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Fabrication and Characterization of Nanocontact Spin-Torque Oscillators
KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101), Material Physics, MF (Closed 20120101).ORCID iD: 0000-0002-3726-9738
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 [en]
Spin-torque oscillators, phase locking, spin wave, giant magnetoresistance, spin transfer torque, thin films.
National Category
Physical Sciences Nano Technology
Identifiers
URN: urn:nbn:se:kth:diva-122292ISBN: 978-91-7501-760-0 (print)OAI: oai:DiVA.org:kth-122292DiVA: diva2:621907
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
List of papers
1. Hole mask colloidal lithography on magnetic multilayers for spin torque applications
Open this publication in new window or tab >>Hole mask colloidal lithography on magnetic multilayers for spin torque applications
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2010 (English)In: INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009) / [ed] Goll, G; Lohneysen, HV; Loidl, A; Pruschke, T; Richter, M; Schultz, L; Surgers, C; Wosnitza, J, BRISTOL: IOP PUBLISHING LTD, DIRAC HOUSE , 2010, Vol. 200, UNSP 072078- p.Conference paper, Published paper (Refereed)
Abstract [en]

We demonstrate the fabrication of metallic nano-contacts on magnetic multilayers using a Hole Mask Colloidal Lithography technique (HCL) based on Polystyrene spheres. The method applies PMMA as a sacrificial layer upon which a hole pattern is formed after liftoff of the spheres. An Au layer functions as a hard mask for the PMMA and the PMMA subsequently masks the SiO(2) during its etching. The resulting pattern is a dense collection of randomly located nano-holes through a SiO(2) film. Final devices are made using traditional photolithography to define a 600 nm circular mesa with about 3 to 4 nano-holes per device, and patterning of a metallic top contact.

Place, publisher, year, edition, pages
BRISTOL: IOP PUBLISHING LTD, DIRAC HOUSE, 2010
Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 200
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-39659 (URN)10.1088/1742-6596/200/7/072078 (DOI)000291321303006 ()2-s2.0-77957035820 (Scopus ID)
Conference
International Conference on Magnetism (ICM 2009). Karlsruhe, GERMANY. JUL 26-31, 2009
Available from: 2011-09-12 Created: 2011-09-12 Last updated: 2013-05-27Bibliographically approved
2. Current induced vortices in multi-nanocontact spin-torque devices
Open this publication in new window or tab >>Current induced vortices in multi-nanocontact spin-torque devices
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2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 7, 07C913- p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate spin transfer torque (STT) switching in multi-nanocontact STT devices fabricated using hole mask colloidal lithography. We also study the STT device resistance and switching properties as a function of applied magnetic field and nanocontact current. At low nanocontact current, magnetoresistance measurements show sharp, single-step switching at low switching fields. When the current is increased, the switching becomes multistep, and the switching field increases dramatically. We explain these results as arising from a transition from a predominantly single domain like switching to switching involving a vortex state. Micromagnetic simulations corroborate this picture, indicating that a single magnetic vortex nucleates in between the nanocontacts through the influence from the total Oersted field generated by the nanocontact ensemble.

Keyword
Phase-locking
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-35134 (URN)10.1063/1.3556960 (DOI)000289952100158 ()2-s2.0-79955442839 (Scopus ID)
Note

QC 20110623

Available from: 2011-06-23 Created: 2011-06-20 Last updated: 2017-12-11Bibliographically approved
3. High frequency operation of a spin-torque oscillator at low field
Open this publication in new window or tab >>High frequency operation of a spin-torque oscillator at low field
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2011 (English)In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 5, no 12, 432-434 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a nano-contact based spin-torque oscillator (STO) combining a high operating frequency with low field operation. The STO is based on an orthogonal spin-valve architecture with an in-plane Co polarizer and an out-of-plane Co/Ni multilayer free layer. High frequency operation at low external fields is achieved by tailoring the Co/Ni layer properties to increase the strength (H(k)) of the perpendicular magnetic anisotropy, while simultaneously reducing the saturation magnetization (M(s)). Our approach emphasizes the importance of mu(0)(H(k)-M(s)) in determining the operating frequency in this system, and suggests that yet higher frequencies should be attainable through further optimization. [GRAPHICS]

Keyword
spin-torque oscillators, perpendicular magnetic anisotropy, Co/Ni multilayers
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-60997 (URN)10.1002/pssr.201105375 (DOI)000298038600007 ()2-s2.0-81355124136 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20120116Available from: 2012-01-17 Created: 2012-01-16 Last updated: 2017-12-08Bibliographically approved
4. Spin Torque-Generated Magnetic Droplet Solitons
Open this publication in new window or tab >>Spin Torque-Generated Magnetic Droplet Solitons
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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.

Keyword
Waves, Excitation, States
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-105274 (URN)10.1126/science.1230155 (DOI)000316053400031 ()2-s2.0-84874995646 (Scopus ID)
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
5. Microwave Signal Generation in Single-Layer Nano-Contact Spin Torque Oscillators
Open this publication in new window or tab >>Microwave Signal Generation in Single-Layer Nano-Contact Spin Torque Oscillators
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2013 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 49, no 7, 4331-4334 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate spin transfer torque (STT) driven microwave signal generation, from about 250 MHz to above 3 GHz, in single perm alloy layers underneath a nano-contact with diameter of 100 nm. The threshold current for signal generation is found to be strongly hysteretic, the microwave signal shows a number of harmonics, zero-field operation is straightforward, and the microwave frequency increases quasi-linearly with drive current. All observations are consistent with STT driven motion of a vortex-antivortex pair nucleated by the Oersted field underneath the nano-contact. While the generated power is about 10 dB smaller than the best GMR based nano-contact spin torque oscillators, the linewidth of 6-100 MHz is of the same order.

Keyword
Single layer, spin torque oscillator, vortex
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-122702 (URN)10.1109/TMAG.2013.2250931 (DOI)000322483200318 ()2-s2.0-84880810559 (Scopus ID)
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20130910. Updated from accepted to published.

Available from: 2013-05-27 Created: 2013-05-27 Last updated: 2017-12-06Bibliographically approved
6. Mutually synchronized bottom-up multi-nanocontact spin-torque oscillators
Open this publication in new window or tab >>Mutually synchronized bottom-up multi-nanocontact spin-torque oscillators
Show others...
2013 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 4, 2731- p.Article in journal (Refereed) Published
Abstract [en]

Spin-torque oscillators offer a unique combination of nanosize, ultrafast modulation rates and ultrawide band signal generation from 100 MHz to close to 100 GHz. However, their low output power and large phase noise still limit their applicability to fundamental studies of spin-transfer torque and magnetodynamic phenomena. A possible solution to both problems is the spin-wave-mediated mutual synchronization of multiple spin-torque oscillators through a shared excited ferromagnetic layer. To date, synchronization of high-frequency spin-torque oscillators has only been achieved for two nanocontacts. As fabrication using expensive top-down lithography processes is not readily available to many groups, attempts to synchronize a large number of nanocontacts have been all but abandoned. Here we present an alternative, simple and cost-effective bottom-up method to realize large ensembles of synchronized nanocontact spin-torque oscillators. We demonstrate mutual synchronization of three high-frequency nanocontact spin-torque oscillators and pairwise synchronization in devices with four and five nanocontacts.

Keyword
Nano-Oscillators, Phase-Locking, Magnetic Multilayers, Polarized Current, Electric-Current, Modulation, Driven, Generation, Excitation, Waves
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-122705 (URN)10.1038/ncomms3731 (DOI)000328023000002 ()2-s2.0-84889253383 (Scopus ID)
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg FoundationEU, European Research Council, ICT-257159
Note

QC 20140114. Updated from submitted to published.

Available from: 2013-05-27 Created: 2013-05-27 Last updated: 2017-12-06Bibliographically approved

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