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Spin dynamics of two-coupled nanomagnets in spin-flop tunnel junctions
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.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0003-2339-1692
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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.

Place, publisher, year, edition, pages
2009. Vol. 80, no 14, 144425-1-144425-6 p.
Keyword [en]
RANDOM-ACCESS MEMORY; MAGNETIC THIN-FILMS; TOGGLE MRAM; REVERSAL
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-8681DOI: 10.1103/PhysRevB.80.144425ISI: 000271351500082Scopus ID: 2-s2.0-71449109792OAI: oai:DiVA.org:kth-8681DiVA: diva2:14065
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: 2010-12-09Bibliographically approved
In thesis
1. Spin transfer torques and spin dynamics in point contacts and spin-flop tunnel junctions
Open this publication in new window or tab >>Spin transfer torques and spin dynamics in point contacts and spin-flop tunnel junctions
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The first part of this thesis is an experimental study of the spin-dependent transport in magnetic point contacts. Nano-contacts are produced micromechanically, by bringing a sharpened non-magnetic (N) tip into contact with a ferromagnetic (F) film. The magnetic and magneto-transport properties of such N/F nanocontacts are studied using transport spectroscopy, spanning the ballistic, diffusive, and thermal transport regimes.

Single N/F interfaces can exhibit current driven magnetic excitations, which are often manifest as peaks in the differential resistance of a point contact defining the N/F interface. Our experiments show that such surface magnetization excitations, and thus the single-interface spin torques, are observed for diffusive and thermal transport regimes where the conduction electrons experience strong scattering near the N/F interface, and are absent for purely ballistic contacts. We conclude that the single-interface spin torque effect is due to impurity scattering at N/F interfaces.

Single N/F interfaces can also exhibit hysteretic conductivity, which is qualitatively similar to the spin-valve effect found in F/N/F trilayers. Based on our measurements of N/F point contacts in the size range of 1-30 nm, we propose two mechanisms of the observed hysteresis. The first mechanism relies on a non-uniform spin distribution near the contact core and is magnetoelastic in origin. This interpretation is in good agreement with some of our experiments on larger point contacts as well as with a numerical micromagnetic model we have developed, where a stress-induced anisotropy creates a non-uniform, domain-wall-like spin distribution in the contact core. The second mechanism we propose is a surface effect which relies on a difference between the surface and interior spins in the ferromagnet in terms of their exchange and anisotropy properties. The surface spin-valve mechanism is in good agreement with the hysteretic magnetoresistance observed for our smallest contacts (~1 nm) and for contacts to nanometer thin ferromagnetic films. This interpretation means that the surface magnetization can be reduced and weakly coupled to the interior spins in the ferromagnet. We find that this surface spin layer can be affected by both external fields and the spin torque of a transport current. The surface magnetization can even form nano-sized spin vorticies at the interface.

The nature of the magnetic excitations induced by by nominally unpolarized currents through single N/F interfaces was probed directly using microwave irradiation. We observed two characteristic high-frequency effects: a resonant stimulation of spin-wave modes by microwaves, and a rectification of off-resonant microwave currents by spin-wave nonlinearities in the point contact conductance. These experiments demonstrate that the effects observed are spin-dynamic in nature.

In the second part of the thesis we study the spin-dynamics in spin-flop tunnel junctions used in toggle magnetic random access memory. Current pulses in the range of 100 ps used to excite the magnetic moments of the two coupled Py free layers into an oscillatory state, in both the antiparallel and scissor states of the cell. These oscillations are detected directly by measuring the junction resistance in real time with a 6 GHz measurement bandwidth. The junctions had the shape of an ellipse, with lateral size ranging from 350x420 to 400x560 nm. The optical and acoustical precession modes of the the spin-flop trilayer are observed in experiment, as expected from single-domain model. The experimental spectra contain additional features, which are explained using numerical micromagnetic simulations, as originating from magnetic state transitions between different magnetization states with non-uniform spin distributions.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. v, 86 p.
Series
Trita-FYS, ISSN 0280-316X ; 2008-26
Keyword
spin dynamics, spin transfer torques, point contacts, magnetization dynamics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4805 (URN)978-91-7415-031-5 (ISBN)
Public defence
2008-06-13, FB52, Albanova Univ. Center, Roslagstullbacken 21, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100818Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2010-08-18Bibliographically approved
2. 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.
Series
Trita-FYS, ISSN 0280-316X ; 2010:74
Keyword
spin-dynamics, MRAM, vortex, switching
National Category
Condensed Matter Physics Condensed Matter Physics Condensed Matter Physics
Identifiers
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)
Opponent
Supervisors
Note
QC 20101209Available from: 2010-12-09 Created: 2010-12-08 Last updated: 2010-12-09Bibliographically approved

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Konovalenko, AlexanderKorenivski, Vladislav

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