Interaction of spin waves with magnetic textures

In this thesis, we present a theoretical study on the scattering of spin waves by magnetic textures, such as domain walls (DWs), in magnetic materials. In the case of an antiferromagnet, we show that, based on the discrete spin Hamiltonian, spin waves are generally reflected by a DW with a reflection coefficient that increases as the DW width decreases. In the interesting case of an atomically sharp DW, the reflection of spin waves exhibits strong dependence on the spin wave's polarization, leading to mainly reflection for one polarization while permitting partial transmission for the other, thus realizing an atomic-scale spin-wave polarizer. In addition, we theoretically investigate magnon heat transport in an antiferromagnetic insulator containing a DW in the presence of a magnetic field applied along the easy axis. We show that the intrinsic spin of the DW couples to the external magnetic field, which modifies the transmission of spin wave through the DW. Applying the magnetic field lifts the degeneracy between two AFM magnon modes and results in different occupation numbers for the two magnon modes. Combined with the finite reflection of a narrow DW, this gives rise to magnon thermal magnetoresistance. In the case of a ferrimagnets, we find that a narrow but atomically smooth FiM DW exhibits a different behavior in comparison with similarly smooth ferromagnetic and antiferromagnetic DWs due to the inequivalence of the two sublattices. Specifically, for sufficiently weak anisotropy, the smaller spin at the center of the DW is found to become precisely normal to the easy axis, selecting an arbitrary direction in the xy plane and thereby breaking the U(1) spin-rotational symmetry spontaneously, which leads to a strong dependence of spin-wave scattering pattern on the spin wave's polarization. Moreover, we find that in the case of an atomically sharp DW, where all the spins point either up or down due to the strong easy-axis anisotropy, the wave vector of the spin-wave changes after passing through the DW, leading to a change in the group velocity of the spin wave, consequently the acceleration or deceleration of the spin waves. Finally, we investigate the interaction of spin wave with a twisted two-dimensional DW in a ferromagnetic spin chain. We demonstrate that a twisted DW gives rise to an effective gauge field, leading to the deflection of magnons. In addition, we show that, in the presence of the in-plane anisotropy, the spin wave Hamiltonian takes a periodic form along the y--direction. This gives rise to multiple diffracted waves on both sides of the DW.