The Verwey transition : neutron scattering studies of the vibrational and magnetic excitations in magnetite

The Verwey transition in magnetite is a remarkable and challenging phenomenon which is still not understood despite many years of intense research. When the temperature is lowered below the transition temperature (Tv = 121 K) there is a sudden drop in the electrical conductivity and the structure- changes from cubic above to monoclinic below. The importance of this phase transition goes further than just this simple material. Magnetite stands as a model system in many areas of the physical sciences, but is probably most known for its use in the theory of 'metal-insulator' transitions, that are known to occur in many of the transition metal oxides. For a long time it was thought that the mechanism responsible for this transition was ionic charge ordering of the iron atoms. This model has since been proven to be incorrect and new models with partial charge ordering have largely taken its place. This work is concerned with using the neutron as a probe to study both the vibrational and magnetic excitations within magnetite and offers insight into the lattice dynamics around the phase transition. The transverse acoustic phonons have been intensively studied and interesting broadening effects observed. Furthermore the entire spin wave spectrum has been observed, with the top mode (known as (1)2) and a direction other than [1 0 0] reported for the ftrst time. From the work presented here it is suggested that the structural phase transition in magnetite is better understood using a band type model where both phases of magnetite (above and below) are in fact semiconducting. The broadening effects of the phonons are described by the presence of an anharmonic potential above the transition temperature and the sudden drop in the electrical conductivity is explained by the widening of the band gap below the transition.EThOS - Electronic Theses Online ServiceGBUnited Kingdo