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Spin valves and spin-torque oscillators with perpendicualr magnetic anisotropy
KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF. (Applied Spintronics Group)
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 [en]
Spin torque oscillator, perpendicular magnetic anisotropy
National Category
Nano Technology
Research subject
SRA - ICT
Identifiers
URN: urn:nbn:se:kth:diva-105223ISBN: 978-91-7501-545-3 (print)OAI: oai:DiVA.org:kth-105223DiVA: diva2:570422
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
List of papers
1. Temperature-dependent interlayer coupling in Ni/Co perpendicular pseudo-spin-valve structures
Open this publication in new window or tab >>Temperature-dependent interlayer coupling in Ni/Co perpendicular pseudo-spin-valve structures
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 17, 174432- p.Article in journal (Refereed) Published
Abstract [en]

The temperature-dependent coupling mechanisms in perpendicular pseudo-spin valves based on the following structure, [Ni/Co](5)/Cu(t(Cu))/[Ni/Co](2), are investigated. Despite a thick (t(Cu) >= 3 nm) Cu spacer, room-temperature measurements reveal complete coupling of the [Ni/Co](5) and [Ni/Co](2) multilayers. This coupling can be attributed to strong long range magnetostatic stray fields that penetrate the spacer layer. This results in magnetic domain imprinting and vertically correlated domains throughout the reversal process. Surprisingly, when the temperature is reduced, a complete decoupling is observed. This somewhat counterintuitive result can be explained by a large difference in the [Ni/Co](5) and [Ni/Co](2) multilayer coercivities at reduced temperatures, which then impedes domain imprinting and promotes decoupling. Finally, the decoupling temperature is found to increase with spacer thickness.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-53400 (URN)10.1103/PhysRevB.84.174432 (DOI)000297294500008 ()2-s2.0-82455194893 (Scopus ID)
Note
QC 20111228Available from: 2011-12-28 Created: 2011-12-28 Last updated: 2017-12-08Bibliographically approved
2. 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
3. Tunable spin configuration in [Co/Ni]-NiFe spring magnets
Open this publication in new window or tab >>Tunable spin configuration in [Co/Ni]-NiFe spring magnets
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2013 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 46, no 12, 125004- p.Article in journal (Refereed) Published
Abstract [en]

We investigate a tunable spin configuration in [Co/Ni](4)/Co-NiFe exchange spring magnets. The strong perpendicular magnetic anisotropy of the Co/Ni multilayer, which competes with the in-plane shape anisotropy of the Ni81Fe19 (Py) layer, allows for unique magnetic configurations. By varying the NiFe thickness (t(NiFe)) from 2.6 to 3.0 nm, we show that the magnetization tilt angle can be easily tuned from completely out-of-plane (0 degrees) to in-plane (90 degrees). This transition, which occurs for a small range of t(NiFe), can be estimated from the major loop remanence and one-dimensional micromagnetic calculations. These tunable magnetization tilt spring magnets are highly promising for future applications in spin-transfer torque-based devices.

Keyword
Torque Oscillator, Layer
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-105237 (URN)10.1088/0022-3727/46/12/125004 (DOI)000315413600005 ()2-s2.0-84874532794 (Scopus ID)
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20130402

Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2017-12-07Bibliographically 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. Magnetostatically driven domain replication in Ni/Co perpendicular pseudo spins valves: the effect of ferromagnetic layer thickness and temperature cycling
Open this publication in new window or tab >>Magnetostatically driven domain replication in Ni/Co perpendicular pseudo spins valves: the effect of ferromagnetic layer thickness and temperature cycling
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-105276 (URN)
Note

QS 2012

Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2012-11-19Bibliographically approved

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