Novel Properties And Manipulation Of Magnetic Skyrmions

The discovery of magnetic Skyrmions in chiral magnetic materials provides great opportunities. On one hand, we may use Skyrmions as information carriers in future spintronic devices. For this purpose, we need to invent precise ways to nucleate, move and excite the Skyrmions by various means. On the other hand, the appearance of Skyrmions in condensed matter systems gives us a particularly convenient playground to test our most fundamental understandings of the physical world, and this line of thinking follows from the original ideas of Tony Skyrme. In the past few years, great progress has been made for both aspects. In this dissertation, we first give an intro-duction to the Skyrmion physics in chapter1, and then present our theoretical studies according to the following outline:In chapter2, the continuum limit of the tilted Heisenberg spin model is shown to lead to a gauge Landau-Lifshitz equation, which provides a unified description for vari-ous spin orders relevant to chiral magnets. For a definite gauge with zero field strength, we find a double periodic solution, where the conical spiral, in-plane spiral, helical, and ferromagnetic spin orders become special cases. For another gauge with nonzero field strength, we obtain the Skyrmion crystal solution. By simulating the influence of magnetic field and temperature for our model, we find a spontaneous formation of a Skyrmion-fragment lattice similar to the spin texture found in the experiment on monolayer Fe; we also obtain a wider range of Skyrmion crystal phase in the parameter space in comparison to the conventional Dzyaloshinski-Moriya model.In chapter3, we propose a mechanism to pin Skyrmions in chiral magnetic thin films by introducing local maxima of magnetic exchange strength as pinning centers. The local maxima can be realized by engineering the local density of itinerant elec-trons. The stationary properties and the dynamical pinning and depinning processes of an isolated Skyrmion around a pinning center are studied. We carry out numerical simulations of the Landau-Lifshitz equation and find a way to control the position of an isolated Skyrmion in a pinning center lattice using electric current pulses. The results are verified by a Thielc equation analysis. We also find that the critical current to depin a Skyrmion, which is estimated to have order of magnitude107-108A-m-2, has linear dependence on the pinning strength.In chapter4, we present the theoretical examination of the magneto-electric re-sponse in the recently discovered multiferroic insulator Cu2OSeO3It is shown that the position-dependent electric field, pointed along the direction of the average induced dipole moment of the Skyrmion, can induce the Hall motion of the Skyrmion with its velocity orthogonal to the field gradient. Finite Gilbert damping produces longitudinal motion. Inter-Skyrmion interaction leads to the reduction of the drift speed. We find a rich variety of resonance modes excited by an ac electric field. On the other hand, by combining Monte Carlo simulation and Ginzburg-Landau analysis, we calculated the thermodynamic magneto-electric susceptibility of Cu2OSeO3in each of the magnetic phases, including helical, conical, ferromagnetic and Skyrmion crystal phases, both for thin film and bulk systems. A common feature for all non-collinear phases is that the magneto-electric susceptibility increases linearly with the applied magnetic field. Be-ing both calculable and measurable, the magneto-electric susceptibility can serve as a new powerful probe to detect and investigate magnetic phases and phase transitions in multiferroic chiral magnets in general.