Strain-Driven Dzyaloshinskii-Moriya Interaction for Room-Temperature Magnetic Skyrmions

Dzyaloshinskii-Moriya interaction in magnets, which is usually derived from inversion symmetry breaking at interfaces or in noncentrosymmetric crystals, plays a vital role in chiral spintronics. Here we report that an emergent Dzyaloshinskii-Moriya interaction can be achieved in a centrosymmetric material, ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$, by a graded strain. This strain-driven Dzyaloshinskii-Moriya interaction not only exhibits distinctive two coexisting nonreciprocities of spin-wave propagation in one system, but also brings about a robust room-temperature magnetic skyrmion lattice as well as a spiral lattice at zero magnetic field. Our results demonstrate the feasibility of investigating chiral spintronics in a large category of centrosymmetric magnetic materials.

Figure 1

Symmetry engineering by graded strain using epitaxial growth. (a) Schematic diagram of epitaxial growth of $\mathrm{LSMO}/\mathrm{NGO}$ (110) heterostructure with $\alpha$ being the angle of monoclinic pseudocubic lattice between $b$ and $c$. (b) The $c$-axis lattice constant as a function of thickness with the blue arrow representing the pseudocubic (pc) lattice constant of bulk LSMO. (c) The tensile strain relaxation (yellow arrow) along out-of-plane direction. (d) Experimental estimation of tensile strain gradient which reaches maximum around 100 nm.

Figure 2

Nonreciprocal spin-wave propagation. (a),(b) Setup for the measurement of nonreciprocal spin wave propagation with in-plane magnetic field (black arrows) perpendicular ($\mathbf{H}\perp \mathbf{k}$) and parallel ($\mathbf{H}\Vert \mathbf{k}$) to the wave vector, respectively. The wave vector $\mathbf{k}$ (white arrows) is along the $x$ direction in all experiments and yellow antennas are coplanar waveguide (CPW1 and CPW2). (c),(d) Transmission spectra for $\mathbf{H}\perp \mathbf{k}$ and $\mathbf{H}\Vert \mathbf{k}$, respectively, at room temperature in 100 nm sample. $S_{21}$ corresponds to propagating spin wave from CPW1 to CPW2, and $S_{12}$ from CPW2 to CPW1. (e),(f) Thickness dependence of the frequency nonreciprocities in $\mathbf{H}\perp \mathbf{k}$ (blue) and $\mathbf{H}\Vert \mathbf{k}$ (orange), respectively. The curves of circles are fitting based on experimental data (squares). And the black-dashed curve replots the magnitude of the estimated strain gradient $E_{zz,z}$.

Figure 3

Strain-driven Dzyaloshinskii-Moriya interaction. (a) Side view of the graded lattice (black-dashed line) of a LSMO thin film with a strain gradient $E_{zz,z}$. The yellow spheres are La or Sr cations and violet spheres are Mn cations. (b) General picture of the strain-driven DMI with three DM vectors (green arrows), ${\mathbf{D}}_1$, ${\mathbf{D}}_2$, and ${\mathbf{D}}_3$ for Mn-Mn pairs in $x$, $y$, and $z$ directions, respectively. (c) The two components $D_{1x}$ and $D_{1y}$ for different characterizations, i.e., helicoid interaction and cycloid interaction, respectively. For the spin wave propagating in the $x$ direction, neighboring spins ${\mathbf{S}}_0$ and ${\mathbf{S}}_1$ (yellow arrows) of Mn ions rotate helicoidally (cyclodially) in the $yz$ ($xz$) plane. Those two different characterizations induce the coexistence of frequency nonreciprocities in both configurations of Figs. 2 and 2.

Figure 4

Topological spin textures at room temperature and topological Hall effect. The zero-field (a) magnetic skyrmion lattice and (b) spin-spiral lattice at room temperature for 100 nm sample. (c) Total Hall resistivity curves of AHE and THE and (d) the net THE contribution as a function of magnetic field at different temperatures. The black and red colors represent the two field-sweeping directions indicated by the colored arrows.

Figure 5

Zero-field MFM images in 50, 150, and 200 nm thin film at room temperature. (a) Tiny wormlike textures in 50 nm thin film. (b) Mixed phase with a few isolated skyrmions in 150 nm thin film. (c) Spin-spiral lattice in 200 nm thin film.