Magnetic ordering phenomena in the hexagonal ABX3 halides, a neutron scattering study

Most ABX3 materials (where A is a 1A group cation, B is a divalent first row transition metal cation and X is a halide ion) possess a hexagonal perovskite structure where chains of face sharing BX6 octahedra align along the c - axis separated by the A ions. This results in an interchain separation that is much larger than the intrachain separation leading to a quasi one-dimensional magnetic behaviour at low temperature. The magnetic character of the compounds is strongly dependent on the nature and strength of the superexchange pathways and the environment around the B+ ion, manipulation of which leads to new and interesting physics. The structural properties of the ABX3 materials allow them to be used as model systems for a variety of conjectures. The chain like structure allows compounds such as CsNiCl3 and RbNiCl3 to be used as model systems for the Haldane conjecture, whilst the triangular arrangement of the spins on the B2+ ions allows compounds such as CsMnBr3 to be used as model systems for the Chiral Universality class conjecture of Kawamura. This thesis is concerned with the investigation of the crystallographic and magnetic properties of some the ABX3 family of compounds by means of neutron scattering; specifically to observe the effects of extreme sample environment on the systems, such as applied hydrostatic pressure, applied magnetic and electric field etc. Magnetostructural correlations have been made for CsFeCl3 and CsFeBr3 on the basis of inelastic and powder neutron diffraction studies on the systems under pressure. Characterisation of the P and -phase of KNiCl3 have been performed using inelastic neutron diffraction measurements and the magnetic and structural phase diagrams of TlFeCl3 have also been mapped out. The magnetic phase diagram of CsNiCl3 has been studied and a proof of the existence of a magnetoelectric effect in CsMnBr3 has been obtained from elastic neutron scattering experiments