Origin of the hump anomalies in the Hall resistance loops of ultrathin SrRuO 3 / SrIrO 3 multilayers

The proposal that very small N\'eel skyrmions can form in ${\mathrm{SrRuO}}_{3}/{\mathrm{SrIrO}}_{3}$ epitaxial bilayers and that the electric field effect can be used to manipulate these skyrmions in gated devices strongly stimulated the recent research of ${\mathrm{SrRuO}}_{3}$ heterostructures. A strong interfacial Dzyaloshinskii-Moriya interaction was considered as the driving force for the formation of skyrmions in ${\mathrm{SrRuO}}_{3}/{\mathrm{SrIrO}}_{3}$ bilayers. Here, we investigated nominally symmetric heterostructures in which an ultrathin ferromagnetic ${\mathrm{SrRuO}}_{3}$ layer is sandwiched between large spin-orbit coupling ${\mathrm{SrIrO}}_{3}$ layers, for which the conditions are not favorable for the emergence of a net interfacial Dzyaloshinskii-Moriya interaction. Previously the formation of skyrmions in the asymmetric ${\mathrm{SrRuO}}_{3}/{\mathrm{SrIrO}}_{3}$ bilayers was inferred from anomalous Hall resistance loops showing humplike features that resembled topological Hall effect contributions. Symmetric ${\mathrm{SrIrO}}_{3}/{\mathrm{SrRuO}}_{3}/{\mathrm{SrIrO}}_{3}$ trilayers do not show hump anomalies in the Hall loops. However, the anomalous Hall resistance loops of symmetric multilayers, in which the trilayer is stacked several times, do exhibit the humplike structures, similar to the asymmetric ${\mathrm{SrRuO}}_{3}/{\mathrm{SrIrO}}_{3}$ bilayers. The origin of the Hall effect loop anomalies likely resides in unavoidable differences in the electronic and magnetic properties of the individual ${\mathrm{SrRuO}}_{3}$ layers rather than in the formation of skyrmions.