Magnetism and transport in nanostructured domain wall systems

  • Hywel Roberts

Student thesis: Doctoral ThesisPhD

Abstract

The precise control of ferromagnetic domain wall formation opens up exciting avenues of research and potential application in spintronics — the manipulation of charge carriers via their spin properties. Recent experiments on Cobalt-Platinum multilayers containing artificially created domains provide the motivation for this work. In this thesis the electronic structure of CoPt multilayers are calculated by an ab initio multiple scattering method, and attempts are made at replicating the systems used in experiments, including lattice relaxations and the effects of substitutional alloying. The magnetic reversal process in Pt/Co/Pt trilayer systems is studied in the framework of micromagnetics, in which effects such as exchange, magnetocrystalline anisotropy and the demagnetising field are treated phenomenologically. The results are compared to recent experiments and the switching mechanism can be understood in terms of domain growth and domain wall nucleation. A ballistic transport framework is outlined in terms of a tight binding Green function method. The domain wall is modelled as a change in the local spin reference frame. The method is applied to Cobalt Platinum trilayers, and it is found that the resulting domain wall resistances are sensitive to the details of the Fermi energy bands. Subsequently, the angular dependence of domain wall resistivity in Pt/Co/Pt systems is studied by a model based on the anisotropic resistivity tensor that is expected in a domain wall. The results are used to extract resistivity parameters from experimental data, and a positive domain wall resistivity is identified, whilst resulting arguments provide supporting evidence for the Levy-Zhang theory of domain wall resistance. Finally, recent experiments on the dilute magnetic semiconductor (Ga,Mn)As have provided evidence for a negative intrinsic domain wall resistance in this material. By applying a similar model to that used on the CoPt systems, it is shown that the anomalous magnetoresistance effect can also provide a significant negative contribution to the measured resistance via induced eddy current at the domain wall boundary.
Date of Award1 May 2008
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorSimon Crampin (Supervisor)

Keywords

  • ferromagnetism
  • domain walls

Cite this

Magnetism and transport in nanostructured domain wall systems
Roberts, H. (Author). 1 May 2008

Student thesis: Doctoral ThesisPhD