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Magnetodynamics in orthogonal nanocontact spin-torque nano-oscillators based on magnetic tunnel junctions
KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Material- och nanofysik.
Department of Physics, University of Gothenburg.
Department of Physics, University of Gothenburg.
International Iberian Nanotechnology Laboratory (INL), Braga, Portugal.
Vise andre og tillknytning
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
Abstract [en]

We demonstrate the magnetodynamics of nanocontact spin-torque nano-oscillators based on magnetic tunnel junctions, with a strong perpendicular magnetic anisotropy free layer and an easy-plane reference layer. First, the static magnetic properties are studied by conducting hysteresis loop and magnetoresistance measurements. Then, we characterize the generated microwave signals by applying dc currents. Field-sweep signals at xed current show a typical FMR-like frequency dependence. Interestingly, for current-sweep measurements, the plot of frequency versus dc current can be fitted well with a symmetric parabolic and an asymmetric linear term. The symmetric behavior is ascribed to the current-induced Joule heating and Oersted elds. The asymmetric dependence is due to the linear perpendicular spin torque (or eld-like torque) and the voltage-controlled magnetic anisotropy. Our observation hints at a route to improving the frequency tunability in future spintronic applications by selectively adding all these contributions.

HSV kategori
Forskningsprogram
Fysik
Identifikatorer
URN: urn:nbn:se:kth:diva-234662OAI: oai:DiVA.org:kth-234662DiVA, id: diva2:1246470
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.
HSV kategori
Forskningsprogram
Fysik
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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