Dr Nic Wilson

Thesis entitled Magnetic properties of the M₃V₂O₈compounds (download 37Mb)

Supervisor: OA Petrenko

Research Summary

Neutron scattering techniques available at central facilities such as ISIS, the ILL and the LLB were used alongside in-house measurements to investigate highly degenerate magnetic systems. Single crystal samples were prepared from powder using IR image furnaces.

Abstract

The Kagome lattice is a very important structure in the field of frustrated magnetism. The lattice has a two dimensional corner-sharing triangular structure, which creates macroscopic degeneracy when combined with antiferromagnetic interactions. Experimentally it is diffcult to find a perfect example of this lattice. Ni₃V₂O₈ and Co₃V₂O₈ are the first compounds found to adopt a buckled version of the Kagome lattice by their magnetic atoms, called the Kagome staircase. The staircase lattice differs from the two dimensional lattice in a number of ways: the magnetic layers have lower symmetry and the further neighbour interactions, with the resulting anisotropic exchange interactions, become more important. These factors cause the relief of geometrical frustration and establish long-range magnetic order.

Powders and large single crystals, both of very high quality, of these compounds have been prepared. Magnetisation and specific heat measurements have been used to produce the magnetic field vs temperature phase diagrams and neutron diffraction has been used to determine the different magnetic structures present in the materials.

Inelastic neutron scattering was also employed to investigate the magnetic excitations of the systems. Co₃V₂O₈ possesses a very complex phase diagram where the sequence of the phase transitions is of particular interest. The system has two main phases below 11 K: a spin density wave structure with k = (0; delta; 0), where delta varies around 1 2 above 6 K and a commensurate k = 0 ferromagnetic structure below. The narrow region between 5.8 and 6.1 K in zero applied field was found to have Bragg peaks at positions forbidden by c-centring symmetry restrictions. Diffraction measurements in an applied field uncovered a further magnetic structure not found in zero field. Multiple magnetic excitations have been observed using inelastic neutron scattering measurements on powder and single crystals. Below 5 meV, four excitations branches were observed and a further collection of at least five branches were seen between 10 meV and 68 meV in energy transfer.

The phase diagram of Ni₃V₂O₈ shows that four magnetic phases exist in this ma- terial in applied fields of up to 120 kOe. On cooling, the material is paramagnetic then enters a high temperature incommensurate phase, next is a low temperature incommensurate phase, then a commensurate antiferromagnetic phase, and finally a very similar commensurate phase where a slight increase of the magnetic moments is observed. The inelastic measurements have detected up to five excitation branches below 6 meV. They also show that the quasi-elastic scattering is Q dependent in the incommensurate phases of the material.

Current Role

Nic is currently Director of Stripey Llama Ltd.

Write to

SaaS Programme Manager
Eurotherm by Schneider Electric
Worthing, UK

Contact Details

E-Mail

nicolaruthwilson at gmail dot com

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