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Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven magnetic nanocontacts
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
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2010 (English)In: Physical Review Letters, ISSN 0031-9007, 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.

Place, publisher, year, edition, pages
2010. Vol. 105, no 21, 217204- p.
Keyword [en]
electric-current, excitation, multilayer
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-26958DOI: 10.1103/PhysRevLett.105.217204ISI: 000284308500024ScopusID: 2-s2.0-78349249735OAI: diva2:373170
Swedish Research CouncilKnut and Alice Wallenberg Foundation
QC 20110104Available from: 2010-11-30 Created: 2010-11-30 Last updated: 2011-04-15Bibliographically approved
In thesis
1. Magnetization Dynamics in Nano-Contact Spin Torque Oscillators: Solitonic bullets and propagating spin waves
Open this publication in new window or tab >>Magnetization Dynamics in Nano-Contact Spin Torque Oscillators: Solitonic bullets and propagating spin waves
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.
Trita-ICT/MAP AVH, ISSN 1653-7610
magnetism, spintronics, thin magnetic films, spin transfer torque, magnetization dynamics, spin waves, microwave oscillators, giant magneto-resistance
National Category
Condensed Matter Physics
urn:nbn:se:kth:diva-26955 (URN)978-91-7415-820-5 (ISBN)
Public defence
2010-12-10, C2, Isafjordsgatan 26, KTH-Electrum, Kista, 10:00 (English)
Swedish Research Council
QC 20101130Available from: 2010-11-30 Created: 2010-11-30 Last updated: 2011-01-12Bibliographically approved

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