Theoretical Investigation Of Magnetism,Spin Transport And Dynamic Properties In Magnetic Materials

In this thesis,we study the magnon transport in the magnetic heterostructure composed of ferromagnetic insulator/antiferromagnetic insulator/ferromagnetic insulator/antiferromagnetic insulator/ferromagnetic insulator,the spin transport and dynamic properties of two-dimensional spin-momentum locked states,and the effect of magnetic disorder and spin current injection on the two-dimensional magnetism as follows:(1)We predict the magnon resonant tunneling effect in the magnon double-barrier tunnel junctions of FI/AFI/FI/AFI/FI,where FI and AFI denote the ferromagnetic and antiferromagnetic insulating layers,respectively.Similar to the electron tunneling in the double-barrier tunnel junctions,each AFI layer acts as an effective potential barrier for magnons owing to its intrinsically large magnon gap of the order of THz.Based on the magnon tunneling effect,we further propose a novel magnon field effect transistor which is capable to realize a gate-tunable transmitted magnon flow by tuning the resonant tunneling via a gate electric field induced Dzyaloshinskii-Moriya interaction in the middle FI layer.The advantages of such transistors include the low dissipation without Joule heating loss and operating at room temperature.(2)We theoretically investigate the spin transport and dynamic properties of twodimensional spin-momentum locked states.To characterize their non-equilibrium spin transport and dynamic properties in the presence of a time-dependent external magnetic field and a spin injection,we introduce three macroscopic variables: a vectorial helical magnetization,a scalar helical magnetization,and the conventional magnetization.We derive a set of closed dynamic equations for these variables by using the spinor Boltzmann equation.By solving the dynamic equations,we predict several intriguing magnetic and spin transport phenomena,including magnetic resonant response to an AC applied magnetic field,charge-spin conversion,and spin current pumped by the dynamics of helical magnetizations.(3)We study the effect of magnetic disorder and spin current injection on the twodimensional magnetism,where the disorder is modelled by an uniaxial random field along the magnetization direction.We find that the magnetic disorder decreases the Curie temperature via reducing the magnon gap in second-order perturbation,and the phase transition evolves from second order to first order in the presence of magnetic disorder,where the discontinuity of magnetization at the phase transition point is proportional to the strength of magnetic disorder.Furthermore,we investigate the two-dimensional magnetism in the presence of spin current injection.We show that the injected spin current whose polarization is parallel to the two-dimensional magnetization can effectively increase the Curie temperature through suppressing the thermal fluctuation of magnetization,while the injected spin current whose polarization is antiparallel to the magnetization enhances the thermal fluctuation of magnetization and switches the magnetization when the spin current injection is large enough.In contrast to the case of three dimension,we show that the critical switching spin current density is highly temperature-dependent due to the more remarkable thermal fluctuation of magnetization in two dimension.In addition,we find that there can exist the magnetic order along the polarization of the injected spin current for two-dimensional isotropic magnets,i.e.,the Mermin-Wagner theorem becomes invalid in the presence of the spin current injection.The reason is that the spin current injection introduces an effective gap to the magnon spectrum and thereby results in the convergence of Goldstone-mode magnon number.