Interface-driven noncollinear magnetic structure and phase transition of Fe thin films

We investigate the magnetic structure and its temperature dependence of Fe thin films grown on MgO(001) and ${\mathrm{Al}}₂{\mathrm{O}}₃$(0001) surfaces by means of nuclear resonant x-ray scattering. By fabricating Fe films $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{doped}$ with $⁵⁷\mathrm{Fe}$ at the interface and middle of the film, depth-resolved analysis of the magnetic structure is performed. On MgO(001), the magnetization is dominantly out-of-plane at the interface, whereas it is mainly in-plane at the middle of the film, indicating that the Fe film has a noncollinear magnetic structure. On ${\mathrm{Al}}₂{\mathrm{O}}₃$(0001), on the other hand, the magnetization is mainly in-plane in the entire film. The noncollinear magnetic structure on MgO(001) is confirmed to be energetically stable with the aid of first-principles calculations. We also investigate the temperature dependence of the internal magnetic field in a depth-resolved way. The experimental data suggest the internal magnetic field at the interface is smaller than that of the middle of the film on both substrates, suggesting that the magnetic phase transition starts at the interface at a lower temperature than the entire film.