Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Static and dynamic properties of uniform- and vortex-states in synthetic nanomagnets
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0002-9993-4748
2016 (English)Doctoral 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.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. , 74 p.
Series
TRITA-FYS, ISSN 0280-316X
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-187473ISBN: 978-91-7729-018-6 (print)OAI: oai:DiVA.org:kth-187473DiVA: diva2:930458
Public defence
2016-06-15, FB54, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2014-4548Stiftelsen Olle Engkvist Byggmästare, 2014-STE
Note

QC 20160524

Available from: 2016-05-24 Created: 2016-05-24 Last updated: 2016-05-25Bibliographically approved
List of papers
1. Resonant Activation of a Synthetic Antiferromagnet
Open this publication in new window or tab >>Resonant Activation of a Synthetic Antiferromagnet
2011 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 7, 077202- p.Article in journal (Refereed) Published
Abstract [en]

The magnetic decay time of a synthetic antiferromagnet comprised of two closely spaced magnetic dipoles is measured in the presence of microwave excitation. The system is known to be highly stable with respect to switching between its two antiparallel ground states under quasistatic magnetic fields. We show that an order of magnitude lower field can switch the pair, provided the field is applied in resonance with the optical eigenmode of the collective spin dynamics in the system. We furthermore show that thermal agitation can play an essential role in spin-flop switching for resonant excitations of near-or subcritical amplitude.

Keyword
coupled nanomagnets, tunnel-junctions, toggle mram, field, model
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-39043 (URN)10.1103/PhysRevLett.107.077202 (DOI)000293705600023 ()2-s2.0-80051511197 (Scopus ID)
Note

QC 20150623

Available from: 2011-09-07 Created: 2011-09-07 Last updated: 2017-12-08Bibliographically approved
2. Core-Core Dynamics in Spin Vortex Pairs
Open this publication in new window or tab >>Core-Core Dynamics in Spin Vortex Pairs
Show others...
2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 9, 097204- p.Article in journal (Refereed) Published
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.

Keyword
Magnetic Vortex, Motion, Excitations, Reversal, Permalloy, Vortices, Dots
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-27197 (URN)10.1103/PhysRevLett.109.097204 (DOI)000308016200009 ()2-s2.0-84865603452 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20101209. Updated from manuscript to article in journal.

Available from: 2012-06-13 Created: 2010-12-09 Last updated: 2017-12-11Bibliographically approved
3. Demonstration of bi-directional microwave-assisted magnetic reversal in synthetic ferrimagnets
Open this publication in new window or tab >>Demonstration of bi-directional microwave-assisted magnetic reversal in synthetic ferrimagnets
Show others...
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 14, 142408- p.Article in journal (Refereed) Published
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.

Keyword
Random-Access Memory, Giant Magnetoresistance, Tunnel-Junctions, Multilayers, Mram
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-133647 (URN)10.1063/1.4824016 (DOI)000325488500057 ()2-s2.0-84885578329 (Scopus ID)
Note

QC 20131111

Available from: 2013-11-11 Created: 2013-11-08 Last updated: 2017-12-06Bibliographically approved
4. Quantitative magnetic characterization of synthetic ferrimagnets for predictive spin-dynamic behavior
Open this publication in new window or tab >>Quantitative magnetic characterization of synthetic ferrimagnets for predictive spin-dynamic behavior
2014 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 11, 6971569Article in journal (Refereed) Published
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.

Keyword
Magnetic multilayers, magnetization reversal, microwave magnetics, spin valves
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-161498 (URN)10.1109/TMAG.2014.2316596 (DOI)000349465900269 ()2-s2.0-84915820027 (Scopus ID)
Note

QC 20150313

Available from: 2015-03-13 Created: 2015-03-12 Last updated: 2017-12-04Bibliographically approved
5. Nonlinear dynamics in spin vortex pairs with strong core-core coupling
Open this publication in new window or tab >>Nonlinear dynamics in spin vortex pairs with strong core-core coupling
2014 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 11, 6971321Article in journal (Refereed) Published
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.

Keyword
Microwave spectra, nonlinear dynamics, rotational and gyrational resonances, spin vortex pairs
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-161496 (URN)10.1109/TMAG.2014.2327157 (DOI)000349465900272 ()2-s2.0-84916204514 (Scopus ID)
Note

QC 20150313

Available from: 2015-03-13 Created: 2015-03-12 Last updated: 2017-12-04Bibliographically 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
Show others...
2016 (English)In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 6, no 5, 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
Keyword
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: 2017-11-30Bibliographically approved
7. 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
(English)Manuscript (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.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-187472 (URN)
Funder
Swedish Research Council, 2014-4548
Note

QC 20160524

Available from: 2016-05-24 Created: 2016-05-24 Last updated: 2016-05-24Bibliographically approved

Open Access in DiVA

fulltext(13477 kB)178 downloads
File information
File name FULLTEXT01.pdfFile size 13477 kBChecksum SHA-512
ee1e379878320bdc1ee5d937d167b4baef25e04cd70be3b6fc02c7c59b1a4ceb4ce7fabbeb9720410cdec1dea1975e55b15f53b871177b1014c48d9138fa3697
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Koop, Björn
By organisation
Nanostructure Physics
Condensed Matter Physics

Search outside of DiVA

GoogleGoogle Scholar
Total: 178 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 700 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf