The Electro-magneto-elastic Coupling Properties Of Topological Magnetic Structures

Topological magnetic structures such as domain wall,vortices and skyrmions have considerable potential applications in advanced functional devices,due to their unique electro-magneto-elastic coupling properties.Based on Landau phase transition theory,this thesis proposes a real-space phase field model for multiferroic heterostructures,which includes the spin-orbit torque and spin-transfer torque induced by current.A nonlinear finite element method has been derived to solve the governing equations of the phase field model,which makes this method more efficient in the complicated structures.By using the phase field model,the following multi-physics coupling properties of the topological magnetic structures have been investigated.Firstly,we study the mechanical controlled properties of magnetic vortices in ferromagnetic nanowires.Because of the strain relaxation effect,the misfit strain due to the mismatch of lattices between nanowire and substrate will change with the size and shape of specimens.We simulate the magnetic domain distributions in different sizes of ferromagnetic nanowires,which have different strain distributions.The simulation results show that magnetic vortices are only stable in specific strain distribution areas,which is consistent with the experimental results.Secondly,we study the control of electric and magnetic fields on the properties of magnetic skyrmions in multiferroic heterostructures by phase field model.The results show that the magnetic skyrmions are stable without external field due to the nonuniform strain distribution,which is generated by the spontaneous electric polarizations of ferroelectric part via electrostrictive effect.Besides,the skyrmions can be changed to the helical phase by changing the distributions of polarizations via external electric field or by current.Lastly,we use the phase field model to study the switching-induced changes of fracture behaviors by topological domain structures in the multiferroic composites.We calculate the stress intensity factors(SIFs)of specimens with different cracks and external loads.And we compare the changes of domain structures with the SIFs curves.We demonstrate the domain switching will toughen or weaken the cracks in the multiferroic composites depending on the loading types.Based on the present thesis,we extended the multiferroic phase field method for multiferroic heterostructures.It provides the theoretical basis of the research of multi-physics properties in topological magnetic structures.