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Static and dynamic properties of vortex pairs in asymmetric nanomagnets
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0002-9993-4748
KTH.
KTH.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0002-9092-093x
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2016 (English)In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 6, no 5, article id 056406Article in journal (Refereed) Published
Resource type
Text
Abstract [en]

Stacked spin-vortex pairs in magnetic multilayered nanopillars, with vertical separation between the vortices small compared to the vortex core size and pure magnetostatic coupling, exhibit spin dynamics absent in individual vortices. This dynamics is nonlinear and is due to the strong direct core-core coupling in the system, dominating energetically for small-signal excitation. We observe and explain the appearance of spin resonance modes, forbidden within linear dynamics, and discuss how they depend on the magnetic and morphological asymmetries in the samples.

Place, publisher, year, edition, pages
American Institute of Physics Inc. , 2016. Vol. 6, no 5, article id 056406
Keywords [en]
Spin dynamics, Dynamic property, Linear dynamics, Magneto-static couplings, Multi-layered, Signal excitation, Spin resonance, Vertical separation, Vortex cores, Vortex flow
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-186979DOI: 10.1063/1.4944515ISI: 000377962500262Scopus ID: 2-s2.0-84961589525OAI: oai:DiVA.org:kth-186979DiVA, id: diva2:929909
Conference
13th Joint Magnetism and Magnetic Materials (MMM)/Intermag Conference, Jan 11-15, 2016, San Diego, CA
Funder
Swedish Research Council, 2014-4548
Note

QC 20160520

Available from: 2016-05-20 Created: 2016-05-16 Last updated: 2019-04-02Bibliographically approved
In thesis
1. Static and dynamic properties of uniform- and vortex-states in synthetic nanomagnets
Open this publication in new window or tab >>Static and dynamic properties of uniform- and vortex-states in synthetic nanomagnets
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Synthetic antiferromagnets (SAFs) consist of two thin ferromagnetic particles separated by a thin nonmagnetic spacer. The magnetic moments of the two particles couple antiparallel via dipolar interactions, with the interlayer exchange interaction suppressed by a suitable choice of the spacer material. The SAF system studied in this thesis contains thin elliptical-in-the-plane permalloy particles magnetized uniformly and mutually antiparallel in the ground state. A SAF can also exhibit long-lived metastable nonuniform magnetization states, such as spin-vortex pairs. The thesis explores hysteresis and spin dynamics in: (i) uniformly magnetized SAFs and (ii) SAFs in the vortex-pair state.

The uniformly magnetized antiparallel ground state of a symmetrical SAF, having identical ferromagnetic particles, is double  degenerate. The resonance modes are in-phase (acoustical) and out-of-phase (optical) oscillations of the magnetic moments. Asymmetry between the two magnetic layers is shown to lift the degeneracy of the antiparallel ground state, which in the static regime results in unequal stability of the two states. In the dynamic regime, the asymmetries are shown to result in a splitting of the resonance frequency of the new non-degenerate ground states. The resulting resonant-mode splitting can be used to selectively switch between the antiparallel ground states by resonant microwave or thermal activation of the system.

The static and dynamic properties of the vortex pairs in SAFs were found to be strongly dependent on the relative orientation of the vortex chiralities and vortex-core polarizations in the two ferromagnetic particles of the SAF. For parallel core polarizations, a strong monopole-like core-core interaction is found to dominate the magnetic properties of the system, increasing the characteristic resonance frequency by an order of magnitude.  Analytical theory and numerical micromagnetic simulations are used to explain the measured responses.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. p. 74
Series
TRITA-FYS, ISSN 0280-316X
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-187473 (URN)978-91-7729-018-6 (ISBN)
Public defence
2016-06-15, FB54, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2014-4548Stiftelsen Olle Engkvist Byggmästare, 2014-STE
Note

QC 20160524

Available from: 2016-05-24 Created: 2016-05-24 Last updated: 2016-05-25Bibliographically approved
2. Nonlinear dynamics of strongly-bound magnetic vortex pairs
Open this publication in new window or tab >>Nonlinear dynamics of strongly-bound magnetic vortex pairs
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This work is a study of nonlinear phenomena in vertically stacked pairs of magnetic vortices. New dynamic regimes are uncovered with a decrease in the inter-vortex separation to below the lateral vortex-core size. These include linear, non-linear, and chaos dynamics of the coupled vortex cores, as well as core-core coupling/decoupling driven by resonant microwave fields. In addition to the direct advantages gained from the favorable symmetry of the system, which includes the fringing flux closure, new ways of exciting and controlling the motion of the vortex cores are shown. The dynamics of the vortex stack show promising improvements over those of a single vortex, in particular the characteristic speed of operation can be increased by an order of magnitude. The system therefore is viewed to have the potential for applications in data storage and oscillators.

A combination of experimental, analytical, and numerical methods is used. A theoretical framework based on the quasiparticle Thiele-equation approach, extended to thermally driven dynamics by using the Monte Carlo method, is constructed and extensively tested experimentally and numerically. In-depth micromagnetic simulations are performed and show consistency with the results obtained analytically, both successfully validated against the measured data collected in a series of experiments on spin vortex pairs. Among these are microwave spectroscopy, transient dynamics, thermal decay, and pinning spectroscopy measurements.

In particular, it is shown that the nonlinear frequency response of a two-vortex system exhibits a fold-over and an isolated rotational core-core resonance. A parametric inter-modal interaction is shown to induce hybrid dynamic regimes of the vortex-core oscillation when the system is subject to high excitation amplitudes.

An intrinsic bi-stability of the core positions in the structure is found and investigated as a candidate for a memory element. The bi-stability is pronounced at lower temperatures. The rates of thermal switching were investigated in order to find the optimum operating DC-bias conditions.

It is found that parametric interactions play a big role in the otherwise frustrated dynamics of essentially a 1D system. The parameters of the short excitation pulses for switching between the core-core states are optimized to achieve switching probabilities of over 90% in the experiment, with the pulses only a few nanoseconds long.

Vortex pairs are demonstrated to be sensitive to the presence of defects in the ferromagnetic layers of the nanostructure. It is shown that the key factor in this sensitivity lies in the vortex' flux closure. Binding of a core-core pair to a defect is observed experimentally. A model is developed to describe the changes in the dynamical characteristics of the defect-pinned vortex pair. The capabilities of the model for characterizing magnetic and morphological defects in nanostructures are demonstrated.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. p. 84
Series
TRITA-SCI-FOU ; 2019:11
Keywords
magnetic vortex, nonlinear mechanics, isolated resonance, nonlinear frequency tuning, chaotic dynamics, parametric interaction, Thiele equation, Monte Carlo simulations, Ito processes, bistability, tunneling magneto-resistance, Magnetisk-virvel, icke-linjär dynamik, isolerad resonans, icke-linjär frekvens-optimering, kaotisk dynamik, parametrisk interaktion, Thiele-ekvation, Monte Carlo-simuleringar, Ito-processer, bistabilitet, tunnelmagneto-resistans.
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-248020 (URN)978-91-7873-136-7 (ISBN)
Public defence
2019-03-29, FP41, Albanova Universitetscentrum, Roslagstullsbacken 33, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20190402

Available from: 2019-04-02 Created: 2019-04-02 Last updated: 2019-04-02Bibliographically approved

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Koop, Björn C.Bondarenko, ArtemKorenivski, Vladislav

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