52 53 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Skyrmion lattice observation of ultra-thin SrRuO 3 heterostructure Related publication: Sohn, B., Kim, B., Park, S. Y., Choi, H. Y., Moon, J. Y., Choi, T., Choi, Y. J., Zhou, H., Choi, J.W., Bombardi, A., Porter, D. G., Chang, S. H., Han, J. H., & Kim, C. Stable humplike Hall effect and noncoplanar spin textures in SrRuO 3 ultrathin films. Physical ReviewResearch 3 , (2021). 023232. DOI: 10.1103/PhysRevResearch.3.023232 Publication keywords: Skyrmion; Magnetic texture; Topological Hall effect T he search is on for the next generation of information carriers. Ideal candidates need to be stable against external disturbances such as temperature or field fluctuations, allow for low-power manipulation and easy readout, and have a small footprint that allows for high packing densities. Magnetic skyrmions – swirls of magnetic moments – have unique topological properties that make thempromising candidates for the next generation of computer storage devices. Crystalline magnetic skyrmions are among the most promising replacements for conventional ferromagnetic computer memory. Chiral spin structures have non-trivial Berry curvature in real space or emergent electromagnetic fields, leading to topological Hall effects. Recent studies show that chiral spin structures can exist in oxide thin film heterostructures with Dzyaloshinskii-Moriya Interactions (DMI) generated due to its Inversion Symmetry Breaking (ISB). Previous research reported progress in observing topological Hall effects in a few- layer thick SrRuO 3 (SRO) grown on top of a single crystal SrTiO 3 (001) substrate without any heavy-metal capping layer. Researchers looking for direct evidence of the existence of magnetic skyrmions in ultra-thin SrRuO 3 films used temperature-dependent Resonant Elastic X-ray Scattering (REXS) on the Materials and Magnetism Beamline (I16). They observed exciting Hall effect data, the fingerprint of magnetic skyrmions. Their results showed a newmagnetic order with the same temperature dependence topological Hall effects, which they believe corresponds to skyrmion lattice structure. This new quasi quantumparticle could soon be used for magnetic racetrack memory. Skyrmions are topologically stable spin whirling configurations in magnetic materials. In addition to their importance in fundamental science, they pos- sess high potential for future device applications. It is known that skyrmions in a magnetic system are stabilised by the Dzyaloshinskii-Moriya Interaction (DMI), forming a unique topological spin whirling texture. A notable example is SrRuO 3 (SRO), a prototypical ferromagnet with inversion symmetry. SRO in heterostructures was recently shown to be a platform for skyrmion formation. Here, the authors present a resonant X-ray diffraction experiment, using resonant scattering at the Ru L 2, 3 absorption edge to observe the spin texture of an ultra-thin SRO system. They believe that this experiment will be a critical building block for the study of skyrmion structure in a real space system. Illustrated in Fig. 1(a) are the electric transport results on SRO. The Topological Hall Effect (THE) exists as clear humps and disappears near the coercive field where the Anomalous Hall Effect (AHE) undergoes a sign change. The THE is a novel quantum state where electrons moving through a non- coplanar magnetic field acquire a Berry curvature and are only accessible under particular circumstances and is often formed by the swirling magnetic texture of skyrmion quasi-particles. At the lowest temperature of 2 K, the THE persists over the 0.6 - 1.6 T range, demonstrating that the THE in SRO thin films is extremely robust compared to other materials that host skyrmions over much weaker fields. The Hall effect data under the external magnetic field tilting are plotted in two different ways in Fig. 1(b), using either the total magnetic field or the perpendicular field as the variable for the plot. As we can see, the THE still exists up to 85 degrees in the in-plane geometry of the external magnetic field. Compared to similar field-tilt experiments in EuO 2 , FeGe 2 , and GaV 4 , the ultra-thin SRO shows an exceptionally wide phase diagram region of THE stability under the field tilting. Figure 1(c) represents the phase diagram at 10 K for the 4 u.c. SRO film, as deduced from the Hall effect measurement data. The two components refer to the in-plane and out-of-plane magnetic field, respectively. As presented in this phase diagram, THE area is extremely wide compared to the other Skyrmion system. Therefore, this ultra-thin SRO system is critically important to study magnetic skyrmions for real application considering this robustness. A Resonant Elastic X-ray Scattering (REXS) experiment was performed at beamline I16 at Diamond Light Source. REXS was measured at the Ru L 2 absorption edge with linear polarisation. A 6-circle diffractometer allowed the authors to optimise scattering from this thin-film sample using a grazing incidence diffraction geometry around the forbidden reflection (1, 1, 1/2). The X-ray path length at this energy and angle is roughly 30 nm and in this geometry, the horizontal X-ray polarisation is ∼ 60 degrees from the scattering plane. Air scattering was reduced with use of a helium-filled bag and an ultra- high gain, vacuum-enclosed Pilatus3-100 K area detector. A closed-cycle cryo- cooler was used to control temperature. SRO thin films were mounted directly to a copper plate using conductive silver paint. A permanent magnet was mounted directly below the plate, with a measured field on the sample H = 0.47 T, providing a steady magnetic field in order to measure the temperature dependence of the topological Hall effect in this system. It has been demonstrated that hump structures in Hall Effect measurement results could originate from noncoplanar spin order such as a skyrmion lattice 1 . However, there is another possibility which is inhomogeneity of Anomalous Hall Effect. Thus, if there is a real noncoplanar spin structure, hidden magnetic orders might emerge with humplike features in a 4 u.c. SRO ultrathin film. And the REXS is a powerful and critical experimental method to directly observe a magnetic order in reciprocal space 2 . Figure 2(a) represents the experimental range of the REXS measurements in the temperature-field phase diagram of Hall measurement for the 4 u.c. SRO film. Figures 2(b) and 2(c) show Reciprocal Space Mapping (RSM) results of REXS measurement under 0.47 T at 120 and 32 K, respectively. At (1, 1, 1/2) pc (pc denotes pseudocubic), the authors observed very weak and nearly temperature-independent diffraction patterns, which can originate from the crystal truncation rods or cation displacement. As shown in Fig. 2(b), only stripy patterns are observed at 120 K. The separation between the stripes in the reciprocal space is 0.0028 Å −1 and can be converted into the distance of ∼ 220 nm in real space, which is consistent with the width of step terraces measured by Atomic Force Microscopy. Upon decreasing the temperature to 32 K, the SRO film entered into the hump phase in Fig. 2(a). Then, extra peaks around the stripes appear in the RSM as indicated by red arrows in Fig. 2(c) and Fig. 2(d) illustrates that black and red peaks resulted from the step terraces and emergent magnetic order at low temperature, respectively. One set of magnetic wavevectors is aligned parallel to the step-terace direction and another is tilted from the step-terrace. To confirm the correlation between hump phase and the emergent magnetic peaks, the authors plotted the intensity of emergent magnetic peaks as a function of temperature (Fig. 2(e)). The phase crossover temperatures from hump to ferromagnetic and from ferromagnetic to nonmagnetic phases were approximately 15 and 60 K, respectively, with μ 0 H ∼ 0.47 T. At both of the crossover temperatures, they clearly observed the abrupt change of intensity of the emergent magnetic peaks. Considering the strong correlation between the hump phase and the intensity of emergent peaks, the authors propose that the emergent magnetic order is responsible for the humplike feature. Based on this REXS observations presenting emergent magnetic order and theoretical analysis with recent XMCD imaging studies 3, 4 , the authors believe that the humplike feature in Hall effect measurement is attributed to the non- coplanar spin order and can be interpreted as Topological Hall Effect. References: 1. Yu, X. Z. et al. Real-space observation of a two-dimensional skyrmion crystal. Nature 465 , 901–904 (2010). DOI : 10.1038/nature09124 2. Zhang, S. L. et al. Direct experimental determination of spiral spin structures via the dichroism extinction effect in resonant elastic soft X-ray scattering. Physical Review B 96 , 094401 (2017). DOI: 10.1103/ PhysRevB.96.094401 3. Huang, H. et al. Detection of the chiral spin structure in ferromagnetic SrRuO 3 thin film. ACS Applied Materials & Interfaces 12 , 37757–37763 (2020). DOI: 10.1021/acsami.0c10545 4. Sohn, B. et al. Hump-like structure in Hall signal from ultra-thin SrRuO 3 films without inhomogeneous anomalous Hall effect. Current Applied Physics 20 , 186–190 (2020). DOI: 10.1016/j.cap.2019.10.021 Funding acknowledgement: This work is supported by IBS-R009-D1 through the IBS Center for Correlated Electron Systems. The work at beamline I16 of the Diamond Light Source was performed under Proposals MM22181. Corresponding author: Dr. Bongju Kim, Seoul National University,
[email protected] MagneticMaterials Group Beamline I16 Figure 1: (a) Hall effect measured for 5 u.c. SRO at 30 K. The curves contain contributions from both AHE and THE, while the humps are due to the THE; (b) Hall resistivity at various angles of field inclination. The same data is plotted using the x-axis of total external magnetic field, and out-of-plane magnetic field; (c) Phase diagram in the plane of parallel and perpendicular components of the external magnetic field. (Inset) Schematics of our experiment with the tilt angle indicated. Figure 2: Hump structure related magnetic order in a SRO ultrathin film. (a) Phase diagram of 4 u.c. SRO in the (T, μ 0 H) plane; Resonant Elastic X-ray Scattering (REXS) was performed as a function of temperature with μ 0 H = 0.47 T; (b); (c) Reciprocal Space Mapping (RSM) of REXS around SRO (1, 1, 1/2) pc reflection under a magnetic field of 0.47 T at 120 and 32 K, where pc denotes pseudocubic. Additional peaks are shown at 32 K (red arrows); (d) Schematic description of REXS features around SRO (1, 1, 1/2) pc reflection. Black stripy (red dotted) patterns are structural (hump-related magnetic) peaks. An incommensurate magnetic modulation vector, q1, is aligned along the direction of the step terrace, whereas q2 is tilted from the step terrace; (e) Temperature-dependent intensity of hump-related magnetic peaks [white dotted circle in panel (c)].