Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Constable, Evan, Terahertz spectroscopy of magnetic metal oxides, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2015. https://ro.uow.edu.au/theses/4359
The optical excitation and control of spins in magnetic materials promises new avenues for devices that couple photonic and spintronic functionality, with the prospects of a new paradigm in information processing. An understanding of the behaviour of the magnetic interactions in candidate materials is essential to the design of devices that exhibit these desirable properties. Terahertz radiation presents an ideal medium for the study of spin dynamics in magnetic materials. Its low energy in the meV range is equivalent to the many weak magnetic coupling mechanisms present. Terahertz radiation is therefore also complementary to established methods that probe these mechanisms such as inelastic neutron scattering and magnetometry. The electromagnetic nature of terahertz radiation means that the magnetic field component can couple directly to the ordered spin moments. Furthermore, the electric field component may interact with lattice coupled magneto-electric excitations. The metal oxides, which combine magnetic super-exchange, strongly correlated electrons and low symmetry crystalline environments, are good candidates for technological applications invoking these interactions. This is because they often feature exploitable properties such as antiferromagnetism, ferroelectricity, semiconducting behaviour and magneto-electric coupling. They are also typically insulating allowing for terahertz transmission investigations. In this thesis terahertz transmission spectroscopy is performed on a number of magnetic metal oxides as well as a non-magnetic semiconductor. Specifically, a non-magnetic semiconductor (ZnTe); a geometrically frustrated metamagnet (Cu3Bi(SeO3)2O2Cl); a canted antiferromagnet (NdFeO3) and a quantum spin ladder system (Sr14Cu24O41) are investigated. Collectively, the work establishes terahertz radiation as an e↵ective probe of material properties. The results reveal a diverse range of magnetic excitations, characterising their temperature and external magnetic field dynamics. The physical interactions probed are commented on regarding their relevance to the emerging field of spintronics.