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Spin Dynamics and Magnetic Multilayers
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
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. , p. x, 74
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 326
Keywords [en]
Physics, magnetism, electronic structure, density functional theory, spin dynamics, spin-transfer torque, spin-density wave, multilayer, interface structure
Keywords [sv]
Fysik
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-8168ISBN: 978-91-554-6944-3 (print)OAI: oai:DiVA.org:uu-8168DiVA, id: diva2:170558
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
List of papers
1. A method for atomistic spin dynamics simulations: implementation and examples
Open this publication in new window or tab >>A method for atomistic spin dynamics simulations: implementation and examples
2008 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 20, no 31, p. 315203-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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96079 (URN)10.1088/0953-8984/20/31/315203 (DOI)000257759600032 ()
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
2. Spin dynamics of a Heisenberg model in an external and uniaxial anisotropy field
Open this publication in new window or tab >>Spin dynamics of a Heisenberg model in an external and uniaxial anisotropy field
(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96080 (URN)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2012-03-26Bibliographically approved
3. Atomistic spin dynamics simulations of diluted magnetic semiconductors using MnxGa1-xAs as a prototype
Open this publication in new window or tab >>Atomistic spin dynamics simulations of diluted magnetic semiconductors using MnxGa1-xAs as a prototype
(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96081 (URN)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2012-03-26Bibliographically approved
4. Atomistic spin dynamics of the Cu-Mn spin-glass alloy
Open this publication in new window or tab >>Atomistic spin dynamics of the Cu-Mn spin-glass alloy
Show others...
2009 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 79, no 2, p. 024411-Article in journal (Refereed) Published
Abstract [en]

We demonstrate the use of Langevin spin dynamics for studying dynamical properties of an archetypical spin-glass system. Simulations are performed on CuMn (20% Mn) where we study the relaxation that follows a sudden quench of the system to the low-temperature phase. The system is modeled by a Heisenberg Hamiltonian where the Heisenberg interaction parameters are calculated by means of first-principles density-functional theory. Simulations are performed by numerically solving the Langevin equations of motion for the atomic spins. It is shown that dynamics is governed, to a large degree, by the damping parameter in the equations of motion and the system size. For large damping and large system sizes, we observe the typical aging regime.

Keywords
Spin glasses and other random magnets, Numerical simulation studies, Spin-glass and other random models
National Category
Engineering and Technology
Research subject
Solid State Physics
Identifiers
urn:nbn:se:uu:diva-122318 (URN)10.1103/PhysRevB.79.024411 (DOI)000262978100056 ()
Available from: 2010-04-08 Created: 2010-04-08 Last updated: 2017-12-12Bibliographically approved
5. Current Driven Magnetization Dynamics in Helical Spin Density Waves
Open this publication in new window or tab >>Current Driven Magnetization Dynamics in Helical Spin Density Waves
2006 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, no 25, article id 256601Article in journal (Refereed) Published
Abstract [en]

A mechanism is proposed for manipulating the magnetic state of a helical spin density wave using a current. It is shown that a current through a bulk metal with a helical spin density wave induces a spin transfer torque, which gives rise to a rotation of the order parameter. The use of spin transfer torque to manipulate the magnetization in bulk systems does not suffer from the obstacles seen for magnetization reversal using interface spin transfer torque in multilayered systems. The effect is demonstrated by a quantitative calculation of the current induced magnetization dynamics of a rare earth metal, Er. Finally, we propose a setup for experimental verification.

Keywords
Spin transfer torque
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-96083 (URN)10.1103/PhysRevLett.96.256601 (DOI)000238697000042 ()16907330 (PubMedID)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
6. Spin-Transfer Torque in Helical Spin Density Waves
Open this publication in new window or tab >>Spin-Transfer Torque in Helical Spin Density Waves
2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 10, p. 104433-Article in journal (Refereed) Published
Abstract [en]

The current driven magnetization dynamics of a helical spin-density wave is investigated. Expressions for calculating the spin-transfer torque of real systems from first-principles density-functional theory are presented. These expressions are used for calculating the   spin-transfer torque for the spin spirals of Er and fcc Fe at two different lattice volumes. It is shown that the calculated torque induces a rigid rotation of the order parameter with respect to the spin spiral axis. The torque is found to depend on the wave vector of the spin spiral and the spin polarization of the Fermi surface states. The resulting dynamics of the spin spiral is also discussed.

Keywords
density functional theory, electron spin polarisation, erbium, Fermi surface, ferromagnetic materials, iron, magnetisation, spin density waves
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96084 (URN)10.1103/PhysRevB.79.104433 (DOI)000264768600089 ()
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
7. Spin spiral ordering in Er/Lu multilayers
Open this publication in new window or tab >>Spin spiral ordering in Er/Lu multilayers
(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96085 (URN)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2012-04-05Bibliographically approved
8. Competing Exchange Interactions in Magnetic Multilayers
Open this publication in new window or tab >>Competing Exchange Interactions in Magnetic Multilayers
Show others...
2006 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 96, p. 057205-Article in journal (Refereed) Published
Abstract [en]

We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-96086 (URN)10.1103/PhysRevLett.96.057205 (DOI)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
9. Role of magnetic impurities in Fe/V multilayers
Open this publication in new window or tab >>Role of magnetic impurities in Fe/V multilayers
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 14, p. 144408-Article in journal (Refereed) Published
Abstract [en]

We have studied the Fe/V bcc (100) multilayers and the effect of alloying the V spacer layers with various amounts of magnetic impurities (Fe, Co, Ni, and Cr). The study was performed by means of total energy electronic structure calculations. We compare the effect of the different types of impurities and discuss the interlayer exchange coupling in terms of Fermi surface topology and bulk magnetic order. The effect of interface roughness and interface intermixing on the phase diagrams was also studied.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-96087 (URN)10.1103/PhysRevB.77.144408 (DOI)000255457300043 ()
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
10. On the sharpness of the interfaces in metallic multilayers
Open this publication in new window or tab >>On the sharpness of the interfaces in metallic multilayers
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2004 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 101, no 14, p. 4742-4745Article in journal (Refereed) Published
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-7275 (URN)10.1073/pnas.0305313101 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2017-12-14Bibliographically approved
11. Structural and magnetic aspects of multilayer interfaces
Open this publication in new window or tab >>Structural and magnetic aspects of multilayer interfaces
2004 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 272-276, no Suppl., p. E941-E942Article in journal (Refereed) Published
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96089 (URN)10.1016/j.jmmm.2003.12.1165 (DOI)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
12. Magnetic moments and exchange interactions in Fe0.82Ni0.18/V bcc(001) multilayers
Open this publication in new window or tab >>Magnetic moments and exchange interactions in Fe0.82Ni0.18/V bcc(001) multilayers
Show others...
2004 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 9, p. 094421-Article in journal (Refereed) Published
Abstract [en]

The magnetic ordering and average magnetization of multilayers are greatly affected by the interface structure of the system. In this paper, interface effects are discussed, and their impact on the amplitude and period of the interlayer exchange coupling and the magnetic moments are investigated for the Fe0.82Ni0.18∕V body-centered-cubic (001) system. By modeling these effects by first principles calculations we find good agreement with experiments over the whole investigated range of layer thicknesses. In addition, as different interface effects give different fingerprints on the interlayer exchange coupling and magnetization, we are able to make an estimate of the interface structure of the different experimental samples by comparing experiment to theory.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-91263 (URN)10.1103/PhysRevB.70.094421 (DOI)
Available from: 2004-01-19 Created: 2004-01-19 Last updated: 2017-12-14Bibliographically approved
13. Magnetic phase diagram of Fe0.82Ni0.18/V(0 0 1) superlattices
Open this publication in new window or tab >>Magnetic phase diagram of Fe0.82Ni0.18/V(0 0 1) superlattices
Show others...
2004 (English)In: J. Magn. Magn. Mater., Vol. 280, p. 346-Article in journal (Refereed) Published
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96091 (URN)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2013-03-21
14. Influence of interface mixing on the magnetic properties of BCC Fe0.82Ni0.18/V (0 0 1) superlattices
Open this publication in new window or tab >>Influence of interface mixing on the magnetic properties of BCC Fe0.82Ni0.18/V (0 0 1) superlattices
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2003 In: J. Magn. Magn. Mater., Vol. 267, p. 234-Article in journal (Refereed) Published
Identifiers
urn:nbn:se:uu:diva-96092 (URN)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2013-03-21
15. Exchange coupling and exchange bias in FM/AFM bilayers for a fully compensated AFM interface
Open this publication in new window or tab >>Exchange coupling and exchange bias in FM/AFM bilayers for a fully compensated AFM interface
2009 (English)In: Acta Physica Polonica. A, ISSN 0587-4246, E-ISSN 1898-794X, Vol. 115, no 1, p. 25-29Article in journal (Refereed) Published
Abstract [en]

We address the interlayer coupling in a ferromagnet/antiferromagnet bilayer where the interface of theantiferromagnet is fully compensated. We discuss the role of di®erent types of exchange interaction for theinterlayer coupling and exchange bias. We propose two types of corrections to the ideal Heisenberg Hamiltonianwhich may explain exchange bias. The ¯rst is a correction for the angular dependence of the exchange interactionsand the second a correction due to magnetostriction and interface imperfections. The ¯rst correction contributes toan anisotropy at the interface and favors either parallel or perpendicular coupling across the interface. The secondcorrection contributes to the exchange bias. Our analysis is based on atomic spin dynamics simulations, and ourresults show that small corrections to the ideal Heisenberg Hamiltonian may have macroscopic consequences insystems with frustrated interatomic interactions.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96093 (URN)000263757900004 ()
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
16. Spin dynamics simulations on an electron beam switching experiment
Open this publication in new window or tab >>Spin dynamics simulations on an electron beam switching experiment
(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96094 (URN)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2012-03-26Bibliographically approved
17. Ultrafast switching in AFM bilayers; a first principles spin dynamics simulation
Open this publication in new window or tab >>Ultrafast switching in AFM bilayers; a first principles spin dynamics simulation
(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96095 (URN)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2012-04-05Bibliographically approved

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