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Experimental evidence of tunable nonlinearity in He+ irradiated spin-torque oscillators
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.ORCID-id: 0000-0003-1909-437X
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik. Department of Physics and Astronomy, Uppsala University.
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.ORCID-id: 0000-0001-9107-3309
Institut d'Electronique Fondamentale, CNRS, Universite Paris-Sud, Universite Paris-Saclay.
Vise andre og tillknytning
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
Abstract [en]

Spin-torque nano-oscillators (STNOs) are among the most promising candidates for nanoscale broadband microwave generators. Before this application can be realized, however, enormous efforts are required of researchers to meet the commercial requirements of high-frequency tunability, high power, and narrow linewidth. As the performance of STNOs is mainly attributed to the nonlinearity on the basis of nonlinear auto-oscillator theory, we here systematically study how this nonlinearity is affected by the free-layer's effective magnetization Meff (i.e., the perpendicular magnetic anisotropy, PMA) in STNOs. The PMA is gradually tuned by using different fluences of He+ irradiation. Consequently, the nonlinearity can be continuously tailored from positive to negative. In addition, due to the almost zero nonlinearity, the linewidth shows an improvement of more than two orders of magnitude. This experimental observation is in strong agreement with the theory of nonlinear auto-oscillators. Our study not only confirms the theoretical prediction of nonlinearity, but also indicates the route to be taken towards realizing commercial microwave generators.

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Identifikatorer
URN: urn:nbn:se:kth:diva-234653OAI: oai:DiVA.org:kth-234653DiVA, id: diva2:1246461
Merknad

QC 20180907

Tilgjengelig fra: 2018-09-07 Laget: 2018-09-07 Sist oppdatert: 2018-09-11bibliografisk kontrollert
Inngår i avhandling
1. Engineering Magnetic Droplets in Nanocontact Spin-Torque Nano-Oscillators
Åpne denne publikasjonen i ny fane eller vindu >>Engineering Magnetic Droplets in Nanocontact Spin-Torque Nano-Oscillators
2018 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2018. s. 74
Serie
TRITA-SCI-FOU ; 2018:36
Emneord
Magnetic Droplet, Magnetic Tunnel Junction, Perpendicular Magnetic Anisotropy, Spin-torque Nano-Oscillators, Spin-Valve.
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Forskningsprogram
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Identifikatorer
urn:nbn:se:kth:diva-234464 (URN)978-91-7729-920-2 (ISBN)
Disputas
2018-10-04, Hall C, ​Electrum 229, Kista, 09:00 (engelsk)
Opponent
Veileder
Forskningsfinansiär
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Merknad

QC 20180907

Tilgjengelig fra: 2018-09-10 Laget: 2018-09-06 Sist oppdatert: 2018-09-18bibliografisk kontrollert

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