Unconventional anomalous Hall effect driven by oxygen-octahedra-tailoring of the SrRuO structure

The anomalous Hall effect of SrRuO is of special interest, since Weyl nodes appear in the band structure and lead to an unconventional temperature dependence of the anomalous Hall constant. Moreover, it has been proposed that coupling of SrRuO films to materials with strong spin-orbit coupling or with ferroelectric or ferromagnetic order might lead to the formation of skyrmions and a topological contribution to the Hall effect. This latter conjecture is strongly debated. We probed this proposal by interfacing thin SrRuO layers to PrCaMnO, since it is known that the strong antiferromagnetic coupling between these two ferromagnets leads to complex magnetization states. Superlattices with sharp interfaces were grown by pulsed-laser deposition. The epitaxial interfacing with the PrCaMnO layers led to major modifications of the structural symmetry of the SrRuO layers. High resolution scanning transmission electron microscopy revealed that the individual SrRuO layers of the superlattices had heterogeneous structure with varying oxygen octahedral tilt angles across the layers, turning their structure to be tetragonal-like, with largely suppressed octahedral tilts when the thickness of the neighboring PrCaMnO layers was increased. These structural modifications were accompanied by major changes in the field dependence of the Hall signal with the mainly tetragonal SrRuO layers showing features strongly reminiscent of a topological Hall effect. However, since there was an intimate link between Hall effect and structure, the Hall data were interpreted as arising from a superposition of Hall effect contributions from tetragonal and orthorhombic SrRuO sub-layers.