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Resonant vortex-pair dynamics and magnetocalorics in magnetic nanostructures
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
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: urn:nbn:se:kth:diva-250386ISBN: 978-91-7873-177-0 (print)OAI: oai:DiVA.org:kth-250386DiVA, id: diva2:1307890
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
List of papers
1. 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
2. Relaxation-Free and Inertial Switching in Synthetic Antiferromagnets Subject to Super-Resonant Excitation
Open this publication in new window or tab >>Relaxation-Free and Inertial Switching in Synthetic Antiferromagnets Subject to Super-Resonant Excitation
2017 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, no 11, article id 4300505Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2017
Keywords
Magnetization dynamics, synthetic antiferromagnets, ultrafast switching
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-217737 (URN)10.1109/TMAG.2017.2707589 (DOI)000413981300189 ()2-s2.0-85032895060 (Scopus ID)
Conference
IEEE International Magnetics Conference (Intermag), APR 24-28, 2017, Dublin, Ireland
Funder
Swedish Research Council, 2014-4548
Note

QC 20171122

Available from: 2017-11-22 Created: 2017-11-22 Last updated: 2019-04-29Bibliographically approved
3. 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
4. Thermally induced antiferromagnetic exchange in magnetic multilayers
Open this publication in new window or tab >>Thermally induced antiferromagnetic exchange in magnetic multilayers
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 10, article id 104427Article in journal (Refereed) Published
Abstract [en]

We demonstrate sharp thermally induced switching between ferromagnetic and antiferromagnetic RKKY ( Ruderman-Kittel-Kasuya-Yosida) exchange in a spin-valve with the spacer incorporating a thin diluted ferromagnetic layer as the core. We illustrate the mechanism behind the effect as being due to a change in the effective thickness of the spacer induced by the Curie transition into its paramagnetic state. The ability to switch between ferromagnetic and antiferromagnetic states in a magnetic multilayer by a slight change in temperature may lead to new types of spin-thermoelectronic devices for use in such applications as memory or oscillators.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-215448 (URN)10.1103/PhysRevB.96.104427 (DOI)000411320900002 ()2-s2.0-85029945218 (Scopus ID)
Note

QC 20171019

Available from: 2017-10-19 Created: 2017-10-19 Last updated: 2019-04-29Bibliographically approved
5. 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
6. 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 ()
Note

QC 20180529

Available from: 2018-05-29 Created: 2018-05-29 Last updated: 2019-04-29Bibliographically approved
7. Giant magnetocaloric effect driven by indirect exchange in magnetic multilayers
Open this publication in new window or tab >>Giant magnetocaloric effect driven by indirect exchange in magnetic multilayers
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2018 (English)In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 2, no 11, article id 114402Article in journal (Refereed) Published
Abstract [en]

Indirect exchange coupling in magnetic multilayers, also known as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, is highly effective in controlling the interlayer alignment of the magnetization. This coupling is typically fixed at the stage of the multilayer fabrication and does not allow ex situ control needed for device applications. In addition to the orientational control, it is highly desirable to also control the magnitude of the intralayer magnetization, ideally, being able to switch it on/off by switching the relevant RKKY coupling. Here we demonstrate a magnetic multilayer material incorporating thermally and field-controlled RKKY exchange, focused on a dilute ferromagnetic alloy layer and driving it though its Curie transition. Such on/off magnetization switching of a thin ferromagnet, performed repeatedly and fully reproducibly within a low-field sweep, results in a giant magnetocaloric effect, with an estimated isothermal entropy change of Delta S approximate to -10 mJ cm(-3) K(-1 )under an external field of similar to 10 mT, which greatly exceeds the performance of the best rare-earth based materials used in the adiabatic-demagnetization refrigeration systems.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-239779 (URN)10.1103/PhysRevMaterials.2.114402 (DOI)000450572600002 ()2-s2.0-85060615826 (Scopus ID)
Note

QC 20190108

Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-04-29Bibliographically approved
8. Effects of asymmetry in strongly coupled spin vortex pairs
Open this publication in new window or tab >>Effects of asymmetry in strongly coupled spin vortex pairs
2019 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 52, no 10, article id 105001Article in journal (Refereed) Published
Abstract [en]

Effects of magnetic asymmetry on strongly coupled spin-vortex pairs with parallel core polarization and antiparallel chirality in synthetic nanomagnets are investigated. This includes vortex-core length asymmetry, biasing field asymmetry, and pinning of one of the two vortex cores. Our experimental observations as well as analytical and micromagnetic modeling show how magnetic asymmetry can be used to differentiate magneto-resistively otherwise degenerate multiple stable states of a vortex pair. These results expand the knowledge base for spin vortex arrays in nanostructures and should be useful in light of the recent proposals on coding information into multiple topological spin states, such as single and multiple vortex core/chirality states.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
vortex core pinning, magnetic vortex memory, magnetic vortex pairs
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-241298 (URN)10.1088/1361-6463/aaf8f7 (DOI)000455128200001 ()2-s2.0-85060232971 (Scopus ID)
Note

QC 20190222

Available from: 2019-02-22 Created: 2019-02-22 Last updated: 2019-04-29Bibliographically approved
9. 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

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