Novel Phases and Dynamical Phenomena in Frustrated Magnets

Frustrated magnets are a class of substances in which exchange interactions between neighboring spins (magnetic moments) cannot be simultaneously satisfied, leading to an extensive degeneracy in the ground state manifold. Frustration gives rise to many exotic phenomena, a prominent one being classical spin liquids (CSLs).

In this thesis, we present an extensive numerical study on finding a new classical spin liquid in which the collective flux degrees of freedom break the translation symmetry of the honeycomb lattice. This exotic phase exists in frustrated spin-orbit magnets where a dominant off-diagonal exchange, called Gamma term, results in a macroscopic ground-state degeneracy at the classical level. We demonstrate that the system undergoes a phase transition corresponding to plaquette ordering of hexagonal fluxes, driven by thermal order-by-disorder at a critical temperature TC ~ 0.04. We perform extensive Monte Carlo simulations and finite-size analysis to investigate the nature of the plaquette-ordering transition. We also study the dynamical behavior of fluxes and the influence of other types of interactions on the phase transition.

Next, we investigate the spin dynamics of a classical Heisenberg antiferromagnet with nearest-neighbor interactions on a quasi-two-dimensional kagome bilayer. This geometrically frustrated lattice consists of two kagome layers connected by a triangular-lattice layer. We combine Monte Carlo method with precessional spin dynamics simulations to compute the dynamical structure factor of the classical spin liquid and study the thermal and dilution effects. The low frequency and long wavelength dynamics of the classical spin liquid in kagome bilayer is dominated by spin diffusion. We discuss the implications of our work for the glassy behaviors observed in the frustrated magnet SrCr9pGa12-9pO19 (SCGO).