Manipulation Of Magnetic Domains In A Kagome-lattice Magnet Fe₃Sn₂

Skyrmions are localized vortex-like spin textures with the advantages of nanometric size and easy dynamic manipulations by electrical methods.Recently,skyrmions are promising information carriers applied in building spintronic devices and have attracted much attention.In non-central symmetry chiral magnets,skyrmions can be stabilized by Dzyaloshinskii-Moriya interaction.In central symmetry uniaxial magnets,skyrmions are stabilized by uniaxial magnetic anisotropies.In the traditional achiral uniaxial magnets,there are two types of bubbles,i.e.type-I topologically non-trivial and type-II topologically trivial bubbles.Among them,the type-I bubbles share the same topology as chiral skyrmions.Thus,type-I bubbles are renamed skyrmions.The sizes(～50 nm to sub-millimeter)of skyrmions and bubbles in achiral uniaxial magnets are highly tunable.Besides,skyrmions and bubbles in achiral uniaxial magnets have excellent thermal stabilities at room temperature.In combination with modern electrical methods with achiral skyrmions and bubbles,we expect to develop distinct spintronic devices.To achieve this purpose,we used a Lorentz transmission electron microscopy(Lorentz-TEM),a focused-ion beam and scanning electron microscopy dual beam system,and a micromagnetic simulation approach to study magnetic domains in confined Kagome-latticed Fe3Sn2 systems,a typical skyrmion-bubble-hosting kagome-lattice magnet.We mainly focus on the stabilizations,topological magnetic transformations.and current-induced dynamics of skyrmions and bubbles in the centrosymmetric uniaxial magnetic confined structures.The main results of this thesis are as follows:1.Fe3Sn2 has a high Curie temperature(～650 K)and a temperature-induced spin reorientation transition at～100 K.Easy-plane and uniaxial magnetocrystalline anisotropies dominate at low and high temperatures,respectively.At room temperature,the quality factor η of Fe3Sn2 is smaller than 1.Because of the temperature-driven spin reorientation in Fe3Sn2,we realized temperature-induced transformation between soft magnetic vortex and target bubbles at zero magnetic fields.Further,we also studied the field-driven core reversals of magnetic vortices in Fe3Sn2 disks and observed a skyrmion-like spin texture during the vortex core reversal.Because of the weak quality factor of Fe3Sn2 at room temperature,we obtained depth-induced spin twistings of skyrmions and bubbles in combination with differential phase contrast of Lorentz-TEM and three-dimensional micromagnetic simulations.We thus well explained the complex multi-ring and arc-shaped vortices in Fe3Sn2 by three-dimensional skyrmions and bubbles.2.Skyrmion and bubbles in the centrosymmetric uniaxial kagome-lattice magnet Fe3Sn2 are entirely different localized magnetic objects,which naturally could be taken as binary bits "1" and "0" in magnetic memories.We proposed a skyrmionbubble-based memory,which uses a single skyrmion-bubble chain to represent a data bitstream.We further experimentally realized a single skyrmion-bubble chain in Fe3Sn2 nanostripe,proving the feasibility of skyrmion-bubble-based memory.Because of the skyrmion interaction and easy motions of skyrmions in ferromagnetic background magnetizations,pinning sites such as notch-like defects must be fabricated in skyrmion-based memories to avoid the unexpected motion of skyrmions.However,the interaction between skyrmions and bubbles could suppress unexpected motion data bits to avoid the additional pinning sites in skyrmionbubble-based memories.3.We studied the stabilization of skyrmions and bubbles in confined Fe3Sn2 disks.Because of different stabilization mechanisms,the maximum skyrmion and bubble counts are quadratic and linear dependent on disk size,respectively.A single skyrmion and a single bubble at most can be obtained in a～540-nm Fe3Sn2 disk.By tuning the in-plane magnetic field,we further demonstrated the controlled single skyrmion-bubble topological transformations in the～540-nm Fe3Sn2 disk.4.We studied the current-induced dynamics of achiral skyrmions in uniaxial ferromagnetic nanostripes using micromagnetic simulations.In uniaxial ferromagnets with weak quality factors,the skyrmion prefers to stay at the central location of the nanostripe due to the dipole-dipole interaction.The skyrmion moves away from the central location when the spin-polarized current is on and moves back to the initial central location when the current is off.Thus,the current-driven dynamics of achiral skyrmions in magnets with weak quality factors could mimic the leakage-integrate-fire behavior of spiking neurons.In uniaxial ferromagnets with strongly enhanced quality factors,we obtained the current-driven coherent motion of achiral skyrmions,which is applicable for skyrmion-based racetrack memory applications.