial coherency strain, and that it naturally explains the complex magnetic switching behavior reported recently in CrO2 films with thicknesses of 50 nm <= d <= 250 nm.