Title: The implications of geometric frustration and orbital degeneracies on the evolution of magnetism in Na4Ir3O8 and α-NaMnO2
Speaker: Rebecca Dally, NIST
Spin-orbit intertwined order gives rise to many novel phenomena with a broad phase space spanned by the competing energy scales within a system. This work 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 = ½ 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.
The second spin-orbit intertwined system, α-NaMnO2, undergoes a cooperative Jahn-Teller distortion of the MnO6 octahedra arising from an orbital degeneracy in the Mn3+ cations, and 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.