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Effect of canted magnetic field on magnetic droplet nucleation boundaries
KTH, School of Engineering Sciences (SCI), Applied Physics. Department of Physics, University of Gothenburg.
KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.ORCID iD: 0000-0003-1909-437X
KTH, School of Electrical Engineering and Computer Science (EECS), Electronics, Integrated devices and circuits.ORCID iD: 0000-0003-1271-1814
Department of Applied Mathematics, University of Colorado.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The influence on magnetic droplet nucleation boundaries by canted magnetic elds are investigated and reported. The nucleation boundary condition, In = αAH + BH + C, is determined at different canted angles (0°< θH<20°) using magnetoresistance (MR) and microwave measurements in nanocontact spintorque oscillators (NC-STOs). As θH increased, the nucleation boundary shifts gradually towards higher In and H. The coefficient B of the nucleation boundary equation also nearly doubled as θH increases. On theother hand, the coefficient αA remained constant for all values of θH. These observations can be explained by considering the drift instability of magnetic droplets and the different tilt behaviour of the Co fixed layer induced by different θH.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-234651OAI: oai:DiVA.org:kth-234651DiVA, id: diva2:1246368
Note

QC 20180907

Available from: 2018-09-07 Created: 2018-09-07 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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Eklund, AndersLe, Quang TuanMazraati, HamidÅkerman, Johan

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