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

Abstract

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.

Observation of magnetization surface textures of the van der Waals antiferromagnet FePS3 by spin Hall magnetoresistance

“…SMR has been observed in paramagnetic CoCr 2 O 4 [10], amorphous yttrium iron garnet (YIG) [11], Cr 2 O 3 [12], NdGaO 3 [13], and gadolinium gallium garnet (GGG) [14]. The SMR revealed magnetic phase transitions in α-Fe 2 O 3 [15], CoCr 2 O 4 [10], and DyFeO 3 [16] and magnetic domain structures [17]. The SMR in the longitudinal resistance of a metal contact on an antiferromagnet with two sublattices A and B reads…”

Unusual ferrimagnetism in CaFe2O4

“…Both possibilities imply a small coercive field of Fe 2+ moments and intrinsic contribution of Fe 3+ to M from Fe 2+ . Therefore, the uncoupled spins from the AFM phase, i.e., from the main Fe 3+ spins, reported previously [13] should be ascribed to defective Fe 2+ that can easily respond to H.…”

“…Magnetic diffuse scattering experiments have attributed M to the presence of FM domain boundaries which also display an enhancement in H resulting from an increase in domain density [8]. Recently, uncoupled spins from the AFM domains have been discussed as a possible origin, based on spin Hall magnetoresistance measurements [13]. Note that the small M is not due to a spin canting since M and the Néel vector are both along [010] [12].…”

Calcium ferrites for phosphate adsorption and recovery from wastewater

“…Spin-polarized DFT + U calculations were performed, and the systems were initialized using antiferromagnetic ground state spin orderings with a Fe-sextet multiplicity . 29,34,35 In this study, the effective Hubbard parameter ( U eff ) was included following the DFT + U calculations were performed within the rotationally invariant formulation by Dudarev et al 36 by applying an anisotropic Hubbard correction 37,38 to Fe-3d states with self-consistent U values computed via DFPT. 39 Hubbard parameters were computed by perturbing all inequivalent Fe-sites.…”