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Register a new userThermally Driven Topology in Chiral Magnets
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Chiral magnets give rise to the anti-symmetric Dzyaloshinskii-Moriya (DM) interaction, which induces topological nontrivial textures such as magnetic skyrmions. The topology is characterized by integer values of the topological charge. In this work, we performed the Monte-Carlo calculation of a two-dimensional model of the chiral magnet. A surprising upturn of the topological charge is identified at high fields and high temperatures. This upturn is closely related to thermal fluctuations at the atomic scale, and is explained by a simple physical picture based on triangulation of the lattice. This emergent topology is also explained by a field-theoretic analysis using $CP^{1}$ formalism.
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Non-trivial topology in a two-dimensional frustrated spin system with the Dzyaloshinskii-Moriya (DM) interaction was investigated by Monto Carlo simulations. At finite temperatures, thermally driven topology was discovered and was found to be dominant at low magnetic field. This topological charge has a quadratic relation with the DM interaction and linear realtions with the external magnetic field or the uniaxial magnetic anisotropy. We also proposed a real frustrated system, the Mn-Bi mono-layer film with exceedingly large DM interaction, to enable thermally driven topology. Other topological non-trivial phases in high magnetic field region were also discussed in this real system.
In chiral magnets a magnetic helix forms where the magnetization winds around a propagation vector $mathbf{q}$. We show theoretically that a magnetic field $mathbf{B}_{perp}(t) perp mathbf{q}$, which is spatially homogeneous but oscillating in time, induces a net rotation of the texture around $mathbf{q}$. This rotation is reminiscent of the motion of an Archimedean screw and is equivalent to a translation with velocity $v_{text{screw}}$ parallel to $mathbf{q}$. Due to the coupling to a Goldstone mode, this non-linear effect arises for arbitrarily weak $mathbf{B}_{perp}(t) $ with $v_{text{screw}} propto |mathbf{B}_{perp}|^2$ as long as pinning by disorder is absent. The effect is resonantly enhanced when internal modes of the helix are excited and the sign of $v_{text{screw}}$ can be controlled either by changing the frequency or the polarization of $mathbf{B}_{perp}(t)$. The Archimedean screw can be used to transport spin and charge and thus the screwing motion is predicted to induce a voltage parallel to $mathbf{q}$. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing $mathbf{B}_{perp}$ forming a `time quasicrystal which oscillates in space and time for moderately strong drive.
Superpositions of spin helices can yield topological spin textures, such as two-dimensional vortices and skyrmions, and three-dimensional hedgehogs. Their topological nature and spatial dimensionality depend on the number and relative directions of the constituent helices. This allows mutual transformation between the topological spin textures by controlling the spatial anisotropy. Here we theoretically study the effect of anisotropy in the magnetic interactions for an effective spin model for chiral magnetic metals. By variational calculations for both cases with triple and quadruple superpositions, we find that the hedgehog lattices, which are stable in the isotropic case, are deformed by the anisotropy, and eventually changed into other spin textures with reduced dimension, such as helices and vortices. We also clarify the changes of topological properties by tracing the real-space positions of magnetic monopoles and antimonopoles as well as the emergent magnetic field generated by the noncoplanar spin textures. Our results suggest possible control of the topological spin textures, e.g., by uniaxial pressure and chemical substitution in chiral materials.
We show that the stability (existence/absence) and interaction (repulsion/attraction) of chiral solitons in monoaxial chiral magnets can be varied by tilting the direction of magnetic field. We, thereby, elucidate that the condensation of attractive chiral solitons causes the discontinuous phase transition predicted by a mean field calculation. Furthermore we theoretically demonstrate that the metastable field-polarized-state destabilizes through the surface instability, which is equivalent to the vanishing surface barrier for penetration of the solitons. We experimentally measure the magnetoresistance (MR) of micrometer-sized samples in the tilted fields in demagnetization-free configuration. We corroborate the scenario that hysteresis in MR is a sign for existence of the solitons, through agreement between our theory and experiments.
We propose a mechanism to pin skyrmions in chiral magnets by introducing local maximum of magnetic exchange strength, which can be realized in chiral magnetic thin films by engineering the local density of itinerate electrons. Thus we find a way to artificially control the position of a single skyrmion in chiral magnetic thin films. The stationary properties and the dynamical pinning and depinning processes of an isolated skyrmion around a pinning center are studied. We do a series of simulations to show that the critical current to depin a skyrmion has linearly dependence on the pinning strength. We also estimate the critical current to have order of magnitude 10^{7}sim10^{8}A/m^{2} .
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