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In-plane field angle dependence of mutually synchronized constriction based spin Hall nano-oscillators
KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We study mutual synchronization phenomena in multiple nanoconstriction-based SHNOs under weak in-plane fields down to μ0H = 30 mT and investigate the angular dependence of the synchronization condition. We compare double nanoconstriction and multiple nanoconstrictions with different spacings of 300 and 900 nm between the constrictions. For all the tested devices, we observe clear evidence of mutual synchronization of individual nanoconstrictions (NCs) only for angles smaller than a critical angle. This critical angle is higher for the 300 nm spacing than for the 900 nm spacing as a result of the stronger synchronization arising from the shorter distance. Direct inspection of the spin waves using μ-BLS maps confirms synchronization of the double nanoconstrictions. Alongside the synchronization, we observe a strong second harmonic that could be interpreted as a sign that the synchronization is mediated by the propagation of the second harmonic of the spin waves. Micromagnetic simulation explains the synchronization at the lower angles by the direction of the spatial profile of the modes and confirms the role of exchange coupling in the synchronization of nanoconstriction-based SHNOs.

Keywords [en]
Synchronization, in-plane field, spin Hall effect, nano-oscillators, SHNO
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-228044OAI: oai:DiVA.org:kth-228044DiVA, id: diva2:1206400
Note

QC 20180524

Available from: 2018-05-16 Created: 2018-05-16 Last updated: 2018-10-03Bibliographically approved
In thesis
1. Determining and Optimizing the Current and Magnetic Field Dependence of Spin-Torque and Spin Hall Nano-Oscillators: Toward Next-Generation Nanoelectronic Devices and Systems
Open this publication in new window or tab >>Determining and Optimizing the Current and Magnetic Field Dependence of Spin-Torque and Spin Hall Nano-Oscillators: Toward Next-Generation Nanoelectronic Devices and Systems
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Spin-torque and spin Hall nano-oscillators are nanoscale devices (about 100 nm) capable of producing tunable broadband high-frequency microwave signals ranging from 0.1 GHz to over 65 GHz that several research groups trying to reach up to 200 - 300 GHz. Their development is ongoing for applications in high-frequency nanoelectronic devices and systems, such as mobile phones, wireless networks, base stations, vehicle radars, and even medical applications.

This thesis covers a wide range of characterizations of spin-torque and spin Hall nano-oscillator devices that aim to investigate their current and magnetic field dependency, as well as to suggest improvements in these devices to optimize their application in spintronics and magnonics. The work is primarily based on experimental methods for characterizing these devices by building up new measurement systems, but it also includes numerical and micromagnetic simulations.

Experimental techniques: In order to characterize the fabricated nanodevices in a detailed and accurate manner through their electrical and microwave responses, new measurement systems capable of full 3D control over the external magnetic fields will be described. In addition, a new method of probing an operational device using magnetic force microscopy (MFM) will be presented.

Spin-torque nano-oscillators: We will describe remarkable improvements in the performance of spin-torque nano-oscillators (STNOs) that enhance their integration capability with applications in microwave systems. In nanocontact (NC-)STNOs made from a conventional spin-valve stack, though with thicker bottom electrodes, it is found the auto-oscillations can be excited with higher frequencies at lower threshold currents, and with higher output powers. We also find that this idea is useful for tuning spin-wave resonance and also controlling the thermal budget. Furthermore, a detailed study of magnetic droplet solitons and spin-wave dynamics in NC-STNOs will be described. Finally, we demonstrate ultra-high frequency tunability in low-current STNOs based on perpendicular magnetic tunnel junctions(p-MTJs).

Spin Hall nano-oscillators: Characterizations of spin Hall nano-oscillator(SHNO) devices based on different structures and materials with both conventional and novel methods will be described. A detailed study of the current, temperature, and magnetic field profiles of nanogap SHNOs will be presented. In addition, we show the current and magnetic field dependence of nanoconstriction-based SHNOs.Moreover, it is shown that multiple SHNOs can be serially synchronized, thereby increasing their output power and enhancing the usage of these devices in applications such as neuromorphic computing. We show synchronization of multiple nanoconstriction SHNOs in the presence of a low in-plane magnetic field. Finally, there is a demonstration of the results of a novel method for probing an operationalSHNO using MFM.

Abstract [sv]

Spinntroniska oscillatorer är ca 100 nm stora nano-komponenter som kan generera avstämningsbara mikrovågssignaler över ett mycket stort frekvensområde. Frekvensområdet sträcker sig i dagsläget från 0,1 GHz till över 65 GHz och flera forskningsgrupper försöker att nå upp till 200-300 GHz. De spinntroniska oscillatorerna baseras på en effekt som kallas spinnvridmoment och de första oscillatorerna kallades därför spinnvridmomentsnano-oscillatorer (eng. spin torque nano-oscillators) som vanligtvis förkortas STNO:er. De senaste åren har man även använt den s.k. spinn-Hall-effekten och oscillatorer baserade på detta förkortas därför SHNO:er. Båda sorternas oscillatorer är under kraftig utveckling för att kunna användas inom olika högfrekvenstillämpningar som t.ex. mobiltelefon, trådlösa nätverk, basstationer, fordonsradar och även medicinska tillämpningar.

Denna avhandling täcker ett brett spektrum av olika mätningar på STNO:er och SHNO:er för att bestämma deras ström- och magnetfältsberoenden samt föreslå förbättringar av deras design för att använda dem inom spinntronik och magnonik. Arbetet bygger i första hand på experimentella metoder för att utveckla nya mätsystem, men det innehåller också numeriska och mikromagnetiska simuleringar.

Experimentella tekniker: För att kunna göra detaljerade och noggranna mätningar, som funktion av ström genom komponenten samt magnetfält runt komponenten, har två nya mätuppställningar utvecklats, båda med målet att enkelt kunna variera styrka och riktning på det magnetiska fältet i tre dimensioner. Dessutom presenteras en ny metod för att studera komponenterna med s.k. magnetkraftsmikroskopi(MFM).

STNO:er: Avhandlingen presenterar väsentliga förbättringar av prestanda hos STNO:er genom att öka tjockleken på det understa metall-lager som hela STNO:n är uppbyggd på. I sådana förbättrade STNO:er kan mikrovågssignaler med högre frekvens, högre uteffekt, och lägre tröskelström realiseras. Dessutom får komponenterna bättre värmeledningsförmåga så att de kan klara högre drivströmmar. Vidare beskrivs en detaljerad studie av magnetdroppsolitoner och spinnvågsdynamik i STNO:er. Slutligen beskrivs en ultrahög frekvensavstämbarhet i STNO:er baserade på magnetiska tunnlingselement med s.k. vinkelrät anisotropi.

SHNO:er: Avhandlingen beskriver också mätningar på SHNO:er baserade på olika strukturer och material, studerade med både konventionella och nya metoder. En detaljerad studie av temperatur och magnetfältsprofiler i s.k. nano-gap SHNO:er presenteras. Dessutom presenterar avhandlingen detaljerade studier av magnetfältsberoendet hos s.k. nano-förträngnings-SHNO:er. Vidare har det visat sig att flera sådana SHNO:er kan synkroniseras seriellt och därigenom få en kraftigt ökad uteffekt. Detta möjliggör i förlängningen också s.k. neuromorfiska beräkningar. Avhandlingen visar att en kedja av sådana SHNO:er också kan synkroniseras även vid låga magnetfält. Slutligen beskrivs de första mätningarna på SHNO:er med hjälp av MFM.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 101
Series
TRITA-SCI-FOU ; 2018:26
Keywords
nanoelectronics, spintronics, nanomagnetism, ferromagnetic materials, microwave oscillators, magnetization dynamics, spin waves, giant magneto-resistance, spin Hall effect, spin-torque nano-oscillators, spin Hall nano-oscillators, numerical modeling, electrical characterization, microwave characterization, magnetic force microscopy.
National Category
Nano Technology Other Electrical Engineering, Electronic Engineering, Information Engineering Signal Processing Physical Sciences Condensed Matter Physics
Research subject
Physics; Materials Science and Engineering; Information and Communication Technology; Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-228391 (URN)978-91-7729-824-3 (ISBN)
Public defence
2018-06-15, Sal B Electrum, Kistagången 16, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilGöran Gustafsson Foundation for Research in Natural Sciences and MedicineSwedish Foundation for Strategic Research Knut and Alice Wallenberg FoundationEU, FP7, Seventh Framework Programme
Note

QC 20180524

Available from: 2018-05-24 Created: 2018-05-23 Last updated: 2018-11-02Bibliographically approved
2. Linear, Non-Linear, and Synchronizing Spin Wave Modes in Spin Hall Nano-Oscillators
Open this publication in new window or tab >>Linear, Non-Linear, and Synchronizing Spin Wave Modes in Spin Hall Nano-Oscillators
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Spin Hall nano-oscillators (SHNOs) are nanoscale spintronic devices that generate microwave signals with highly tunable frequency. This thesis focuses on improving the signal quality of nanoconstriction-based SHNOs and also on developing a better understanding of their magnetization dynamics.

In the first part of the thesis, we fabricate and characterize low-threshold current SHNOs using NiFe/β-W bilayers. Due to the high spin Hall angle of the β-phase W, the auto-oscillation threshold current is improved by 60% over SHNOs based on NiFe/Pt. We also demonstrate low operational current by utilizing W/Co20Fe60B20/MgO stacks on highly resistive silicon substrates. Thanks to the moderate perpendicular magnetic anisotropy (PMA) of Co20Fe60B20, these SHNOs show much wider frequency tunability than SHNOs based on NiFe with no PMA. Performance is further improved by using highly resistive silicon substrates with a high heat conductance, dissipating the generated excess heat much better than sapphire substrates. Moreover, it also means that the fabrication of SHNOs is now compatible with conventional CMOS fabrication, which is necessary if SHNOs are to be used in integrated circuits. In another approach, we attempt to decrease the threshold current of SHNOs based on an NiFe/Pt stack by inserting an ultra-thin Hf layer in the middle of the stack. This Hf dusting decreases the damping of the bilayer linearly but also degrades its spin Hall efficiency. These opposing trends determine the optimum Hf thickness to ≈0.4 nm, at which the auto-oscillation threshold current is minimum. Our achievements arising from these three approaches show a promising path towards the realization of low-current SHNO microwave devices with highly efficient spin-orbit torque.

In the next chapter, we use both electrical experimentation and micromagnetic simulation to study the auto-oscillating spin wave modes in nanoconstriction-based SHNOs as a function of the drive current and the applied field. First, we investigate the modes under an in-plane low-range field of 40-80 mT, which is useful for developing low-field spintronic devices with applications in microwave signal generation. It is also essential for future studies on the synchronization of multiple SHNOs. Next, using an out-of-plane applied magnetic field, we observe three different modes and demonstrate switching between them under a fixed external field by tuning only the drive current. The flexibility of these nanopatterned spin Hall nano-oscillators is desirable for implementing oscillator-based neuromorphic computing devices.

In the final part, we study the synchronization of multiple nanoconstriction-based SHNOs in weak in-plane fields. We electrically investigate the synchronization versus the angle of the field, observing synchronization for angles below a threshold angle. In agreement with the experimental results, the spatial profile of the spin waves from the simulations shows that the relative angle between the modes from the nanoconstrictions decreases with decreasing the field angle, thus facilitating synchronization. The synchronization observed at low in-plane fields improves the microwave signal quality and could also be useful for applications such as neuromorphic computing.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 58
Series
TRITA-SCI-FOU ; 2018:37
Keywords
spin Hall effect, spin Hall nano-oscillators, threshold current, microwave, spin wave, synchronization
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-234657 (URN)978-91-7729-929-5 (ISBN)
Public defence
2018-10-05, Sal Sven-Olof Öhrvik, Kistagången 16, Electrum 1, floor 2, KTH Kista, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20180907

Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2018-09-10Bibliographically approved

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Citation style
  • apa
  • harvard1
  • ieee
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  • Other locale
More languages
Output format
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