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ACS omega 5(11), 5824-5833 (2020). doi:10.1021/acsomega.9b03996, Published by ACS Publications, Washington, DC

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.

Magnetic interlayer coupling is a key ingredient in designing magnetic multilayers with functionalities that reach out to the realm of applications. In epitaxial ferromagnetic (FM) oxide multilayers, the magnetic interlayer coupling is, however, less studied and its prediction is often a challenging task. Ultrathin FM ${\mathrm{SrRuO}}_{3}$ epitaxial films with perpendicular magnetic anisotropy, interfaced with suitable oxides, may be susceptible of forming skyrmions. Hence, a strong FM interlayer coupling would be beneficial to achieve uniform switching behavior of a ${\mathrm{SrRuO}}_{3}$-based multilayer. Previous studies reported that the coupling of two ${\mathrm{SrRuO}}_{3}$ layers separated by a non-FM oxide spacer is at best weakly FM and the two FM layers switch at markedly different fields. Here we study the magnetic interlayer coupling between two FM ${\mathrm{SrRuO}}_{3}$ layers separated by ultrathin ${\mathrm{LaNiO}}_{3}$ in epitaxial heterostructures grown on ${\mathrm{SrTiO}}_{3}$(100) single crystals. We found that FM ${\mathrm{SrRuO}}_{3}$ layers separated by 2 monolayers (MLs) thick ${\mathrm{LaNiO}}_{3}$ show weak FM interlayer coupling of about $106\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{J}/{m}^{2}$ at 10 K. The coupling becomes strongly FM for four MLs thick (about 1.6 nm) ${\mathrm{LaNiO}}_{3}$ spacers and the two ${\mathrm{SrRuO}}_{3}$ layers reverse their magnetization at a common value of the perpendicular magnetic field. This is likely due to a transition of the ${\mathrm{LaNiO}}_{3}$ spacer from insulating to metallic, as its thickness increases.

SrRuO3, a 4d ferromagnet with multiple Weyl nodes at the Fermi level, offers a rich playground to design epitaxial heterostructures and superlattices with fascinating magnetic and magnetotransport properties. Interfacing ultrathin SrRuO3 layers with large spin–orbit coupling 5d transition-metal oxides, such as SrIrO3, results in pronounced peaklike anomalies in the magnetic field dependence of the Hall resistivity. Such anomalies have been attributed either to the formation of Néel-type skyrmions or to modifications of the Berry curvature of the topologically nontrivial conduction bands near the Fermi level of SrRuO3. Here, epitaxial multilayers based on SrRuO3 interfaced with 5d perovskite oxides, such as SrIrO3 and SrHfO3, were studied. This work focuses on the magnetotransport properties of the multilayers, aiming to unravel the role played by the interfaces with 5d perovskites in the peaklike anomalies of the Hall resistance loops of SrRuO3 layers. Interfacing with large band gap insulating SrHfO3 layers did not influence the anomalous Hall resistance loops, while interfacing with the nominally paramagnetic semimetal SrIrO3 resulted in pronounced peaklike anomalies, which have been lately attributed to a topological Hall effect contribution as a result of skyrmions. This interpretation is, however, under strong debate and lately alternative causes, such as inhomogeneity of the thickness and the electronic properties of the SrRuO3 layers, have been considered. Aligned with these latter proposals, our findings reveal the central role played in the anomalies of the Hall resistivity loops by electronic inhomogeneity of SrRuO3 layers due to the interfacing with semimetallic 5d5 SrIrO3.