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Register a new userMelting a skyrmion lattice topologically: through the hexatic phase to a skyrmion liquid
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Skyrmions are twirling magnetic textures whose non-trivial topology leads to particle-like properties promising for information technology applications. Perhaps the most important aspect of interacting particles is their ability to form thermodynamically distinct phases from gases and liquids to crystalline solids. Dilute gases of skyrmions have been realized in artificial multilayers, and solid crystalline skyrmion lattices have been observed in bulk skyrmion hosting materials. Yet, to date melting of the skyrmion lattice into a skyrmion liquid has not been reported experimentally. Through direct imaging with cryo-Lorentz transmission electron microscopy, we demonstrate that the skyrmion lattice in the material Cu$_2$OSeO$_3$ can be dynamically melted. Remarkably, we discover this melting process to be a topological defects mediated two-step transition via a theoretically hypothesized hexatic phase to the liquid phase. The existence of hexatic and liquid phases instead of a simple fading of the local magnetic moments upon thermal excitations implies that even in bulk materials skyrmions possess considerable particle nature, which is a pre-requisite for application schemes.
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Skyrmions represent topologically stable field configurations with particle-like properties. We used neutron scattering to observe the spontaneous formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortices, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice stabilizes at the border between paramagnetism and long-range helimagnetic order perpendicular to a small applied magnetic field regardless of the direction of the magnetic field relative to the atomic lattice. Our study experimentally establishes magnetic materials lacking inversion symmetry as an arena for new forms of crystalline order composed of topologically stable spin states.
Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric bulk materials is a rewarding challenge, where the released helicity / vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. We report here on the experimental observation of a skyrmion lattice (SkL) phase with large topological Hall effect and an incommensurate helical pitch as small as 2.8 nm in metallic Gd3Ru4Al12, which materializes a breathing kagome lattice of Gadolinium moments. The magnetic structure of several ordered phases, including the SkL, is determined by resonant x-ray diffraction as well as small angle neutron scattering. The SkL and helical phases are also observed directly using Lorentz transmission electron microscopy. Among several competing phases, the SkL is promoted over a low-temperature transverse conical state by thermal fluctuations in an intermediate range of magnetic fields.
We present a comprehensive analysis of high resolution neutron scattering data involving Neutron Spin Echo spectroscopy and Spherical Polarimetry which confirm the first order nature of the helical transition and reveal the existence of a new spin liquid skyrmion phase. Similar to the blue phases of liquid crystals this phase appears in a very narrow temperature range between the low temperature helical and the high temperature paramagnetic phases.
We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu2OSeO3. The magnetic phase diagram associated with the helimagnetic modulation inferred from small angle neutron scattering and magnetisation measurements includes a skyrmion lattice phase and is strongly reminiscent of MnSi, FeGe and Fe1-xCoxSi, i.e., binary isostructural siblings of Cu2OSeO3 that order helimagnetically. The temperature dependence of the specific heat of Cu2OSeO3 is characteristic of nearly critical spin fluctuations at the helimagnetic transition. This provides putative evidence for effective spin currents as the origin of enhancements of the magneto-dielectric response instead of atomic displacements considered so far.
The magnetic inhomogeneity of the A-phase in MnSi chiral magnet is identified for the first time from the precise measurements of transverse magnetoresistance (MR) anisotropy. The area inside the A-phase (A-phase core) corresponds to isotropic MR having no confinement to the MnSi crystal lattice. Per contra, the MR becomes anisotropic both on the border of the A-phase and in other magnetic phases, the strongest magnetic scattering being observed when external magnetic field applied along [001] or [00-1] directions. We argue here that the established MR features prove the presence of two different types of the skyrmion lattices inside the A-phase, and the dense skyrmion state of the A-phase core is built from individual skyrmions similar to Abrikosov-type magnetic vortexes.
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