Novel spin dynamics of noncollinear antiferromagnetic systems

Abstract

This dissertation studies novel spin dynamics in two classes of complex magnetic systems: noncollinear antiferromagnets (AFMs) and patterned magnonic crystals. In the first chapter, we provide some important background physics in a pedagogical manner, including details on continuum models of magnetism, the Holstein-Primakoff transformation, and spin currents. In Chapter 2 we study the conserved Noether spin current arising from spin-rotation symmetry in noncollinear AFMs. We find that a Hall component of the spin current can be generically created by a longitudinal driving force associated with a propagating spin wave, inherently distinguishing noncollinear AFMs from collinear ones. The corresponding Hall coefficient, which we coin the Hall mass, is an isotropic scalar that generally exists in noncollinear AFMs and their polycrystals. We demonstrate the resulting Hall spin current numerically in FM/noncollinear AFM bilayer structures based on Kagome lattice antiferromagnets Mn3Sn and Mn3Ir via spin pumping, and give the criteria for ideal boundary conditions at the interface. These results shed light on the potential of noncollinear AFMs for manipulating the polarization and flow of spin currents in spintronic devices. We then extend this framework in Chapter 3 by developing a generalized coarse-grained Lagrangian for noncollinear AFMs with an arbitrary number of sublattices. The generalized formalism includes terms second order in the sublattice canting and rotation gradients, leading to additional dynamical terms compared to the three-sublattice theory. We test the coarse-graining formalism by studying the spin-wave modes in CoX3S6 (X = Nb, Ta), where the magnetic state forms an all-in-all-out tetrahedral order. The spin-wave spectrum derived form the coarse-grained Lagrangian is compared to linear spin-wave theory via the Holstein-Primakoff transformation, showing good agreement in the long wavelength approximation. However, limitations of the coarse-graining procedure appear when considering complex spin-wave modes not characterized by uniform rotations or canting, such as when spins in different layers rotate out of phase. Finally, we present analytical calculations and micromagnetic simulations of a patterned yttrium iron garnet (YIG) film with a periodic array of triangular holes, motivated by experimental efforts to realize topological magnon edge modes in this geometry. The analytical model confirms that rotation of the triangular holes opens a band gap at the K point, similar to including a staggered on-site potential in graphene models. Micromagnetic simulations of a finite sample reveal chiral spin-wave modes appearing within a bulk band gap, with opposite chirality at the top and bottom sample edges that reverses upon flipping the applied field. However, the chiral modes are not strongly localized to the sample boundaries in our simulations, making definitive identification of their topological character difficult. The simulated chiral modes therefore give suggestive evidence of topological edge modes and provide concrete guidelines for experimental detection. Together, these works contribute to a broader understanding of spin dynamics in magnetically complex systems. In both noncollinear AFMs and patterned magnonic crystals, the geometry and symmetry of the magnetic structure can give rise to novel spin transport and dynamical phenomena, offering new opportunities for controlling spin currents and spin-wave propagation in next-generation magnetic devices.