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  • 1.
    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.

  • 2.
    Cherepov, Sergiy
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Koop, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Galkin, A. Yu.
    Khymyn, R.S.
    Institute of Magnetism, Ukrainian Academy of Science.
    Ivanov, B. A.
    Institute of Magnetism, Ukrainian Academy of Science.
    Worledge, D. C.
    IBM T.J. Watson Researh Center.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Core-Core Dynamics in Spin Vortex Pairs2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 9, p. 097204-Article in journal (Refereed)
    Abstract [en]

    We investigate nanopillars in which two thin ferromagnetic particles are separated by a nanometer thin nonmagnetic spacer and can be set into stable spin vortex-pair configurations. We find that the previously unexplored limit of strong vortex core-core coupling can dominate the spin dynamics in the system. We observe experimentally and explain analytically and numerically how the 0.2 GHz gyrational resonance modes of the individual vortices are transformed into a 2 GHz collective rotational resonance mode in the configurations where the two cores form a bound pair.

  • 3.
    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.

  • 4.
    Koop, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Static and dynamic properties of uniform- and vortex-states in synthetic nanomagnets2016Doctoral 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.

  • 5.
    Koop, Björn C.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Dzhezherya, Y. I.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Quantitative magnetic characterization of synthetic ferrimagnets for predictive spin-dynamic behavior2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 11, article id 6971569Article in journal (Refereed)
    Abstract [en]

    Geometric or magnetic asymmetries in synthetic antiferromagnetic particles give rise to ferrimagnetic-like magnetization behavior, both quasi-static and dynamic. Such asymmetries in synthetic ferrimagnets can originate from a thickness imbalance or a fringing field from the reference layer in a nanopillar stack. In this paper, we theoretically describe the effects of the corresponding magnetic asymmetry contribution on the structure's static and spin-dynamic behavior. The developed model is then used to experimentally determine the asymmetry parameters of typical nanoscale spin-flop junctions, as well as successfully describe their microwave resonant properties, such as the frequency splitting and the field dependence of the optical spin-resonance spectra.

  • 6.
    Koop, Björn C.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Dzhezherya, Yu. I.
    Demishev, K.
    Yurchuk, V.
    Worledge, D. C.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Demonstration of bi-directional microwave-assisted magnetic reversal in synthetic ferrimagnets2013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 14, p. 142408-Article in journal (Refereed)
    Abstract [en]

    We study spin dynamics in synthetic ferrimagnets, in which two dipole-coupled magnetic nanoparticles are different in thickness or biased asymmetrically with an external field. We observe a splitting of the optical spin-resonance for the two antiparallel ground states of the system and demonstrate how this splitting can be used to deterministically select a particular ground state. The demonstrated resonant switching is a fast and low-field way of controlling the magnetic state of nanodevices currently used in such large scale applications as magnetic random access memory.

  • 7.
    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.

  • 8.
    Koop, Björn C.
    et al.
    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.
    Nonlinear dynamics in spin vortex pairs with strong core-core coupling2014In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 11, article id 6971321Article in journal (Refereed)
    Abstract [en]

    We investigate the dynamics of spin vortex pairs in magnetic multilayer particles, with the vortices closely spaced vertically and therefore with strong core-core coupling. We focus on the spin-dynamic behavior of the system beyond the linear small-signal regime, and on the state with antiparallel vortex chiralities and parallel cores, in which the vortex cores are strongly dipole coupled. The data show a clear transition from the dominant single rotational resonance at 2-3 GHz for small excitation field amplitudes to a dominant gyrational resonance at high excitation fields. The concomitant changes in the microwave spectra, seen as satellite resonances near the rotational peak as well as a pronounced low-frequency resonance, are interpreted as arising from the nonlinearities of the main rotational mode, which also mediate microwave power transfer from the high- to the low-frequency mode.

  • 9.
    Koop, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Descamps, Thomas
    KTH.
    Holmgren, Erik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Relaxation-Free and Inertial Switching in Synthetic Antiferromagnets Subject to Super-Resonant Excitation2017In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, no 11, article id 4300505Article in journal (Refereed)
    Abstract [en]

    Applications of magnetic memory devices greatly benefit from ultra-fast, low-power switching. In this paper, we propose a method for how this can be achieved efficiently in a nano-sized synthetic antiferromagnet by using perpendicular-to-the-plane picosecondrange magnetic-field pulses. Our detailed micromagnetic simulations, supported by analytical results, yield the parameter space where inertial switching and relaxation-free switching can be achieved in the system. We furthermore discuss the advantages of dynamic switching in synthetic antiferromagnets and, specifically, their relatively low-power switching as compared with that in single ferromagnetic particles. Finally, we show how the excitation of spin waves in the system can be used to significantly reduce the post-switching spin oscillations for practical device geometries.

  • 10.
    Koop, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Descamps, Thomas
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Relaxation-free and inertial switching in synthetic antiferromagnets subject to super-resonant excitationManuscript (preprint) (Other academic)
    Abstract [en]

    Applications of magnetic memory devices greatly benefit from ultra-fast, low-power switching. Here we propose how this can be achieved efficiently in a nano-sized synthetic antiferromagnet by using perpendicular-to-the-plane picosecond-range magnetic field pulses. Our detailed micromagnetic simulations, supported by analytical results, yield the parameter space where inertial switching and relaxation-free switching can be achieved in the system. We furthermore discuss the advantages of dynamic switching in synthetic antiferromagnets and, specifically, their relatively low-power switching as compared to that in single ferromagnetic particles. Finally, we show how excitation of spin-waves in the system can be used to significantly reduce the post-switching spin oscillations for practical device geometries.

  • 11.
    Kravets, Anatolii F.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics. National Academy of Sciences of Ukraine, Ukraine.
    Dzhezherya, Yu I.
    Tovstolytkin, A. I.
    Kozak, I. M.
    Gryshchuk, A.
    Savina, Yu O.
    Pashchenko, V. A.
    Gnatchenko, S. L.
    Koop, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Synthetic ferrimagnets with thermomagnetic switching2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 10, p. 104427-Article in journal (Refereed)
    Abstract [en]

    Interlayer exchange coupling in strong/weak/strong ferromagnetic multilayers is investigated as a function of external magnetic field and temperature, with the focus on the magnetization switching near the Curie transition in the spacer composed of a diluted ferromagnet of concentration paramagnetic in the bulk. The effect of an externally applied reversing magnetic field on the width of the thermomagnetic transition is studied experimentally and explained theoretically as a result of the interplay between the proximity-induced exchange and the Zeeman effects in the system. Of high potential for applications should be the ability to switch one of the ferromagnetic outer layers using magnetic field, temperature, or a combination of the two.

1 - 11 of 11
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