Oxidation-linked Anomalous Hall-like Transverse Magnetoresistance in Sputtered Chromium Thin Films

The conversion of spin currents to charge currents, and vice-versa, are a core topic in spintronics with important open fundamental questions and potential applications in information technology and other fields. Heavy metals with large spin-orbit coupling are the traditional route to materials for spin-charge conversion, but are typically expensive. We have recently shown notably high spin-charge conversion efficiency in thermally-driven spin-charge conversion (or spin Seebeck effect, SSE) using Cr films deposited on yttrium iron garnet (YIG) substrates, with thickness and temperature dependence that suggests that antiferromagnetism in Cr could play a role [1,2]. Here we focus on spin Hall magnetoresistance (SMR) data for the same Cr/YIG structures. We compare thermally evaporated and sputtered Cr films, which show symmetrically similar SSE signals though with different temperature dependence. The SMR for these films is dramatically different, with the sputtered films showing an unexpected anomalous Hall-like signal, which would normally be forbidden by the symmetry of the experiment; while the thermally evaporated Cr is consistent with convenional SMR. By comparing structures with capped and un-capped Cr films, both in SMR and via SQUID magnetometry, we have linked the anomalous Hall-like transverse magnetoresistance with oxidation of the sputtered Cr [3]. These results are important for continued fundamental understanding of the role magnetic order plays in spin-charge conversion and spin transport, and are potentially relevant to recent interest in Cr as a material that could potentially show orbital Hall effects.