image: (a) simulated color scale map showing the relaxation of the growth strain in 0.5 μm wide La0.67Sr0.33MnO3 (LSMO) wire, lithographically fabricated from LSMO thin film grown on LaAlO3 (LAO) substrate. (b) Magnetic vortex cluster state observed in LSMO wire using variable temperature magnetic force microscopy (VT-MFM). (c) Phase field simulated image of domains in 0.5 μm wide LSMO wire. (d) Calculated strain profile as a function of normalized position across different widths of LSMO wires (left panel) and corresponding simulated magnetic domains in these wires (right panel). (e) High-angle annular dark-field (HAADF) scanning tunneling electron microscopy (STEM) image of LSMO/LAO. (f) Schematics for possible use of observed vortex cluster state for storage devices.
Topologically stabilized spin structures at the nanoscale magnets, including domain walls, vortices and skyrmions, have recently received much attention. Among the nanoscale non-linear spin textures, vortex is a typical and well-known magnetic domain in dimensionally confined systems with a symmetry determined by its polarity and chirality. Because of its stability at the nanoscale and its robust control on nanosecond timescales, the magnetic vortex can be a promising candidate for next-generation magnetic data-storage devices [1].
Recently, emergent phenomena were discovered in manganites with strong electron correlation, such as the nonvolatile tunable magnetoresistance [2], ultralow-current-induced domain-wall motion [3], anisotropic resistance switching [4], topological Hall effect [5] and the high-frequency spin-wave propagation [6] etc., which are strongly affected by their mesoscopic domains. Magnetic domains in manganites are sensitive to various external stimuli such as strain, size, electric/magnetic fields etc., making them a model system to manipulate their spin textures (e.g. vortex, chiral domain walls). However, magnetic vortices were usually observed in spatially confined nanostructures such as the square-shaped, triangle-shaped, and disc-shaped nano-islands, and the shape-induced magnetic anisotropy is assumed to be the major mechanism for the formation of the magnetic vortex.
Using variable-temperature magnetic force microscopy (VT-MFM) and in-situ magnetoresistance measurements, Chinese researchers in collaboration with German scientists discover that magnetic vortex clusters in epitaxial LSMO structure can be stabilized by artificially engineering its strain state. Phase-field modeling further supports that the vortex state in this one-dimensional manganite originate from the inhomogeneous strain. Enhancement of the uniaxial strain relaxation-induced magnetic anisotropy in wires and its competition with the shape-induced anisotropies plays an important role in stabilizing the flux closure spin structure. This work offers a new strategy to build up emergent spin textures in strongly correlated magnets and may trigger new designs for magnetoelectronic devices.