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Resonant switching and vortex dynamics in spin-flop bi-layers
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is a study of the static and dynamic behavior of the magne-tization in spin-flop bi-layers, which consist of two soft ferromagnetic layerscoupled by dipolar forces through a thin nonmagnetic spacer. The focus ofthe work is three fold: collective spin dynamics in the anti-parallel groundstate; resonant switching in the presence of thermal agitation; and static anddynamic behavior of the system in the vortex-pair state, with a particularemphasis on the interlayer core-core interaction.

Two collective spin-flop resonance modes are observed and interpreted asacoustical and optical spin precessions, in which the moments of the two lay-ers oscillate in phase and out of phase, respectively. An analytical macrospinmodel is developed to analyze the experimental results and is found to ac-curately predict the resonance frequencies and their field dependence in thelow-field anti-parallel state and the high-field near saturated state. A micro-magnetic model is developed and successfully explains the static and dynamicbehavior of the system in the entire field range, including the C- and S-typespin-perturbed scissor state of the bi-layer at intermediate fields.

The optical spin-flop resonance at 3-4 GHz is used to demonstrate resonantswitching in the system, in the range of the applied field where quasi-staticswitching is forbidden. An off-axis field of relatively small amplitude canexcite large-angle scissor-like oscillations at the optical resonance frequency,which can result in a full 180-degree reversal, with the two moments switchingpast each other into the mirror anti-parallel state. It is found that the switch-ing probability increases with increasing the duration of the microwave fieldpulse, which shows that the resonant switching process is affected by thermalagitation. Micromagnetic modeling incorporating the effect of temperature isperformed and is in good agreement with the experimental results.

Vortex pair states in spin-flop bi-layers are produced using high amplitudefield pulses near the optical spin resonance in the system. The stable vortex-pair states, 16 in total, of which 4 sub-classes are non-degenerate in energy, areidentified and investigated using static and dynamic applied fields. For AP-chirality vortex-pair states, the system can be studied while the two vortexcores are coupled and decoupled in a single field sweep. It is found thatthe dynamics of the AP-chirality vortex pairs is critically determined by thepolarizations of the two vortex cores and the resulting attractive or repulsivecore-core interaction. The measured spin resonance modes in the system areinterpreted as gyrational, rotational, and vibrational resonances with the helpof the analytical and micromagnetic models developed herein.

A significant effort during this project was made to build two instrumentsfor surface and transport characterization of magnetic nanostructures: a high-current Scanning Tunneling Microscope for studying transport in magneticpoint contacts, and a Current In Plane Tunneling instrument for characteriz-ing unpatterned magnetic tunnel junctions. The design and implementationof the instruments as well as the test data are presented.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , v, 80 p.
Series
Trita-FYS, ISSN 0280-316X ; 2010:74
Keyword [en]
spin-dynamics, MRAM, vortex, switching
National Category
Condensed Matter Physics Condensed Matter Physics Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-27193ISBN: 978-91-7415-840-3 (print)OAI: oai:DiVA.org:kth-27193DiVA: diva2:375676
Public defence
2010-12-17, FB:42, AlbaNova University Center, KTH,Roslagstullbacken 21, Stockholm, 17:14 (English)
Opponent
Supervisors
Note
QC 20101209Available from: 2010-12-09 Created: 2010-12-08 Last updated: 2010-12-09Bibliographically approved
List of papers
1. Spin dynamics of two-coupled nanomagnets in spin-flop tunnel junctions
Open this publication in new window or tab >>Spin dynamics of two-coupled nanomagnets in spin-flop tunnel junctions
Show others...
2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 14, 144425-1-144425-6 p.Article in journal (Refereed) Published
Abstract [en]

Collective spin dynamics of two dipole-coupled nanomagnets in spin-flop tunnel junctions are studied experimentally and theoretically. The measured GHz magnetization oscillations reveal several collective spin-precessional modes. Analytical macrospin and numerical micromagnetic models of the spin-flop dynamics are developed, which provide a detailed explanation of the observed frequency spectra in terms of optical, acoustical, and micromagnetic modes in the antiparallel, parallel, and scissor magnetization states of the junctions.

Keyword
RANDOM-ACCESS MEMORY; MAGNETIC THIN-FILMS; TOGGLE MRAM; REVERSAL
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-8681 (URN)10.1103/PhysRevB.80.144425 (DOI)000271351500082 ()2-s2.0-71449109792 (Scopus ID)
Note
QC 20100818. Uppdaterad från manuskript till artikel (20100818). Tidigare titel: Spin dynamics of two coupled nanomagnets in spin-flop tunnelAvailable from: 2008-06-04 Created: 2008-06-04 Last updated: 2017-12-14Bibliographically approved
2. Resonant Switching of Two Dipole-Coupled Nanomagnets
Open this publication in new window or tab >>Resonant Switching of Two Dipole-Coupled Nanomagnets
2010 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, 2112-2115 p.Article in journal (Refereed) Published
Abstract [en]

The storage layer of recently developed spin-flop magnetic random-access memory consists of two closely spaced dipole-coupled nanomagnets and is highly stable in the ground state as well as in quasistatic fields applied off the easy axis. We show experimentally and confirm by using micromagnetic simulations that these spin-flop bilayers can be switched relatively easily by dynamic fields, applied at the frequency of the optical spin resonance of the bilayer. The field amplitude sufficient for this resonant switching can be an order of magnitude lower than the fields necessary for quasistatic reversal. Our data and micromagnetic analysis suggest that thermal agitation can play a role in the observed resonant switching behavior.

Keyword
Magnetic random-access memory (MRAM), micromagnetic simulations, resonant switching, spin-flop bilayers
Identifiers
urn:nbn:se:kth:diva-27188 (URN)10.1109/TMAG.2010.2043715 (DOI)000278037800217 ()2-s2.0-77952825643 (Scopus ID)
Note
QC 20101208Available from: 2010-12-08 Created: 2010-12-08 Last updated: 2017-12-11Bibliographically approved
3. Micromagnetics of Spin-Flop Bilayers: S, C, and Vortex Spin States
Open this publication in new window or tab >>Micromagnetics of Spin-Flop Bilayers: S, C, and Vortex Spin States
2010 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, 2124-2127 p.Article in journal (Refereed) Published
Abstract [en]

Spin-flop tunnel junctions subjected to strong gigahertz excitations are found to exhibit highly stable resistance states, intermediate between the two spin-uniform ground states of high and low resistance. The associated spin distributions are necessarily nonuniform and differ significantly from the ground-state anti-parallel spin configuration in their static and dynamic properties. Detailed micromagnetic modeling reveals that inplane spin vortices in dipole-coupled thin-film bilayers are stable spin configurations, where the orientation of the vortex cores and the vortex chirality play an important role in the response of the system to external magnetic fields.

Keyword
MRAM, spin vortices, spin-flop tunnel junctions
Identifiers
urn:nbn:se:kth:diva-27189 (URN)10.1109/TMAG.2010.2043716 (DOI)000278037800220 ()2-s2.0-77952836066 (Scopus ID)
Note
QC 20101208Available from: 2010-12-08 Created: 2010-12-08 Last updated: 2017-12-11Bibliographically approved
4. 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
5. Resonant activation of asynthetic antiferromagnet
Open this publication in new window or tab >>Resonant activation of asynthetic antiferromagnet
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-27198 (URN)
Note
QC 20101209Available from: 2010-12-09 Created: 2010-12-09 Last updated: 2012-02-10Bibliographically approved

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