Antiferromagnetic Ordering and Uncoupled Spins in CaFe₂O₄ Thin Films Probed by Spin Hall Magnetoresistance

CaFe₂O₄ is a uniaxial antiferromagnet displaying two coexisting magnetic orderings, A and B, characterized by ↑↑↓↓ and ↑↓↑↓ spin modulation, respectively, and the emergence of a net magnetization in a limited temperature range, which is not yet understood. The spin Hall magnetoresistance (SMR) is probed at the interface between Pt and CaFe₂O₄ and the crystallographic domain structure of thin film samples is exploited to perform single- and multi-domain scale measurements. The SMR response, upon rotating the magnetic field along three orthogonal planes, shows little effect of the strong magnetocrystalline and shape anisotropies. Together with the response to a varying magnetic field strength, the modulations in the SMR signal allow to extract two contributions: one corresponds to the long-range antiferromagnetic ordering, supporting a single ground state scenario; while the second contribution originates from uncompensated, non-interacting spins. These are expected to exist at the antiphase boundaries between antiferromagnetic domains. Here, it is shown that these are also uncoupled from the antiferromagnetic ordering. Nonetheless, the long range correlations that emerge in the proximity of the critical antiferromagnetic transition can give rise to ordering of the uncompensated spins and be responsible for the net magnetization observed in this antiferromagnet.