Resonant switching and vortex dynamics in spin-flop bi-layers
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
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
Condensed Matter Physics Condensed Matter Physics Condensed Matter Physics
IdentifiersURN: urn:nbn:se:kth:diva-27193ISBN: 978-91-7415-840-3OAI: oai:DiVA.org:kth-27193DiVA: diva2:375676
2010-12-17, FB:42, AlbaNova University Center, KTH,Roslagstullbacken 21, Stockholm, 17:14 (English)
Silva, Thomas, Dr.
Korenivski, Vladislav, Professor
QC 201012092010-12-092010-12-082010-12-09Bibliographically approved
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