The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Ferromagnetism in (Ga,Mn)As and (Ga,Mn)N

Hannes Raebiger

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium E at Helsinki University of Technology (Espoo, Finland) on the 21st of April, 2006, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-8122-8)   [956 KB]
Dissertation is also available in print (ISBN 951-22-8121-X)


(Ga,Mn)As and (Ga,Mn)N are so called diluted magnetic semiconductors, i.e. semiconductor based materials made ferromagnetic by inclusion of a magnetic element—in this case Mn. This type of materials bridge over the incompatibilities in metal–semiconductor interfaces in electronics components and have an enormous potential for future spintronics applications, where both charge and spin degrees of freedom can be employed simultaneously. In order to design new—or employ the existing—diluted magnetic semiconductor materials, the underlying mechanisms of magnetism must be understood. In this work a theoretical study of the structural and magnetic properties of the two most important prototype materials, (Ga,Mn)As and (Ga,Mn)N, is presented.

Ferromagnetism arises from the quantum mechanical exchange interactions, but is by its very nature a macroscopic ordering effect. Therefore a multiscale approach is employed, beginning from quantum mechanical interactions. Both microscopic configurational energies and corresponding magnetic interactions are calculated from first principles. These energies are used in Monte Carlo simulations to study macroscopic and finite temperature properties. Curie temperatures are estimated using the Weiss molecular field theory, as well as a more sophisticated Monte Carlo approach.

We show that both (Ga,Mn)As and (Ga,Mn)N consist largely of Mn clusters, and that the electronic and magnetic properties of these clusters differ significantly from those of single substitutional impurities. For (Ga,Mn)As we also show using lattice kinetic Monte Carlo methods that clustering occurs during growth and annealing via the Ga monovacancy mediated diffusion. For both materials clustering efficiently reduces the Curie temperatures even though the underlying band structure trends are different. The Curie temperatures are estimated for (Ga,Mn)As using the Weiss molecular field theory, while for (Ga,Mn)N we employ Monte Carlo methods in order to obtain the Curie temperatures.

This thesis consists of an overview and of the following 7 publications:

  1. H. Raebiger, A. Ayuela, and R. M. Nieminen, Intrinsic hole localization mechanism in magnetic semiconductors, Journal of Physics: Condensed Matter 16, L457-L462 (2004). © 2004 Institute of Physics Publishing. By permission.
  2. H. Raebiger, A. Ayuela, J. von Boehm, and R. M. Nieminen, Clustering of Mn in (Ga,Mn)As, Journal of Magnetism and Magnetic Materials 290-291, 1398-1401 (2005). © 2005 Elsevier Science. By permission.
  3. H. Raebiger, T. Hynninen, A. Ayuela, and J. von Boehm, Effects of Mn clustering on ferromagnetism in (Ga,Mn)As, Physica B, in press, 4 pages. © 2006 Elsevier Science. By permission.
  4. H. Raebiger, A. Ayuela, and J. von Boehm, Electronic and magnetic properties of substitutional Mn clusters in (Ga,Mn)As, Physical Review B 72, 014465, 7 pages (2005). © 2005 American Physical Society. By permission.
  5. H. Raebiger, M. G. Ganchenkova, and J. von Boehm, Diffusion and clustering of substitutional Mn in (Ga,Mn)As, cond-mat/0603135, 4 pages, submitted for publication.
  6. T. Hynninen, H. Raebiger, A. Ayuela, and J. von Boehm, High Curie temperatures in (Ga,Mn)N from Mn clustering, Applied Physics Letters 88, 122501, 3 pages (2006). © 2006 American Institute of Physics. By permission.
  7. T. Hynninen, H. Raebiger, and J. von Boehm, A multiscale study of ferromagnetism in clustered (Ga,Mn)N, Journal of Physics: Condensed Matter 18, 1561-1567 (2006). © 2006 Institute of Physics Publishing. By permission.

Keywords: diluted magnetic semiconductors, clustering, Curie temperature, density-functional calculation

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© 2006 Helsinki University of Technology

Last update 2011-05-26