Magnetic states at the surface of α−Fe2O3 thin films doped with Ti, Zn, or Sn

The spin states at the surface of epitaxial thin films of hematite, both undoped and doped with 1% Ti, Sn, or Zn, respectively, were probed with x-ray magnetic linear dichroism (XMLD) spectroscopy. Morin transitions were observed for the undoped $({T}_M\ensuremath{\approx}200$ K) and Sn-doped $({T}_M\ensuremath{\approx}300$ K) cases, while Zn- and Ti-doped samples were always in the high- and low-temperature phases, respectively. In contrast to what has been reported for bulk hematite doped with the tetravalent ions ${\mathrm{Sn}}⁴⁺$ and ${\mathrm{Ti}}⁴⁺$, for which ${T}_M$ dramatically decreases, these dopants substantially increase ${T}_M$ in thin films, far exceeding the bulk values. The normalized Fe ${L}_II$-edge dichroism for $T<{T}_M$ does not strongly depend on doping or temperature, except for an apparent increase of the peak amplitudes for $T<100$ K. We observed magnetic field-induced inversions of the dichroism peaks. By applying a magnetic field of 6.5 T on the Ti-doped sample, a transition into the $T>{T}_M$ state was achieved. The temperature dependence of the critical field for the Sn-doped sample was characterized in detail. It was demonstrated the sample-to-sample variations of the Fe ${L}_III$-edge spectra were, for the most part, determined solely by the spin orientation state. Calculations of the polarization-dependent spectra based on a spin-multiplet model were in reasonable agreement with the experiment and showed a mixed excitation character of the peak structures.