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Magnetization dynamics of complex magnetic materials by atomistic spin dynamics simulations
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In recent years, there has been an intense interest in understanding the microscopic mechanism of laser induced ultrafast magnetization dynamics in picosecond time scales. Magnetization switching on such a time scale has potential to be a significant boost for the data storage industry. It is expected that the writing process will become ~1000 times faster by this technology, compared to existing techniques. Understanding the microscopic mechanisms and controlling the magnetization in such a time scale is of paramount importance at present.

In this thesis, laser induced ultrafast magnetization dynamics has been studied for Fe, Co, GdFe, CoMn and Heusler alloys. A multiscale approach has been used, i.e., first-principles density functional theory combined with atomistic spin dynamics utilizing the Landau –Lifshitz-Gilbert equation, along with a three-temperature phenomenological model to obtain the spin temperature. Special attention has been paid to the calculations of exchange interaction and Gilbert damping parameters. These parameters play a crucial role in determining the ultrafast magnetization dynamics under laser fluence of the considered materials.

The role of longitudinal and transversal excitations was studied for elemental ferromagnets, such as Fe and Co. A variety of complex temporal behavior of the magnetic properties was observed, which can be understood from the interplay between electron, spin, and lattice subsystems. The very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale was studied. We have shown that the ultrafast thermal switching process can happen above the compensation temperature in GdFe alloys. It is demonstrated that the exchange frustration via Dzyaloshinskii-Moriya interaction between the atomic Gd moments, in Gd rich area of these alloys, leads to Gd demagnetization faster than the Fe sublattice. In addition, we show that Co is a perfect Heisenberg system. Both Co and CoMn alloys have been investigated with respect to ultrafast magnetization dynamics. Also, it is predicted that ultrafast switching process can happen in the Heulser alloys when they are doped with heavy elements. Finally, we studied multiferroic CoCr2O4 and Ca3CoMnO4 systems by using the multiscale approach to study magnetization dynamics. In summary, our approach is able to capture crucial details of ultrafast magnetization dynamics in technologically important materials.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , 89 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1456
Keyword [en]
Ultrafast remagnetization, ultrafast dynamics, magnetism, multiferroics, amorphous alloys, DFT, spinels magnetostriction
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-308329ISBN: 978-91-554-9763-7 (print)OAI: oai:DiVA.org:uu-308329DiVA: diva2:1049482
Public defence
2017-02-24, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 247062
Available from: 2017-02-03 Created: 2016-11-24 Last updated: 2017-02-23
List of papers
1. Microscopic Model for Ultrafast Remagnetization Dynamics
Open this publication in new window or tab >>Microscopic Model for Ultrafast Remagnetization Dynamics
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2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 15, 157201- p.Article in journal (Refereed) Published
Abstract [en]

In this Letter, we provide a microscopic model for the ultrafast remagnetization of atomic moments already quenched above the Stoner-Curie temperature by a strong laser fluence. Combining first-principles density functional theory, atomistic spin dynamics utilizing the Landau-Lifshitz-Gilbert equation, and a three-temperature model, we analyze the temporal evolution of atomic moments as well as the macroscopic magnetization of bcc Fe and hcp Co covering a broad time scale, ranging from femtoseconds to picoseconds. Our simulations show a variety of complex temporal behavior of the magnetic properties resulting from an interplay between electron, spin, and lattice subsystems, which causes an intricate time evolution of the atomic moment, where longitudinal and transversal fluctuations result in a macrospin moment that evolves highly nonmonotonically.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-185192 (URN)10.1103/PhysRevLett.109.157201 (DOI)000309590300042 ()
Available from: 2012-11-22 Created: 2012-11-21 Last updated: 2017-12-07Bibliographically approved
2. All-thermal switching of amorphous Gd-Fe alloys: Analysis of structural properties and magnetization dynamics
Open this publication in new window or tab >>All-thermal switching of amorphous Gd-Fe alloys: Analysis of structural properties and magnetization dynamics
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 9, 094411Article in journal (Refereed) Published
Abstract [en]

In recent years there has been an intense interest in understanding the microscopic mechanism of thermally induced magnetization switching driven by a femtosecond laser pulse. Most of the effort has been dedicated to periodic crystalline structures while the amorphous counterparts have been less studied. By using a multiscale approach, i.e., first-principles density functional theory combined with atomistic spin dynamics, we report here on the very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale level. Both structural and dynamical properties of Gd-Fe alloys reported in this work are in good agreement with previous experiments. We calculated the dynamic behavior of homogeneous and inhomogeneous amorphous Gd-Fe alloys and their response under the influence of a femtosecond laser pulse. In the homogeneous sample, the Fe sublattice switches its magnetization before the Gd one. However, the temporal sequence of the switching of the two sublattices is reversed in the inhomogeneous sample. We propose a possible explanation based on a mechanism driven by a combination of the Dzyaloshinskii-Moriya interaction and exchange frustration, modeled by an antiferromagnetic second-neighbor exchange interaction between Gd atoms in the Gd-rich region. We also report on the influence of laser fluence and damping effects in the all-thermal switching.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-263429 (URN)10.1103/PhysRevB.92.094411 (DOI)000360884700002 ()
Available from: 2015-10-07 Created: 2015-09-30 Last updated: 2017-12-01Bibliographically approved
3. Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
Open this publication in new window or tab >>Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
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2017 (English)In: Physical review B, ISSN 2469-9950, Vol. 95, no 21, 214417Article in journal (Refereed) Published
Abstract [en]

Temperature-dependent magnetic experiments such as pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum that is more or less dependent on the magnetic configuration, e.g., as reflected by the temperature dependence of the exchange interactions. In this paper, we demonstrate from first-principles theory that the magnetic excitation spectra in Co in fcc, bcc, and hcp structures are nearly identical in a wide range of noncollinear magnetic configurations. This is a curious result of a balance between the size of the magnetic moments and the strength of the Heisenberg exchange interactions, that in themselves vary with configuration, but put together in an effective spin Hamiltonian results in a configuration-independent effective model. We have used such a Hamiltonian, together with ab initio calculated damping parameters, to investigate the magnon dispersion relationship as well as ultrafast magnetization dynamics of Co and Co-rich CoMn alloys.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-308321 (URN)10.1103/PhysRevB.95.214417 (DOI)000404015400003 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020, 2012.0031StandUp
Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2017-09-21Bibliographically approved
4. Ultrafast magnetization dynamics in pure and doped Heusler alloys
Open this publication in new window or tab >>Ultrafast magnetization dynamics in pure and doped Heusler alloys
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-308103 (URN)
Available from: 2016-11-24 Created: 2016-11-23 Last updated: 2016-11-24
5. Overcoming magnetic frustration and promoting half-metallicity in spinel CoCr2O4 by doping with Fe
Open this publication in new window or tab >>Overcoming magnetic frustration and promoting half-metallicity in spinel CoCr2O4 by doping with Fe
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 22, 224417Article in journal (Refereed) Published
Abstract [en]

In this paper, we present a systematic study of the effects of Fe doping on the electronic and magnetic structures of spinel CoCr2O4 by ab initio density functional theory and atomistic spin dynamics calculations. Our calculated magnetic structure for pristine CoCr2O4 correctly reproduces the experimental one with a q-vector of (0.67, 0.67,0.0), establishing the accuracy of the calculated interatomic exchange interactions. We show that the noncollinear spin structure with a nonzero q-vector in the spinel structure is driven towards collinearity by Fe doping by a complex interplay between interatomic exchange interactions. In the inverse spinel structure with 100% Fe doping, a collinear antiferromagnetic order develops along with a half-metallic electronic structure, which evolves due to the chemical disorder between Fe and Co in the B sites described by the coherent potential approximation. This is a comprehensive theoretical study to understand the evolution of magnetic and electronic properties of multiferroic CoCr2O4 doped with Fe.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-274283 (URN)10.1103/PhysRevB.92.224417 (DOI)000366500100006 ()
Funder
Carl Tryggers foundation Swedish Research CouncilEU, European Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2016-02-03 Created: 2016-01-20 Last updated: 2017-11-30Bibliographically approved
6. Temperature dependence of dielectric behavior in Ca3CoMnO6: insights from density functional theory and atomistic spin dynamics simulations
Open this publication in new window or tab >>Temperature dependence of dielectric behavior in Ca3CoMnO6: insights from density functional theory and atomistic spin dynamics simulations
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-308302 (URN)
Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2016-11-24

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