Magnetic states at the surface of α-Fe_2O_3 thin films doped with Ti, Zn, or Sn

摘要

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≈200 K) and Sn-doped (T_M ≈ 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 Sn~(4+) and 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_(11)-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_(111)-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.