The implications of geometric frustration and orbital degeneracies on the evolution of magnetism in Na4Ir3O8 and α-NaMnO2

Thesis advisor: Stephen D. WilsonSpin-orbit intertwined order gives rise to many novel phenomena with a broad phase space spanned by the competing energy scales within a system. This dissertation synthesized and studied two such systems demonstrating different manifestations of spin-orbit interactions, originating from orbital degeneracy effects, on geometrically frustrated magnetic lattices. Firstly, strong spin-orbit coupling in the hyperkagome lattice, Na4Ir3O8, and secondly, the layered material, α-NaMnO2, where single-ion anisotropy and a cooperative Jahn-Teller distortion drive magnetism to the quasi-1D limit. The magnetic ground state of the Jeff = 1/2 spin-liquid candidate, Na4Ir3O8, is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen, state comprised of quasi-static moments develops below a characteristic temperature of TF = 6 K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasi-static, short-range, spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Much of this dissertation focuses on the second spin-orbit intertwined system, α-NaMnO2, where a cooperative Jahn-Teller distortion of the MnO6 octahedra arising from an orbital degeneracy in the Mn3+ cations directly affects the electronic (ferro-orbital) and magnetic (antiferromagnetic) order, which results in an intriguing study of low-dimensional magnetism. Intricacies of the structure, static magnetic order, and magnon dynamics are presented, which heavily relied on neutron scattering techniques. In particular, a longitudinally polarized bound magnon mode is characterized through the use of polarized neutron scattering.Thesis (PhD) — Boston College, 2018.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics