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Direct Observation of Zhang-Li Torque Expansion of Magnetic Droplet Solitons
KTH, School of Engineering Sciences (SCI), Applied Physics. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;Uppsala Univ, Dept Phys & Astron, S-75120 Uppsala, Sweden..
KTH, School of Engineering Sciences (SCI), Applied Physics. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden.;NanOsc AB, S-16440 Kista, Sweden..
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2018 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 120, no 21, article id 217204Article in journal (Refereed) Published
Abstract [en]

Magnetic droplets are nontopological dynamical soli tons that can be nucleated in nanocontact based spin torque nano-oscillators (STNOs) with perpendicular magnetic anisotropy free layers. While theory predicts that the droplet should be of the same size as the nanocontact, its inherent drift instability has thwarted attempts at observing it directly using microscopy techniques. Here, we demonstrate highly stable magnetic droplets in all-perpendicular STNOs and present the first detailed droplet images using scanning transmission X-ray microscopy. In contrast to theoretical predictions, we find that the droplet diameter is about twice as large as the nanocontact. By extending the original droplet theory to properly account for the lateral current spread underneath the nanocontact, we show that the large discrepancy primarily arises from current-in-plane Zhang-Li torque adding an outward pressure on the droplet perimeter. Electrical measurements on droplets nucleated using a reversed current in the antiparallel state corroborate this picture.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC , 2018. Vol. 120, no 21, article id 217204
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-230841DOI: 10.1103/PhysRevLett.120.217204ISI: 000433040200010PubMedID: 29883139Scopus ID: 2-s2.0-85047636345OAI: oai:DiVA.org:kth-230841DiVA, id: diva2:1220636
Note

QC 20180619

Available from: 2018-06-19 Created: 2018-06-19 Last updated: 2018-09-11Bibliographically approved
In thesis
1. Engineering Magnetic Droplets in Nanocontact Spin-Torque Nano-Oscillators
Open this publication in new window or tab >>Engineering Magnetic Droplets in Nanocontact Spin-Torque Nano-Oscillators
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Spin-torque nano-oscillators (STNOs) are nanoscale spintronic devices capable of generating highly tunable broadband microwave signals. In this thesis, I study nanocontact (NC)-based STNOs using strong perpendicular magnetic anisotropy(PMA) free layers, where a novel magnetic soliton—a magnetic droplet—exists. This work is devoted to further understanding the characteristics of the magnetic droplet in diverse magnetic structures, including orthogonal and all-perpendicular(all-PMA) spin valves (SVs) and orthogonal magnetic tunnel junctions (MTJs). The nucleation, transition, and collapse of magnetic droplets are observed, tailored, and analyzed by engineering the magnetic properties of the thin films’ stacks. This thesis consists of three main parts: Orthogonal SVs with [Co/Ni]/Cu/CoxNiFe1−x: Magnetic droplets were first observed in orthogonal SV STNOs. We engineered the fixed layer magnetization Ms,p by cosputtering different compositions of CoxNiFe1−x (x = 0−1). The nucleation boundaries of a magnetic droplet in a current-field phase shift to a lower region as Ms,p decreases. The nucleation boundary is also examined under canted fields in order to better understand the drift instability of the droplets. The observations not only confirm the theoretical predictions of nucleation boundary, but suggest a method for controlling the nucleation boundary. All-PMA SVs with [Co/Ni]/Cu/[Co/Pd]: In contrast to orthogonal SVs,all-PMA NC-STNOs show many novel features. First, thanks to the dramatic improvement in droplet stability that results from using a [Co/Pd] PMA fixed layer, the droplets are directly imaged by a scanning transmission x-ray microscopy(STXM). The transition between the static bubble and magnetic droplet is also observed and imaged. Moreover, to investigate the effect of PMA, He+ irradiation is conducted on the all-PMA NC-STNOs, progressively tuning the PMA. The transitions of the normal FMR-like mode and droplet mode are demonstrated. The behavior of frequency tunability versus PMA is systematically studied. These investigations of all-PMA and irradiated NC-STNOs show that it is feasible to engineer the magnetic properties of STNOs through He+ irradiation. Besides, the dynamic droplets and static bubbles have great potential applications in next-generation information carriers. Orthogonal MTJs with CoFeB/MgO/CoFe: The existence of droplets in orthogonal MTJs is still debated. Instead, the magnetodynamics are investigated here. Very importantly, we find that the frequency tunability is determined by the spin-transfer torque (STT), the voltage-controlled magnetic anisotropy (VCMA), and thermal heating. This paves the way to improving tunability by combining these contributions. This study will contribute greatly to real applications, such as microwave generators and detectors.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 74
Series
TRITA-SCI-FOU ; 2018:36
Keywords
Magnetic Droplet, Magnetic Tunnel Junction, Perpendicular Magnetic Anisotropy, Spin-torque Nano-Oscillators, Spin-Valve.
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-234464 (URN)978-91-7729-920-2 (ISBN)
Public defence
2018-10-04, Hall C, ​Electrum 229, Kista, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20180907

Available from: 2018-09-10 Created: 2018-09-06 Last updated: 2018-09-18Bibliographically approved

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Åkerman, Johan

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