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Nonlinear dynamics of strongly-bound magnetic vortex pairs
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0002-9092-093x
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 [en]
magnetic vortex, nonlinear mechanics, isolated resonance, nonlinear frequency tuning, chaotic dynamics, parametric interaction, Thiele equation, Monte Carlo simulations, Ito processes, bistability, tunneling magneto-resistance
Keywords [sv]
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: urn:nbn:se:kth:diva-248020ISBN: 978-91-7873-136-7 (print)OAI: oai:DiVA.org:kth-248020DiVA, id: diva2:1301593
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
List of papers
1. Chaotic dynamics in spin-vortex pairs
Open this publication in new window or tab >>Chaotic dynamics in spin-vortex pairs
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, article id 054402Article in journal (Refereed) Published
Abstract [en]

We report on spin-vortex pair dynamics measured at temperatures low enough to suppress stochastic core motion, thereby uncovering the highly nonlinear intrinsic dynamics of the system. Our analysis shows that the decoupling of the two vortex cores is resonant and can be enhanced by dynamic chaos. We detail the regions of the relevant parameter space, in which the various mechanisms of the resonant core-core dynamics are activated. We show that the presence of chaos can reduce the thermally induced spread in the decoupling time by up to two orders of magnitude.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-248013 (URN)10.1103/PhysRevB.99.054402 (DOI)000457728700004 ()2-s2.0-85061380893 (Scopus ID)
Funder
Swedish Research Council, 2014-4546Swedish Research Council, 2018-03526
Note

QC 20190402

Available from: 2019-04-02 Created: 2019-04-02 Last updated: 2019-05-20Bibliographically approved
2. Transient dynamics of strongly coupled spin vortex pairs: Effects of anharmonicity and resonant excitation on inertial switching
Open this publication in new window or tab >>Transient dynamics of strongly coupled spin vortex pairs: Effects of anharmonicity and resonant excitation on inertial switching
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2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 19, article id 192405Article in journal (Refereed) Published
Abstract [en]

Spin vortices in magnetic nanopillars are used as GHz oscillators, with frequency however essentially fixed in fabrication. We demonstrate a model system of a two-vortex nanopillar, in which the resonance frequency can be changed by an order of magnitude, without using high dc magnetic fields. The effect is due to switching between the two stable states of the vortex pair, and we show that it can be done with low-amplitude fields of sub-ns duration. We detail the relevant vortex-core dynamics and explain how field anharmonicity and phase control can be used to enhance the performance.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-228429 (URN)10.1063/1.5030855 (DOI)000431980100026 ()2-s2.0-85046887938 (Scopus ID)
Note

QC 20180529

Available from: 2018-05-29 Created: 2018-05-29 Last updated: 2020-03-09Bibliographically approved
3. Resonant pinning spectroscopy with spin-vortex pairs
Open this publication in new window or tab >>Resonant pinning spectroscopy with spin-vortex pairs
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094406Article in journal (Refereed) Published
Abstract [en]

Vortex pairs in magnetic nanopillars with strongly coupled cores and pinning of one of the cores by a morphological defect, are used to perform resonant pinning spectroscopy, in which a microwave excitation applied to the nanopillar produces pinning or depinning of the cores only when the excitation is in resonance with the rotational or gyrational eigenmodes of the specific initial state of the core-core pair. The shift in the eigenmode frequencies between the pinned and depinned states is determined experimentally and explained theoretically, and illustrates the potential for multicore spin-vortex memory with resonant writing of information onto various stable vortex pair states. Further, it is shown how the same resonant spectroscopy techniques applied to a vortex pair can be used as a sensitive nanoscale probe for characterizing morphological defects in magnetic films.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-224680 (URN)10.1103/PhysRevB.97.094406 (DOI)000426749400002 ()2-s2.0-85044007173 (Scopus ID)
Funder
Swedish Research Council, 2014-4548
Note

QC 20180326

Available from: 2018-03-26 Created: 2018-03-26 Last updated: 2019-04-29Bibliographically approved
4. Stochastic dynamics of strongly-bound magnetic vortex pairs
Open this publication in new window or tab >>Stochastic dynamics of strongly-bound magnetic vortex pairs
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2017 (English)In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 7, no 5, article id 056007Article in journal (Refereed) Published
Abstract [en]

We demonstrate that strongly-bound spin-vortex pairs exhibit pronounced stochastic behaviour. Such dynamics is due to collective magnetization states originating from purely dipolar interactions between the vortices. The resulting thermal noise exhibits telegraph-like behaviour, with random switching between different oscillation regimes observable at room temperature. The noise in the system is further studied by varying the external field and observing the related changes in the frequency of switching and the probability for different magnetic states and regimes. Monte Carlo simulations are used to replicate and explain the experimental observations.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
Keywords
Intelligent systems, Monte Carlo methods, Stochastic systems, Collective magnetization, Dipolar interaction, External fields, Magnetic state, Magnetic vortices, Random switching, Stochastic dynamics, Vortex pair
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-200848 (URN)10.1063/1.4974066 (DOI)000402797100164 ()2-s2.0-85009456825 (Scopus ID)
Note

QC 20170206

Available from: 2017-02-06 Created: 2017-02-03 Last updated: 2019-04-29Bibliographically approved
5. Non-Degeneracy and Effects of Pinning in Strongly Coupled Vortex Pairs
Open this publication in new window or tab >>Non-Degeneracy and Effects of Pinning in Strongly Coupled Vortex Pairs
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2017 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, no 11, article id 4400505Article in journal (Refereed) Published
Abstract [en]

We study the effects of pinning on the quasi-static behavior of stacked, strongly coupled spin-vortex pairs in magnetic multilayered nanopillars, with vertical vortex separation small compared with the vortex-core size. The small separation causes the core-core interaction to be the dominant energy contribution for small applied fields and excitations, which results in highly non-linear dynamics. The properties of such a vortex pair are expected to only be dependent on the relative vortex core polarizations and relative chiralities, so that the individual configurations should be degenerated. We show how pinning can lift this degeneracy, which can be used to distinguish the individual chirality configurations.

Place, publisher, year, edition, pages
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2017
Keywords
Asymmetric synthetic antiferromagnets, spin vortices, vortex pinning
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-217736 (URN)10.1109/TMAG.2017.2697204 (DOI)000413981300198 ()2-s2.0-85032946588 (Scopus ID)
Conference
IEEE International Magnetics Conference (Intermag), APR 24-28, 2017, Dublin, IRELAND
Note

QC 20171122

Available from: 2017-11-22 Created: 2017-11-22 Last updated: 2019-04-29Bibliographically approved
6. Static and dynamic properties of vortex pairs in asymmetric nanomagnets
Open this publication in new window or tab >>Static and dynamic properties of vortex pairs in asymmetric nanomagnets
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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
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
Keywords
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:nbn:se:kth:diva-186979 (URN)10.1063/1.4944515 (DOI)000377962500262 ()2-s2.0-84961589525 (Scopus ID)
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

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