Origin of the hump anomalies in the Hall resistance loops of ultrathin ${\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3$ multilayers

The proposal that very small Néel 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.

Figure 1

Microstructure investigations by scanning transmission electron microscopy. (a) Schematics of sample ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_m$ ($m=1$, 3, 6) grown on ${\mathrm{SrTiO}}_3$(100) substrates. (b) An overview HAADF-STEM micrograph of sample ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_6$ indicates the layers are uniform (except for the top layer that was damaged during the FIB processing of the specimen). (c) Schematics of the structure of the trilayer ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_1$, for which a 6 uc ${\mathrm{SrRuO}}_3$ layer is inserted between two ${\mathrm{SrIrO}}_3$ layers (both 2 uc thick). Green, orange, blue, and red dots represent Sr, Ir, Ru, and O atomic column positions, respectively, in the crystal structure drawn using VESTA [41]. In (d) and (e) high magnification micrographs show the quality of the interfaces. EDX elemental mapping across the entire stacks shown in (e) probed the uniformity of chemical element distribution. (f) FFT pattern obtained from the image shown in (d), which shows the spots due to the reflections originating from the orthorhombic distortion (marked by red circles), and confirms the in-plane $c$-axis orientation of the layers [white arrow in (d)].

Figure 2

Temperature dependence of the magnetic moment of the samples (a) ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_1$ and (b) ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_6$ under zero field cooling (ZFC, blue plot) and field cooling (FC, red plot, 0.1 T applied perpendicular to the sample surface) conditions. The dotted red curve shown in (b) is the ${\mathrm{FC}}_1$ curve of the trilayer sample from (a) multiplied by six and plotted for the sake of comparison. The insets in (a) and (b) show the first derivative of the magnetization with respect to temperature, used to determine the Curie transition temperatures of the ${\mathrm{SrRuO}}_3$ layers.

Figure 3

Magnetic moment hysteresis loops (measured by SQUID magnetometry, red loops) and MOKE rotation angle loops for samples (a) ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_1$ and (b) ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_6$, measured in perpendicular magnetic field. (c) Comparison of the coercive fields of these two samples at different temperatures, as obtained from the SQUID and MOKE hysteresis loops. The lines are guides for the eye.

Figure 4

Summary of the anomalous Hall effect (AHE) resistance $R_{yx}$ loops of the ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_m$ ($\mathit{m}=1$, 6) samples, as a function of temperature: (a) for ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_1$ and (b) for ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_6$. In (c) and (d) the anomalous Hall resistance loops (black) and the MOKE rotation angle loops (red) measured at 80 K for sample ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_1$ and ${\mathrm{SrIrO}}_3/[{\mathrm{SrRuO}}_3/{\mathrm{SrIrO}}_3{]}_6$, respectively, are compared.