Identification and investigation of new low-dimensional quantum spin systems

This thesis focuses on one area of modern condensed matter physics, namely low-dimensional magnetism, and more specifically one-dimensional linear chains. The work herein can be split into three parts. The first part provides a tool for the greater community. I herein propose a Pad´e approximation for the temperature dependent magnetic susceptibility of a S = 3/2 spin chain, that is more accurate than those already known. The approximation allows one to fit experimentally measured magnetic susceptibilities and ascertain values such as the near-neighbour spin exchange interaction and the g-factor. The second and third parts of this thesis are both concerned with experimentally and theoretically characterizing two isostructural linear S = 1/2 chain compounds on opposite ends of the 3d transition metal series. The compounds, CuCrO4 (3d9) and TiPO4 (3d1), are shown to have completely different ground states despite both being largely isostructural and S = 1/2 quantum spin systems. In this work and the resulting publications it is shown that CuCrO4 is a one-dimensional S = 1/2 spin chain with anti-ferromagnetic nearest- and next nearest-neighbour spin exchange interactions. The ratio of these spin exchange interactions is shown experimentally and theoretically to be approximately 2, putting CuCrO4 in the vicinity of the Majumdar-Ghosh point, for which the magnetic ground-state can ii be solved analytically. Small ferromagnetic inter-chain coupling leads to long-range ferromagnetic ordering between anti-ferromagnetic chains at 8.2(2) K. At this temperature a spontaneous electrical polarization is observed. This classifies CuCrO4 as a type-II multiferroic. Contrary to CuCrO4, TiPO4 has a non-magnetic ground state. At 111 and 74 K TiPO4 undergoes a two stage phase transition, which is interpreted as a spin-Peierls transition. There is evidence that below 74 K TiPO4 has a new crystal structure, in which there are alternating dimerised chains and two different PO4 tetrahedral units. Currently the new structure has not been identified and no super-structure reflections have been confirmed in either low-temperature neutron or x-ray diffraction. In summary this thesis presents some experimental and theoretical contributions to the field of low-dimensional magnetism.