Quantum mechanical models of nanomagnetism in transition metal clusters and diluted magnetic semiconductors
(2009) Abstract
 This dissertation investigates nanomagnetism in small transition metal clusters and the diluted magnetic semiconductor (Ga,Mn)As. We derive quantum mechanical models aimed at a realistic description of the low energy physics in these systems. The main focus of the work presented is on magnetic anisotropy, the effects of spinorbit interaction and quantum geometric phases. The first part of the thesis gives an introduction to the field of nanomagnetism, surveying both experimental and theoretical background. The second part constitutes the core of the thesis and comprises five publications and one manuscript.
Paper I investigates the magnetic properties of symmetric Co clusters, and is motivated by the anomalously large... (More)  This dissertation investigates nanomagnetism in small transition metal clusters and the diluted magnetic semiconductor (Ga,Mn)As. We derive quantum mechanical models aimed at a realistic description of the low energy physics in these systems. The main focus of the work presented is on magnetic anisotropy, the effects of spinorbit interaction and quantum geometric phases. The first part of the thesis gives an introduction to the field of nanomagnetism, surveying both experimental and theoretical background. The second part constitutes the core of the thesis and comprises five publications and one manuscript.
Paper I investigates the magnetic properties of symmetric Co clusters, and is motivated by the anomalously large anisotropies observed in experiment. Using a tightbinding model equipped with meanfield exchange and spinorbit interaction, we are able to shed light on the origins of the high anisotropy.
Paper II addresses transition metal dimers and the physical limits on magnetic anisotropy per atom. Using ab initio calculations supported by symmetry and perturbational considerations, we argue that the unique dimer symmetry enables a giant anisotropy. The symmetry exceptionally permits a first order spinorbit contribution to the anisotropy energy, representing the physical upper limit on anisotropy per atom.
Paper III combines ab initio calculations for small transition metal clusters with a fieldtheoretical framework to derive effective Hamiltonians for a single giant spin degree of freedom, describing the low energy physics associated with the collective magnetization orientation. The giant spin Hamiltonian is subject to a quantum correction in the form of Berry's phase, which can profoundly modify the anisotropy energy landscape.
In Paper IV we study the magnetic properties of a single Mn in GaAs, in part motivated by recent advances in STM characterization of (Ga,Mn)As. Using a kineticexchange tightbinding model we investigate how the magnetic properties are affected by the presence of a surface and relate our results to experiments.
Paper V addresses the nature of MnMn interactions in GaAs using the kineticexchange model. We calculate effective exchange interactions, acceptor level properties and anisotropy energies for Mn pairs oriented along different crystalline directions. The bonding/antibonding nature of the acceptor wave functions is examined.
Paper VI combines a fieldtheoretical approach with the kineticexchange tightbinding model to derive giant spin Hamiltonians. The effect of quantum mechanical Berry phase corrections on the anisotropy for various (Ga,Mn)As systems is investigated. Chern number theory is employed to elucidate the nature of the acceptor. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/record/1502282
 author
 Strandberg, Olof ^{LU}
 supervisor
 opponent

 Professor Eriksson, Olle, Uppsala universitet, Institutionen för fysik och materialvetenskap, Avdelningen för materialteori
 organization
 publishing date
 2009
 type
 Thesis
 publication status
 published
 subject
 keywords
 diluted magnetic semiconductors, Mn)As, (Ga, transition metal clusters, magnetic anisotropy, Nanomagnetism, quantum geometric phases, Chern numbers, effective spin Hamiltonians, dimers
 pages
 307 pages
 publisher
 Lund University (MediaTryck)
 defense location
 Föreläsningssal B, Mathematical Physics, Sölvegatan 14, Lund
 defense date
 20091211 14:00
 ISBN
 9789162879181
 language
 English
 LU publication?
 yes
 id
 9fe5a3f189994ecc8d81e43ddd520d61 (old id 1502282)
 date added to LUP
 20091116 12:22:38
 date last changed
 20160919 08:45:14
@misc{9fe5a3f189994ecc8d81e43ddd520d61, abstract = {This dissertation investigates nanomagnetism in small transition metal clusters and the diluted magnetic semiconductor (Ga,Mn)As. We derive quantum mechanical models aimed at a realistic description of the low energy physics in these systems. The main focus of the work presented is on magnetic anisotropy, the effects of spinorbit interaction and quantum geometric phases. The first part of the thesis gives an introduction to the field of nanomagnetism, surveying both experimental and theoretical background. The second part constitutes the core of the thesis and comprises five publications and one manuscript.<br/><br> Paper I investigates the magnetic properties of symmetric Co clusters, and is motivated by the anomalously large anisotropies observed in experiment. Using a tightbinding model equipped with meanfield exchange and spinorbit interaction, we are able to shed light on the origins of the high anisotropy. <br/><br> Paper II addresses transition metal dimers and the physical limits on magnetic anisotropy per atom. Using ab initio calculations supported by symmetry and perturbational considerations, we argue that the unique dimer symmetry enables a giant anisotropy. The symmetry exceptionally permits a first order spinorbit contribution to the anisotropy energy, representing the physical upper limit on anisotropy per atom. <br/><br> Paper III combines ab initio calculations for small transition metal clusters with a fieldtheoretical framework to derive effective Hamiltonians for a single giant spin degree of freedom, describing the low energy physics associated with the collective magnetization orientation. The giant spin Hamiltonian is subject to a quantum correction in the form of Berry's phase, which can profoundly modify the anisotropy energy landscape. <br/><br> In Paper IV we study the magnetic properties of a single Mn in GaAs, in part motivated by recent advances in STM characterization of (Ga,Mn)As. Using a kineticexchange tightbinding model we investigate how the magnetic properties are affected by the presence of a surface and relate our results to experiments. <br/><br> Paper V addresses the nature of MnMn interactions in GaAs using the kineticexchange model. We calculate effective exchange interactions, acceptor level properties and anisotropy energies for Mn pairs oriented along different crystalline directions. The bonding/antibonding nature of the acceptor wave functions is examined.<br/><br> Paper VI combines a fieldtheoretical approach with the kineticexchange tightbinding model to derive giant spin Hamiltonians. The effect of quantum mechanical Berry phase corrections on the anisotropy for various (Ga,Mn)As systems is investigated. Chern number theory is employed to elucidate the nature of the acceptor.}, author = {Strandberg, Olof}, isbn = {9789162879181}, keyword = {diluted magnetic semiconductors,Mn)As,(Ga,transition metal clusters,magnetic anisotropy,Nanomagnetism,quantum geometric phases,Chern numbers,effective spin Hamiltonians,dimers}, language = {eng}, pages = {307}, publisher = {ARRAY(0xaa41518)}, title = {Quantum mechanical models of nanomagnetism in transition metal clusters and diluted magnetic semiconductors}, year = {2009}, }