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  • 1.
    Bondarenko, Artem
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Nonlinear dynamics of strongly-bound magnetic vortex pairs2019Doctoral 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.

    Download full text (pdf)
    Bondarenko-thesis
  • 2.
    Bondarenko, Artem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Holmgren, Erik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Koop, Björn C.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Descamps, Thomas
    KTH.
    Ivanov, B. A.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Stochastic dynamics of strongly-bound magnetic vortex pairs2017In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 7, no 5, article id 056007Article in journal (Refereed)
    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.

  • 3.
    Bondarenko, Artem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Holmgren, Erik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Li, Zhong Wei
    Ivanov, B. A.
    Institute of Magnetism, National Academy of Science, 03142 Kiev, Ukraine; National University of Science and Technology “MISiS”, Moscow 119049, Russian Federation..
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Chaotic dynamics in spin-vortex pairs2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, article id 054402Article in journal (Refereed)
    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.

  • 4.
    Holmgren, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Bondarenko, Artem
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. Institute of Magnetism, National Academy of Science, 03142 Kiev, Ukraine.
    Koop, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ivanov, Boris
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Non-Degeneracy and Effects of Pinning in Strongly Coupled Vortex Pairs2017In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, no 11, article id 4400505Article in journal (Refereed)
    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.

  • 5.
    Holmgren, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Bondarenko, Artem
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. Natl Acad Sci, Kiev, Ukraine..
    Persson, M.
    KTH.
    Ivanov, B. A.
    Natl Acad Sci, Inst Magnetism, UA-03142 Kiev, Ukraine.;Natl Univ Sci & Technol MISiS, Moscow 119049, Russia..
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Transient dynamics of strongly coupled spin vortex pairs: Effects of anharmonicity and resonant excitation on inertial switching2018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 19, article id 192405Article in journal (Refereed)
    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.

  • 6.
    Koop, Björn C.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Gruschke, M.
    KTH.
    Descamps, Thomas
    KTH.
    Bondarenko, Artem
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Ivanov, B. A.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Static and dynamic properties of vortex pairs in asymmetric nanomagnets2016In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 6, no 5, article id 056406Article in journal (Refereed)
    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.

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