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Spin Torque-Generated Magnetic Droplet Solitons
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
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2013 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 339, no 6125, 1295-1298 p.Article in journal (Refereed) Published
Abstract [en]

Dissipative solitons have been reported in a wide range of nonlinear systems, but the observation of their magnetic analog has been experimentally challenging. Using spin transfer torque underneath a nanocontact on a magnetic thin film with perpendicular magnetic anisotropy (PMA), we have observed the generation of dissipative magnetic droplet solitons and report on their rich dynamical properties. Micromagnetic simulations identify a wide range of automodulation frequencies, including droplet oscillatory motion, droplet "spinning," and droplet "breather" states. The droplet can be controlled by using both current and magnetic fields and is expected to have applications in spintronics, magnonics, and PMA-based domain-wall devices.

Place, publisher, year, edition, pages
2013. Vol. 339, no 6125, 1295-1298 p.
Keyword [en]
Waves, Excitation, States
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-105274DOI: 10.1126/science.1230155ISI: 000316053400031Scopus ID: 2-s2.0-84874995646OAI: oai:DiVA.org:kth-105274DiVA: diva2:570492
Funder
EU, FP7, Seventh Framework Programme, ICT-257159 "MACALO"Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20130411

Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Spin valves and spin-torque oscillators with perpendicualr magnetic anisotropy
Open this publication in new window or tab >>Spin valves and spin-torque oscillators with perpendicualr magnetic anisotropy
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Researches in spintronics, especially those remarkably classified in the current induced spin-transfer torque (STT) framework, circumvent challenges with different materials and geometries. Perpendicular magnetic anisotropy (PMA) materials are showing capability of holding promise to be employed in STT based spintronics elements, e.g. spin-torque oscillators (STOs), STT-magnetoresistive random access memories (STT-MRAMs) and current induced domain wall motion elements.

This dissertation presents experimental investigations into developing sputter deposited Co/Ni multilayers (MLs) with PMA and employs these materials in nano-contact STOs (NC-STOs) based on giant magnetoresistance (GMR) effect and in pseudo-spin-valve (PSV) structures. The magnetostatic stray field coupling plays an important role in perpendicular PSVs. The temperature dependent coupling mechanism recommends that this coupling can be tailored, by i) the saturation magnetization and coercivity of the individual layers, ii) the coercivity difference in layers, and iii) the GMR spacer thickness, to get a well decoupled and distinguishable switching response. Moreover, this thesis focused on the implementation and detailed characterization of NC-STOs with strong PMA Co/Ni ML free layers and in-plane Co reference layers as orthogonal (Ortho) magnetic geometry in so-called Ortho-NC-STOs. The primary target of reaching record high STO frequencies, 12 GHz, at close to zero field, 0.02 Tesla, was achieved. However, in large external fields, >0.4 Tesla, an entirely new magnetodynamic object, a “magnetic droplet”, theoretically predicted in 1977, was discovered experimentally. Detailed experiments, combined with micromagnetic simulations, demonstrate the formation of a magnetic droplet with a partially reversed magnetization direction underneath the NC, and a zone of large amplitude precession in a region bounding the reversed magnetization. The magnetic droplet exhibits a very rich dynamics, including i) auto-modulation as a combine of droplet frequency with a slow time evolution (few GHz) of un-centering the droplet mode under the NC, ii) droplet breathing as reversible deformation of droplet mode with ½ droplet frequency. All observation of droplet opens a new mechanism of excitation for future fundamental studies as well as experiments especially for domain wall electronics and nano-scopic magnetism.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xii, 73 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:19
Keyword
Spin torque oscillator, perpendicular magnetic anisotropy
National Category
Nano Technology
Research subject
SRA - ICT
Identifiers
urn:nbn:se:kth:diva-105223 (URN)978-91-7501-545-3 (ISBN)
Public defence
2012-12-07, C2, Electrum, KTH-ICT, Isafjordsgatan 26, Kista, 09:00 (English)
Opponent
Supervisors
Note

QC 20121119

Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2012-11-19Bibliographically approved
2. Fabrication and Characterization of Nanocontact Spin-Torque Oscillators
Open this publication in new window or tab >>Fabrication and Characterization of Nanocontact Spin-Torque Oscillators
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The manufacturing of nanocontact-based spin-torque oscillators (NC-STOs)has opened the door for spintronic devices to play a part as active microwaveelements. The NC-STO has the capability of converting a direct current intoa microwave signal, and vice versa, by utilizing the spin transfer torque (STT)in ferromagnetic multilayer systems. However, the high-frequency operation ofNC-STOs typically requires high magnetic fields and the microwave power theygenerate is rather limited. As a result, NC-STOs are not yet commercially used,and they require improvements in both material systems and device geometriesbefore they can find actual use in microwave applications.

In order to improve and advance this technology, NC-STOs are requiredwith both different nanocontact (NC) sizes and geometries, and using differ- ent stacks of magnetic materials. This dissertation presents experimental in- vestigations into the manufacturing of such devices using different fabrication techniques and a number of different magnetic material stacks. Currently, the fabrication of NC-STOs is limited to advanced laboratories, because NC fabri- cation requires high-resolution lithography tools. In the present work, we have developed an alternative method of fabrication, which does not require such tools and has the capability of fabricating NC-STOs having one to hundreds of NCs in a variety of sizes, possibly  down to 20 nm. Devices fabricated with this method have shown mutual synchronization of three parallel-connected NCs, and pairwise synchronization in devices with four and five NCs.

Furthermore, the present work demonstrates low-field operation (down to0.02 Tesla) of NC-STOs at a record high frequency of 12 GHz. This wasachieved by implementing multilayers with a perpendicular magnetic anisotropy(PMA) material in the free layer of the NC-STO. In addition, the fabricateddevices revealed an unexpected dynamic regime under large external appliedfield (above 0.4 Tesla). The new dynamic regime was found to be due to anentirely novel nanomagnetic dynamic object â a so-called magnetic droplet soliton,predicted theoretically in 1977 but not experimentally observed until now.Detailed experiments and micromagnetic simulations show that the droplet hasvery rich dynamics.

Finally,  spin-torque-induced  transverse spin wave instabilities have beenstudied.  A NC-STO with  a material stack consisting of a single ferromag- netic metal sandwiched between two non-ferromagnetic metals was fabricated. Prior to this work, evidence of spin wave instabilities was reported as resis- tance switching in nanopillar- and mechanical point contact based STOs. In the present  work, the fabricated NC-STOs showed actual microwave  signals up to 3 GHz under zero applied field with strong current hysteresis. All  the fabricated NC-STOs open up new means of studying STT in different environ- ments, in order to resolve their current drawbacks for industrial applications.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xvi, 65 p.
Series
TRITA-ICT/MAP AVH, ISSN 1653-7610 ; 2013:04
Keyword
Spin-torque oscillators, phase locking, spin wave, giant magnetoresistance, spin transfer torque, thin films.
National Category
Physical Sciences Nano Technology
Identifiers
urn:nbn:se:kth:diva-122292 (URN)978-91-7501-760-0 (ISBN)
Public defence
2013-06-14, Sal/Hall E, Foru KTH-ITC, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20130527

Available from: 2013-05-27 Created: 2013-05-17 Last updated: 2014-01-14Bibliographically approved

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