Antiferromagnetic ordering and uncoupled spins in CaFe2O4 thin films probed by spin Hall-magnetoresistance measurements

CaFe2O4 is a uniaxial antiferromagnet displaying two coexisting magnetic orderings, A and B, characterized by up-down-up-down and up-up-down-down spin modulation, respectively, and the emergence of a net magnetization in a limited temperature range, which is not yet understood. We investigate the angular dependent magnetoresistance (ADMR) at the interface between Pt and CaFe2O4 and exploit the crystallographic domain structure of thin film samples to probe the spin Hall magnetoresistance (SMR) at a single- and multi-domain scale. Upon rotation of the magnetic field along three orthogonal planes, we observe a sinusoidal modulation of the magnetoresistance indicating that the mechanism for SMR is the rotation of the spins, despite the strong magnetocrystalline and shape anisotropies. This, together with the study of the magnetic field dependence of the response, allows us to extract two contributions to the SMR: one corresponds to the long-range antiferromagnetic ordering, supporting a single ground state scenario. The second contribution originates from uncompensated and uncoupled spins. These are expected to exist at the antiphase boundaries between antiferromagnetic domains. Here we show that these are uncoupled from the antiferromagnetic ordering. Nonetheless, the long range correlations that emerge in the proximity of the critical antiferromagnetic transition could give rise to ordering of the uncompensated spins and be responsible for the net magnetization observed in this antiferromagnet.