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Micromagnetics of Spin-Flop Bilayers: S, C, and Vortex Spin States
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0002-2725-0558
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0003-2339-1692
IBM T.J. Watson Researh Center.
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.

Place, publisher, year, edition, pages
2010. Vol. 46, no 6, 2124-2127 p.
Keyword [en]
MRAM, spin vortices, spin-flop tunnel junctions
URN: urn:nbn:se:kth:diva-27189DOI: 10.1109/TMAG.2010.2043716ISI: 000278037800220ScopusID: 2-s2.0-77952836066OAI: diva2:375602
QC 20101208Available from: 2010-12-08 Created: 2010-12-08 Last updated: 2010-12-09Bibliographically approved
In thesis
1. Resonant switching and vortex dynamics in spin-flop bi-layers
Open this publication in new window or tab >>Resonant switching and vortex dynamics in spin-flop bi-layers
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.
Trita-FYS, ISSN 0280-316X ; 2010:74
spin-dynamics, MRAM, vortex, switching
National Category
Condensed Matter Physics Condensed Matter Physics Condensed Matter Physics
urn:nbn:se:kth:diva-27193 (URN)978-91-7415-840-3 (ISBN)
Public defence
2010-12-17, FB:42, AlbaNova University Center, KTH,Roslagstullbacken 21, Stockholm, 17:14 (English)
QC 20101209Available from: 2010-12-09 Created: 2010-12-08 Last updated: 2010-12-09Bibliographically approved

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