Far infrared spectroscopy of lattice dynamics in oxides with magnetically frustrated geometry

Abstract

Magnetically frustrated materials have raised considerable research interest due to their tendency to exhibit a variety of exotic physical phenomena, such as spin-ice and spin-liquid states. In these systems, magnetic moments cannot minimise their interaction energies with all neighbouring spins simultaneously. Local ordering principles can then often take hold, stabilising a degenerate network of magnetic configurations, from which novel non-conventional magnetism emerges. Since the lattice geometry is a primary source of the frustration in these systems, studying the lattice dynamics directly is a useful way to gain insight about the environment that supports these novel frustrated spin states.The aim of my thesis is to investigate the lattice dynamics of different magnetically frustrated materials to help improving our understanding of the exotic magnetic states they support. The main experimental work was performed using a Fourier transform infrared spectrometer in reflectance mode in combination with a He-ow cryostat to cool the samples down to temperatures approaching 10 K. My investigations cover phonon spectra in the (far-) infrared regime (spectral range: 25 1/cm to 14000 1/cm) in three groups of materials.These include langasite La3Ga5SiO14 which was studied to characterise a low frequency phonon arising at 40 1/cm for E||c polarisation. In this system, theLa ions form a distorted kagome network. Frustration comes into play by replacingLa with magnetic rare-earth ions (R3Ga5SiO14 where R = rare-earth element).A softening of the low-frequency phonon with reduced temperature and increased substituted ion's atomic number is observed, indicating a possible instability of the langasite structure at low temperatures. Calculating the dielectric function highlights this low-frequency phonon as the main contribution of the langasite's large static permittivity, ~100.Francisite Cu3Bi(SeO3)2O2Cl is characterised by a structural phase transition at 115K that makes this material a rare example of a soft-mode driven antiferroelectric. My studies focused on investigating signatures of a close lying ferroelectric phase that represents a polarisation of the antiferroelectric sublattices. A polar soft mode was identified in the infrared regime, its dynamics studied and connected to the lattice dynamics. Francisite is another distorted kagome system that features frustrated Cu2+ spins. The consequences of polar soft modes in close proximity to spin wave excitations are also investigated.The pyrochlore lattice of corner-sharing tetrahedra is the prototype of a three dimensional frustrated system. Rare-earth pyrochlores, R2B2O7, are a hot topic in condensed matter research with Ho2Ti2O7, for example, shown to exhibit novel spin ice properties. On the other hand, the ground state of Tb2Ti2O7 has remained a puzzle for sometime, with many experimental signatures pointing towards a spin liquid state. Thus, I have used infrared spectroscopy to study the lattice dynamics of Tb2Ti2O7 single crystals in comparison with a Tb2Sn2O7 powder pellet and the spin ice Ho2Ti2O7 single crystal. A unique splitting of phonons can be observed inTb2Ti2O7 providing evidence of a lattice distortion that breaks fundamental symmetries related to the magnetism. This observation supports the theory that Tb2Ti2O7 features a novel vibronic spin-liquid ground state.Together, these studies highlight how the lattice and its dynamics can influence the environments that host frustrated spin systems producing unexpected behaviour.