Emergent Chiral Spin Crystal Phase in (111) SrRuO3 Thin Films

TL;DR

The study reports an intrinsic double-$Q$ chiral spin crystal in (111) SrRuO$_3$ thin films, evidenced by a robust topological Hall effect and real-space stripe patterns that arise from two orthogonal spin modulations. Unlike prior work that relies on interfacial DM interactions, the observed phase is stabilized by the interplay of dipolar couplings and magnetic frustration on a buckled honeycomb lattice, enabling stable topological textures in relatively thick films. The combination of MFM imaging, transport measurements, and Monte Carlo micromagnetic simulations shows that the double-$Q$ state can be described as a superposition of two cycloidal spirals with density $D = |1/(Q_1 Q_2)|$ that tracks the THE. This work establishes a new intrinsic mechanism for topological magnetism in perovskites and points to scalable routes for spintronic devices leveraging chiral spin textures without relying on engineered interfacial DM.

Abstract

Perovskite ruthenates are fascinating playgrounds for exploring topological spin textures, but generally rely on extrinsic mechanisms to trigger the noncoplanar states. Here we report the discovery of an emergent chiral spin crystal phase in (111) SrRuO3 epitaxial films, characterized by a significant topological Hall effect and noncoplanar spin arrangements with different propagation vectors along two orthogonal directions. Instead of driven by the enhanced Dzyaloshinskii-Moriya interaction due to broken inversion symmetry at heterointerfaces, this emergent state arises intrinsically from the interplay of dipolar interactions and magnetic frustration, leading to the stabilization of topological phases in much thicker films. These findings open a new pathway for creating and controlling the topological spin states in perovskites, with broad implications for spintronic device design.

Figures (5)

• Figure 1: Schematic of the chiral spin crystal phase. (a) The buckled honeycomb lattice constructed by two neighboring Ru layers of SrRuO$_3$ along the [111] direction. Ru$_1$ and Ru$_2$ refer to the Ru atoms of respective layers. (b) The dipolar interaction and the nearest-neighbor interactions up to the third order. (c) Illustration of the competition between Zeeman energy and tensile-strain induced magnetic anisotropy. (d) Overview of the double-$Q$ chiral spin crystal phase on the buckled honeycomb lattice. (e) Schematics of the cycloidal spin arrangements along two orthogonal $Q_1$ and $Q_2$ directions.
• Figure 2: THE in (111) SrRuO$_3$ thin films. (a) The Hall resistivity $\rho_{xy}$ and the longitudinal magnetoresistance MR at different temperatures across T$_C$. The blue curves refer to scans recorded by sweeping magnetic field from positive to negative, whereas the red ones from negative to positive. The hump-like THE contribution at each temperature is shadowed in gray. (b) The peak value of THE as a function of temperature. (c) Mapping of the extracted topological Hall resistivity as a function of temperature and magnetic field.
• Figure 3: Evolution of magnetic domains in (111) SrRuO$_3$ thin films at 4 K. (a) MFM images recorded in regime (I) at 0.3 and 1.1 T on the initial magnetization process. The color bars stand for a frequency range from -1.0 Hz to 1.0 Hz, covering a total scanning area of 5$\times$7 $\mu$m. The outlined area on each figure in the black box highlights the splitting of the magnetic bubbles. (b) MFM images recorded in regime (II) at a set of magnetic fields on the hysteretic branch with backward sweeping direction. The color bars stand for a frequency range from -0.35 Hz to 0.35 Hz. The crystallographic [1-10] and [11-2] directions are marked on the image of 6 T. (c) The full hysteresis loop of Hall resistivity at 4 K. The dark blue and pink circles refer to where the MFM scans were recorded in regime (I) and (II), respectively.
• Figure 4: Fourier analyses and model illustration of the double-$Q$ chiral spin crystal phase. (a) The 2.5 T MFM image after noise filter. (b) Emergence of the double-$Q$ feature at 2.5 T on the FFT image, with the overall appearance shown at the left corner. The slight deviation from perfect orthogonality of $Q_1$ and $Q_2$ is due to the global averaging nature of FFT over the large scanning area. (c) Isolated MFM images along each $Q$ direction by inverse FFT. (d) Simulated spin arrangements as a superposition of two orthogonal cycloidal spin spirals, projected along the out-of-plane direction. The propagation vectors $Q_1$ and $Q_2$ are approximately aligned along the crystallographic [10-1] and [1-21] directions. (e) MFM data extracted from the line scans marked on (a) and the corresponding spin z component from the line cuts on (d). (f) Plot of the Hall resistivity $\rho_{\text{xy}}$ and the density of the double-$Q$ spin crystal as a function of magnetic field at 4 K.
• Figure 5: Monte Carlo micromagnetic simulations on the buckled honeycomb lattice. The simulated spin structures are obtained in the cases of (a) no dipolar interactions and bulk-like parameters; (b) no dipolar interactions and manipulated exchange interactions; (c) with dipolar interactions and bulk-like parameters; (d) with dipolar interactions and manipulated exchange interactions. The corresponding FFT images in each case are displayed at the bottom.