  Antiferromagnetic materials promise improved performance for spintronic
applications, as they are robust against external magnetic field perturbations
and allow for faster magnetization dynamics compared to ferromagnets. The
direct observation of the antiferromagnetic state, however, is challenging due
to the absence of a macroscopic magnetization. Here, we show that the spin Hall
magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin
structure via simple electrical transport experiments by investigating the
easy-plane antiferromagnetic insulators $\alpha$-Fe2O3 (hematite) and NiO in
bilayer heterostructures with a Pt heavy metal top electrode. While rotating an
external magnetic field in three orthogonal planes, we record the longitudinal
and the transverse resistivities of Pt and observe characteristic resistivity
modulations consistent with the SMR effect. We analyze both their amplitude and
phase and compare the data to the results from a prototypical collinear
ferrimagnetic Y3Fe5O12/Pt bilayer. The observed magnetic field dependence is
explained in a comprehensive model, based on two magnetic sublattices and
taking into account magnetic field-induced modifications of the domain
structure. Our results show that the SMR allows us to understand the spin
configuration and to investigate magnetoelastic effects in antiferromagnetic
multi-domain materials. Furthermore, in $\alpha$-Fe2O3/Pt bilayers, we find an
unexpectedly large SMR amplitude of $2.5 \times 10^{-3}$, twice as high as for
prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for
room-temperature antiferromagnetic spintronic applications.
