  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 Sn4+ and Ti4+, for which T_M dramatically decreases,
these dopants substantially increase T_M in thin films, far exceeding the bulk
values. The normalized Fe LII-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 LIII-edge spectra were,
for the most part, determined solely by the spin orientation state.
Calculations of the polarization-depedent spectra based on a spin-multiplet
model were in reasonable agreement with the experiment and showed a mixed
excitation character of the peak structures.
