Probing defect and magnetic structures on the nanoscale

  • Alexis Kallis

Student thesis: Doctoral ThesisPhD


This thesis reports on experimental research on structural defects and magnetic species on the nanoscale. The latter project involved considerable development work on the production of a spin-polarised mono-energetic positron beam. The construction of the system is described through various trial steps with emphasis on the methods of maximum practical polarization of the positron beam and of electrons in the sample with the smallest possible loss of beam intensity. A new sodium-22 source capsule was purchased, having beryllium foil backing to minimise the depolarisation effects of backscattering, and the source-moderator spacing was increased. Different types of sample were tested, varying in atomic structure, purity, magnetic susceptibility, electronic structure, and electric conductivity - including iron of different purity and structure, mu metal and solid oxygen. After these tests measurements were taken on single crystal iron, and the results suggest that the positron response to magnetic structures is very small, and that prospects for depth profiling of dilute magnetic systems are not favourable at this time.A large number of other investigations have been performed on non-magnetic defect structures in various materials. Variable Energy Positron Annihilation Spectroscopy – here involving beam-based Doppler broadening – was applied to novel materials of relevance to photonic or electronic structures on the nanoscale. These included thin films of technological interest such as AlGaN and Ar plasma-treated TiO2: silicon and silicon-on-insulator samples implanted with He and Si ions to engineer vacancies: Si-rich SiO2 and SiN to form nanocrystals for photonic applications in which new findings on the evolution of the nanocrystals, and the role of the nanocrystal-oxide interface in optical emission, could be very useful in the technological development of such systems: and a study of the structural phase and nano-pore properties of water ice films grown from vapour on a cold copper surface.The variety of these experimental studies serves to underline the wide applicability of positron beam spectroscopy in research on defect and nanostructure structures. A list of papers published to date resulting from this work is given at the end of the thesis; a number of others are planned.
Date of Award1 Sept 2010
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorPaul Coleman (Supervisor)


  • positron annihilation
  • defects
  • nanostructures

Cite this