Mechanical Control On Topological Magnetic Structure In Ferromagnetic Materials

Because of their intriguing multi-filed coupling properties,ferromagnets are widely employed in different functional devices such as sensors and memories.The topological feature of magnetic structure has a great influence on the physical properties of ferromagnetic materials.In this thesis,a real-space phase field model is employed to investigate two kinds of topological magnetic structures,i.e.magnetic vortex and magnetic skyrmions,in ferromagnetic materials subjected to different mechanical loads.Firstly,by using phase field simulation,this thesis investigates the evolution of magnetic vortex and its influence on the fracture behavior in cracked ferromagnetic materials.The calculation results show that an external stress will transform the initial two-vortices state into six-vortices structure and hence reduce the stress intensity factor of the crack tip.Secondly,Dzyaloshinskii-Monya interaction and spin-transfer torque are introduced into the phase filed model,which can be used to investigate the effect of uniaxial strain on the magnetic order in chiral magnets.The simulation results demonstrate that external strain in the ferromagnetic thin films can not only change the size and configuration of the magnetization structure but also induce novel topological phase transitions of skyrmions in chiral magnets.Moreover,these kinds of topological phase transitions in chiral magnets also affect the apparent elastic constants of the materials.Finally,the effect of interface distortion on skyrmions is investigated in ferroelectric-ferromagnetic composite thin films.The phase field simulations show that a spontaneous magnetic skyrmions lattice state can be obtained in ferromagnetic layer through interface distortion induced by ferroelectric skyrmions in ferroelectric layer.The present work provides a magnetoelastic coupling mechanism on the mechanical control of topological magnetic structures and suggests an effective way to tune the electromagnetic properties of ferromagnetic thin films for a new generation of spintronic devices based on topological magnetic structure.