Validity of magnetotransport detection of skyrmions in epitaxial SrRuO3 heterostructures

Abstract

A technically simple way of probing the formation of skyrmions is to measure the topological Hall resistivity that should occur in the presence of skyrmions as an additional contribution to the ordinary and anomalous Hall effect. This type of probing, lately intensively used for thin film samples, relies on the assumption that the topological Hall effect contribution can be extracted unambiguously from the measured total Hall resistivity. Ultrathin films and heterostructures of the $4d$ ferromagnet ${\mathrm{SrRuO}}_{3}$ have stirred up a lot of attention after the observation of anomalies in the Hall resistivity, which resembled a topological Hall effect contribution. These anomalies, first reported for bilayers in which the ${\mathrm{SrRuO}}_{3}$ was interfaced with the strong spin-orbit coupled oxide ${\mathrm{SrIrO}}_{3}$, were attributed to the formation of tiny N\'eel-type skyrmions. Here we present the investigation of heterostructures with two magnetically decoupled and electrically parallel connected ${\mathrm{SrRuO}}_{3}$ layers. The two ${\mathrm{SrRuO}}_{3}$ layers deliberately have different thicknesses, which affects the coercive field and ferromagnetic transition temperature of the two layers, and the magnitude and temperature dependence of their anomalous Hall constants. The ${\mathrm{SrRuO}}_{3}$ layers were separated by ultrathin layers of either the strong spin-orbit coupling oxide ${\mathrm{SrIrO}}_{3}$ or of the large band-gap insulator ${\mathrm{SrZrO}}_{3}$. Our magnetic and magnetotransport studies confirm the additivity of the anomalous Hall transverse voltages for the parallel conducting channels originating from the two ferromagnetic ${\mathrm{SrRuO}}_{3}$ layers as well as the possibility to tune the global anomalous Hall resistivity by magnetic field, temperature, or structural modifications at the epitaxial all-oxide interfaces. The Hall voltage loops of these two-layer heterostructures demonstrate the possibility to generate humplike structures in the Hall voltage loops of ${\mathrm{SrRuO}}_{3}$ heterostructures without the formation of skyrmions and emphasize that the detection of skyrmions only by Hall measurements can be misleading.