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Chaotic dynamics in spin-vortex pairs
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0002-9092-093X
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
Institute of Magnetism, National Academy of Science, 03142 Kiev, Ukraine; National University of Science and Technology “MISiS”, Moscow 119049, Russian Federation..
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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.

Place, publisher, year, edition, pages
2019. Vol. 99, article id 054402
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-248013DOI: 10.1103/PhysRevB.99.054402ISI: 000457728700004Scopus ID: 2-s2.0-85061380893OAI: oai:DiVA.org:kth-248013DiVA, id: diva2:1301562
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
In thesis
1. 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
2. Resonant vortex-pair dynamics and magnetocalorics in magnetic nanostructures
Open this publication in new window or tab >>Resonant vortex-pair dynamics and magnetocalorics in magnetic nanostructures
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates the dynamics of spin vortex pairs in nanopillars andthermal effects in magnetic multilayers with RKKY-like indirect exchange.

Spin vortices are being investigated as storage elements for memory applicationsas well as for GHz oscillators and signal processing devices. In this work, spin vortex pairs in synthetic antiferromagnets (SAF’s) are studied. A SAF consists of two magnetic layers separated by a thin nonmagnetic spacer. The two magnetic layers can be set in to a vortex-pair state, one of which, with parallel core polarizationand antiparallel chirality (the P-AP pair-state), is rather unique as regards tomagnetostatics and spin dynamics. We show experimentally how the low temperaturecore-core hysteresis is modified by the presence of three types of asymmetryin the magnetic layers of the SAF: bias-field asymmetry, thickness imbalance, andcore pinning. Of special interest is that suitably designed core pinning can lift thedegeneracy of the different chirality states of the vortex pair.

We show that decoupling of strongly bound cores in a P-AP vortex pair can be resonantly enhanced by microwave fields of frequency matching the rotationalresonance of the pair at 3 GHz. As the excitation amplitude increases the pairexperience a period-doubling cascade, which eventually results in chaotic dynamics. Still higher microwave amplitudes result in dynamic deterministic decoupling, which takes place independently of thermal fluctuations. Introducing anharmonicity into the excitation reduces the duration required for the core-core decoupling by an order of magnitude. Defects within the magnetic layers modify the system’s spectral properties, which are in-depth investigated.

RKKY-like indirect exchange interaction in ferromagnetic/nonmagnetic multilayers is an interface effect, the sign of which depends on the thickness of the nonmagnetic spacer separating the ferromagnetic layers. This interaction is known to be insensitive to external control once the multilayer is fabricated. In this thesis, novel thermal control of indirect exchange coupling in specially designed multilayersis demonstrated. Two materials systems are investigated. The first incorporates a uniform Fe-Cr spacer separating two Fe layers, with the thickness chosen to correspond to the antiferromagnetic peak in the RKKY. Direct exchange across the spacer strongly couples the two Fe layers at low temperatures. Heating the system above the Curie temperature of the spacer results in antiferromagnetic indirect RKKY interlayer coupling. A rather strong magnetic proximity effect at the interfaces broadens the transition and weakens the indirect exchange. Introducing thin Cr layers at the two Fe-Cr interfaces suppresses the direct ferromagnetic exchange and sequentially transmits indirect RKKY exchange. We show that in this gradient-spacer multilayer design the thermal phase transition is significantly narrower, allowing thermal switching between indirect ferromagnetic and indirect antiferromagnetic exchange coupling of the outer magnetic layers.

We demonstrate, using RKKY exchange biasing, a reversible Curie transition in weakly ferromagnetic spacer layers. Thermal on/off switching of the spacer magnetization results in two different entropy states and a strong magnetocaloric effect. The low-field magnitude of the effect rivals that in many advanced bulk rare-earthbased materials, with the full effect being achieved in the 10 mT field range.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. p. 92
Series
TRITA-SCI-FOU ; 2019:21
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-250386 (URN)978-91-7873-177-0 (ISBN)
Public defence
2019-05-24, FB52, AlbaNova universitetscentrum, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20190430

Available from: 2019-04-30 Created: 2019-04-29 Last updated: 2019-04-30Bibliographically approved

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Bondarenko, ArtemHolmgren, ErikKorenivski, Vladislav

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