The quasi-one-dimensional magnets AFeX3 have aroused much interest in recent years
due to their unique magnetic properties. It has been shown that the magnetic
excitations in linear chain systems with half-integer and integer spin value differ
fundamentally. Haldane conjectured that an energy gap will arise in linear chain
systems with integer spin. The magnetic systems studied in this thesis: CsFeBr3 and
CsFeCl3 are linear chain compounds which have a singlet ground state and are
described by an effective spin S=1 formalism. [Continues.]
A Master's Thesis. Submitted in partial fulfilment of the requirements for the award of Master of Philosophy at Loughborough University.