  We report on the chemical structure and spin Hall magnetoresistance (SMR) in
epitaxial $\alpha$-Fe$_2$O$_3$(hematite)(0001)/Pt(111) bilayers with hematite
thicknesses of 6 nm and 15 nm grown by molecular beam epitaxy on a MgO(111)
substrate. Unlike previous studies that involved Pt overlayers on hematite, the
present hematite films were grown on a stable Pt buffer layer and displayed
structural changes as a function of thickness. These structural differences
(the presence of a ferrimagnetic phase in the thinner film) significantly
affected the magnetotransport properties of the bilayers. We observed a sign
change of the SMR from positive to negative when the thickness of hematite
increased from 6 nm to 15 nm. For $\alpha$-Fe$_2$O$_3$(15 nm)/Pt, we
demonstrated room-temperature switching of the N\'eel order with rectangular,
nondecaying switching characteristics. Such structures open the way to
extending magnetotransport studies to more complex systems with double
asymmetric metal/hematite/Pt interfaces.
