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A method for atomistic spin dynamics simulations: implementation and examples
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
2008 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 31, 315203- p.Article in journal (Refereed) Published
Abstract [en]

We present a method for performing atomistic spin dynamic simulations. A comprehensive summary of all pertinent details for performing the simulations such as equations of motions, models for including temperature, methods of extracting data and numerical schemes for performing the simulations is given. The method can be applied in a first-principles mode, where all interatomic exchange is calculated self-consistently, or it can be applied with frozen parameters estimated from experiments or calculated for a fixed spin-configuration. Areas of potential application to different magnetic questions are also discussed. The method is finally applied to one situation where the macrospin model breaks down, magnetic switching in ultra strong magnetic fields.

Place, publisher, year, edition, pages
2008. Vol. 20, no 31, 315203- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-96079DOI: 10.1088/0953-8984/20/31/315203ISI: 000257759600032OAI: oai:DiVA.org:uu-96079DiVA: diva2:170541
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Spin Dynamics and Magnetic Multilayers
Open this publication in new window or tab >>Spin Dynamics and Magnetic Multilayers
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Theoretical studies based on first-principles theory are presented for a number of different magnetic systems. The first part of the thesis concerns spin dynamics and the second part concerns properties of magnetic multilayers. The theoretical treatment is based on electronic structure calculations performed by means of density functional theory.

A method is developed for simulating atomistic spin dynamics at finite temperatures, which is based on solving the equations of motion for the atomic spins by means of Langevin dynamics. The method relies on a mapping of the interatomic exchange interactions from density functional theory to a Heisenberg Hamiltonian. Simulations are performed for various magnetic systems and processes beyond the reach of conventional micromagnetism. As an example, magnetization dynamics in the limit of large magnetic and anisotropy fields is explored. Moreover, the method is applied to studying the dynamics of systems with complex atomic order such as the diluted magnetic semiconductor MnGaAs and the spin glass alloy CuMn. The method is also applied to a Fe thin film and a Fe/Cr/Fe trilayer system, where the limits of ultrafast switching are explored. Current induced magnetization dynamics is investigated by calculating the current induced spin-transfer torque by means of density functional theory combined with the relaxation time approximation and semi-classical Boltzmann theory. The current induced torque is calculated for the helical spin-density waves in Er and fcc Fe, where the current is found to promote a rigid rotation of the magnetic order.

Properties of magnetic multilayers composed of magnetic and nonmagnetic layers are investigated by means of the Korringa-Kohn-Rostocker interface Green's function method. Multilayer properties such as magnetic moments, interlayer exchange coupling and ordering temperatures are calculated and compared with experiments, with focus on understanding the influence of interface quality. Moreover, the influence on the interlayer exchange coupling of alloying the nonmagnetic spacer layers with small amounts of a magnetic impurity is investigated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. x, 74 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 326
Keyword
Physics, magnetism, electronic structure, density functional theory, spin dynamics, spin-transfer torque, spin-density wave, multilayer, interface structure, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-8168 (URN)978-91-554-6944-3 (ISBN)
Public defence
2007-09-21, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15
Opponent
Supervisors
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2012-03-28Bibliographically approved
2. Atomistic Spin Dynamics, Theory and Applications
Open this publication in new window or tab >>Atomistic Spin Dynamics, Theory and Applications
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The topic of this Thesis is magnetization dynamics on atomic length scales. A computational scheme, Atomistic Spin Dynamics, based on density functional theory, the adiabatic approximation and the atomic moment approximation is presented. Simulations are performed for chemically disordered systems, antiferromagnets and ferrimagnets and also systems with reduced dimensionality

The autocorrelation function of the archetypical spin glass alloy CuMn is sampled in simulations following a quenching protocol. The aging regime can be clearly identified and the dependence of the relaxation on the damping parameter is investigated. The time evolution of pair correlation and autocorrelation functions has been studied in simulations of the dilute magnetic semiconductor GaMnAs. The dynamics reveal a substantial short ranged magnetic order even at temperatures at or above the ordering temperature. The dynamics for different concentrations of As antisites are discussed.

Antiferromagnets offer opportunities for ultrafast switching, this is studied in simulations of an artificial antiferromagnet. For the right conditions, the cooperative effect of applied field torque and and the torque from the other sublattice enables very fast switching. The dynamics of bcc Fe precessing in a strong uniaxial anisotropy are investigated. It is demonstrated that the magnetization can shrink substantially due to a spin wave instability. The dynamics of a two-component model ferrimagnet at finite temperature are investigated. At temperatures where the magnetic system is close to the magnetic and angular momentum compensations points of the ferrimagnet, the relaxation in a uniaxial easy exis anisotropy resembles results in recent experiments on ferrimagnetic resonance.

The different cases of uniaxial or colossal magnetic anisotropy in nanowires at different temperatures are compared. The magnon softening in a ferromagnetic monolayer is investigated, giving results that compare well with recent experiments. The effect of lattice relaxation can be treated in first principles calculations. Subsequent simulations captures the softening of magnons caused by reduced dimensionality and temperature.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 108 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 776
Keyword
ab initio calculations, spin dynamics, magnetization, magnetic switching, diluted magnetic semiconductors, spin glasses, ferromagnetic relaxation, ferrimagnets, antiferromagnets
National Category
Condensed Matter Physics Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-120103 (URN)978-91-554-7921-3 (ISBN)
Public defence
2010-12-03, Polhemsalen, Ångström laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Note
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 706Available from: 2010-11-11 Created: 2010-03-07 Last updated: 2011-03-21Bibliographically approved

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Hellsvik, JohanNordström, LarsEriksson, Olle

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