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Magnetization Dynamics in Nano-Contact Spin Torque Oscillators: Solitonic bullets and propagating spin waves
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetization dynamics in nano-contact spin torque oscillators (STOs) is investigated from an experimental and theoretical point of view. The fundamentals of magnetization dynamics due to spin transfer torque are given.

A custom-made high frequency (up to 46 GHz) in large magnetic fields (up to 2.2 T) microwave characterization setup has been built for the purpose and described in this thesis. A unique feature of this setup is the capability of applying magnetic fields at any direction θe out of the sample plane, and with high precision.

This is particularly important, because the (average) out-of-plane angle of the STO free magnetic layer has fundamental impact on spin wave generation and STO operation.

By observing the spin wave spectral emission as a function of θe, we find that at angles θe below a certain critical angle θcr, two distinct spin wave modes can be excited: a propagating mode, and a localized mode of solitonic character (so called spin wave bullet). The experimental frequency, current threshold and frequency tuneability with current of the two modes can be described qualitatively by analytical models and quantitatively by numerical simulations. We are also able to understand the importance, so far underestimated, of the Oersted field in the dynamics of nano-contact STOs. In particular, we show that the Oersted field strongly affects the current tuneability of the propagating mode at subcritical angles, and it is also the fundamental cause of the mode hopping observed in the time-domain. This mode hopping has been observed both experimentally using a state-of-the-art real-time oscilloscope and corroborated by micromagnetic simulations. Micromagnetic simulations also reveal details of the spatial distribution of the spin wave excitations.

By investigating the emitted power as a function of θe, we observed two characteristic behaviors for the two spin wave modes: a monotonic increase of the power for increasing out-of-plane angles in the case of the propagating mode; an increase towards a maximum power followed by a drop of it at the critical angle for the localized mode. Both behaviors are reproduced by micromagnetic simulations. The agreement with the simulations offers also a way to better understand the precession dynamics, since the emitted power is strongly connected to the angular variation of the giant magnetoresistance signal.

We also find that the injection locking of spin wave modes with a microwave source has a strong dependence on θe, and reaches a maximum locking strength at perpendicular angles. We are able to describe these results in the theoretical framework of non-linear spin wave dynamics.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , xvii, 125 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610
Keyword [en]
magnetism, spintronics, thin magnetic films, spin transfer torque, magnetization dynamics, spin waves, microwave oscillators, giant magneto-resistance
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-26955ISBN: 978-91-7415-820-5 (print)OAI: oai:DiVA.org:kth-26955DiVA: diva2:373156
Public defence
2010-12-10, C2, Isafjordsgatan 26, KTH-Electrum, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note
QC 20101130Available from: 2010-11-30 Created: 2010-11-30 Last updated: 2011-01-12Bibliographically approved
List of papers
1. Spin torque oscillator frequency versus magnetic field angle: The prospect of operation beyond 65 GHz
Open this publication in new window or tab >>Spin torque oscillator frequency versus magnetic field angle: The prospect of operation beyond 65 GHz
2009 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 10Article in journal (Refereed) Published
Abstract [en]

We study the impact of the magnetic field angle on the oscillation frequency of a nanocontact spin torque oscillator (STO) in magnetic fields up to 2.1 T. A model based on a single nonlinear, nonpropagating spin wave mode is found to explain the experimental data. We observe oscillation frequencies as high as 46 GHz in high magnetic fields applied normal to the film plane, and we are able to extrapolate the maximum expected operating frequency to beyond 65 GHz for in-plane magnetic fields. The STO signal remains surprisingly strong at these conditions, which opens up for possible millimeter-wave applications.

Keyword
cobalt alloys, copper, iron alloys, nanocontacts, nanoelectronics, nickel alloys, oscillators, spin valves, spin waves, microwave generation, driven, multilayers, emission
Identifiers
urn:nbn:se:kth:diva-18250 (URN)10.1063/1.3097238 (DOI)000264280000058 ()2-s2.0-62549110885 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven magnetic nanocontacts
Open this publication in new window or tab >>Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven magnetic nanocontacts
Show others...
2010 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 21, 217204- p.Article in journal (Refereed) Published
Abstract [en]

Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.

Keyword
electric-current, excitation, multilayer
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-26958 (URN)10.1103/PhysRevLett.105.217204 (DOI)000284308500024 ()2-s2.0-78349249735 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20110104Available from: 2010-11-30 Created: 2010-11-30 Last updated: 2017-12-12Bibliographically approved
3. Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators
Open this publication in new window or tab >>Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators
Show others...
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 17, 174427- p.Article in journal (Refereed) Published
Abstract [en]

The integrated power and linewidth of a propagating and a self-localized spin-wave mode excited by spin-polarized current in an obliquely magnetized magnetic nanocontact are studied experimentally as functions of the angle theta(e) between the external bias magnetic field and the nanocontact plane. It is found that the power of the propagating mode increases monotonically with theta(e), while the power of the self-localized mode has a broad maximum near theta(e) = 40 degrees and exponentially vanishes near the critical angle theta(e) = 58 degrees, at which the localized mode disappears. The linewidth of the propagating mode in the interval of angles 58 degrees < theta(e) < 90 degrees, where only this mode is excited, is adequately described by the existing theory, while in the angular interval where both modes can exist the observed linewidth of both modes is substantially broadened due to the telegraph switching between the modes. Numerical simulations and an approximate analytical model give a good semiquantitative description of the observed results.

Keyword
Polarized Current, Microwave Generation, Driven, Emission, Waves
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-28215 (URN)10.1103/PhysRevB.85.174427 (DOI)000304251700004 ()2-s2.0-84861716952 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20120612. Updated from manuscript to article in journal.Available from: 2011-01-12 Created: 2011-01-12 Last updated: 2017-12-11Bibliographically approved
4. Phase locking of nanocontact spin torque osillators as a function of applied field angle
Open this publication in new window or tab >>Phase locking of nanocontact spin torque osillators as a function of applied field angle
Show others...
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-28217 (URN)
Note
QC 20110112Available from: 2011-01-12 Created: 2011-01-12 Last updated: 2011-01-12Bibliographically approved
5. Time-space analysis of spin wave  dynamics in nano-contact spin torque oscillators
Open this publication in new window or tab >>Time-space analysis of spin wave  dynamics in nano-contact spin torque oscillators
Show others...
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-28216 (URN)
Note
QC 20110112Available from: 2011-01-12 Created: 2011-01-12 Last updated: 2011-01-12Bibliographically approved

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