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Magnetism and ultrafast magnetization dynamics of Co and CoMn alloys at finite temperature
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Univ Fed Para, Fac Fis, Belem, PA, Brazil.
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2017 (English)In: Physical review B, ISSN 2469-9950, Vol. 95, no 21, article id 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.

Place, publisher, year, edition, pages
2017. Vol. 95, no 21, article id 214417
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-308321DOI: 10.1103/PhysRevB.95.214417ISI: 000404015400003OAI: oai:DiVA.org:uu-308321DiVA, id: diva2:1049399
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020, 2012.0031StandUpAvailable from: 2016-11-24 Created: 2016-11-24 Last updated: 2018-04-07Bibliographically approved
In thesis
1. Magnetization dynamics of complex magnetic materials by atomistic spin dynamics simulations
Open this publication in new window or tab >>Magnetization dynamics of complex magnetic materials by atomistic spin dynamics simulations
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. p. 89
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1456
Keywords
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:nbn:se:uu:diva-308329 (URN)978-91-554-9763-7 (ISBN)
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
2. Electronic structure and exchange interactions from ab initio theory: New perspectives and implementations
Open this publication in new window or tab >>Electronic structure and exchange interactions from ab initio theory: New perspectives and implementations
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the magnetic properties of several materials were investigated using first principle calculations. The ab initio method named real space linear muffin-tin orbitals atomic sphere approximation (RS-LMTO-ASA) was used to calculate the electronic structure and magnetic properties of bulk systems, surface and nanostructures adsorbed on surfaces.

We have implemented new features in the RS-LMTO-ASA method, such as the calculation of (a) Bloch Spectral Function (BSF), (b) orbital resolved Jij and (c) Dzyaloshinskii-Moriya interaction (DMI). Using (a), we have shown that one can calculate the dispersion relation for bulk systems using a real space method. Furthermore, the dispersion relation was revealed to be existent even for finite one-dimensional structures, such as the Mn chain on Au(111) and Ag(111) surfaces. With (b), we have investigated the orbital resolved exchange coupling parameter Jij for 3d metals. It is demonstrated that the nearest neighbor (NN) interaction for bcc Fe has intriguing behavior, however, the contribution coming from the T2g orbitals favours the anti-ferromagnetic coupling behavior. Moreover, the Fermi surface for bcc Fe is formed mostly by the T2g orbitals and these are shown to be highly Heisenberg-like, i.e. do not depend significantly on the magnetic configuration. Later, the same approach was used to study other transition metals, such as Cr, Mn, Co and Ni. In the end, we have presented the results obtained with the implementation (c). Our results have shown the large dependence of the DMI values, both the strength and direction, with respect to which magnetic configuration they are calculated from. We argue that, for the investigated systems, the non-collinearity induces currents (spin and charge) that will influence directly the DMI vectors.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 85
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1663
Keywords
ab initio, exchange interactions, non-collinear magnetism
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-347812 (URN)978-91-513-0315-4 (ISBN)
Public defence
2018-05-29, Seminar Room, Universidade Federal do Pará, Av. Augusto Correa 01, Belém, PA, Brazil, Belém, 10:00 (English)
Opponent
Supervisors
Note

The public defence can also be followed on livestream at Rosetta room, Ång/10239, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala

Available from: 2018-05-07 Created: 2018-04-07 Last updated: 2018-05-07

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Chimata, RaghuveerDelczeg-Czirjak, Erna KrisztinaSzilva, AttilaKvashnin, YaroslavPereiro, ManuelThonig, DannySanyal, BiplabEriksson, Olle

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Chimata, RaghuveerDelczeg-Czirjak, Erna KrisztinaSzilva, AttilaKvashnin, YaroslavPereiro, ManuelThonig, DannySanyal, BiplabEriksson, Olle
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