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Determining and Optimizing the Current and Magnetic Field Dependence of Spin-Torque and Spin Hall Nano-Oscillators: Toward Next-Generation Nanoelectronic Devices and Systems
KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.ORCID iD: 0000-0003-4253-357X
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 [en]
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: urn:nbn:se:kth:diva-228391ISBN: 978-91-7729-824-3 (print)OAI: oai:DiVA.org:kth-228391DiVA, id: diva2:1209697
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
List of papers
1. Microwave probe stations with three-dimensional control of the magnetic field to study high frequency dynamics in nanoscale devices
Open this publication in new window or tab >>Microwave probe stations with three-dimensional control of the magnetic field to study high frequency dynamics in nanoscale devices
2018 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623Article in journal (Refereed) Accepted
Abstract [en]

We present two microwave probe stations with motorized rotary stages for adjusting the magnitude and angle of the applied magnetic field. In the first system, the magnetic field is provided by an electromagnet and can be adjusted from 0 to ~ 1.4 T while its polar angle (θ) can be varied from 0o to 360o. In the second system the magnetic field is provided by a Halbach array permanent magnet, which can be rotated and translated to cover the full range of polar (θ) and azimuthal (φ) angles with a tunable field magnitude up to ~ 1 T. Both systems are equipped with microwave probes, bias-Ts, amplifiers, and spectrum analyzers, to allow for microwave characterization up to 40 GHz, as well as software to automatically perform continuous large sets of electrical and microwave measurements.

Keywords
Autonomous, microwave, probe station, RF measurement, spin-torque nano-oscillator, spin-Hall nano-oscillator
National Category
Condensed Matter Physics Nano Technology
Research subject
Physics; Production Engineering; Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-228238 (URN)
Note

QC 20180524

Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2018-05-24Bibliographically approved
2. Magnetic force microscopy of an operational nanodevice
Open this publication in new window or tab >>Magnetic force microscopy of an operational nanodevice
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present a new method for probing the spatial profile of an operational magnetic nanodevice using magnetic force microscopy (MFM). We have developed an MFM system by adding a microwave probe station equipped with microwave probe, bias-T, and amplifier to allow electrical and microwave characterization up to 40 GHz during the MFM process. The nanoscale spintronic devices---spin Hall nano-oscillators (SHNOs) based on Pt/NiFe bilayers with a specific design compatible with the developed system---were fabricated and scanned using a Co magnetic force microscopy tip with 10 nm spatial resolution, while a DC current sufficient to exert auto-oscillation flowed. Our results show that this method of developed provides a promising path for the characterization of the spatial profiles of operational nano-oscillators.

National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228242 (URN)
Note

QC 20180524

Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2018-05-24Bibliographically approved
3. Order of magnitude improvement of nano-contact spin torque nano-oscillator performance
Open this publication in new window or tab >>Order of magnitude improvement of nano-contact spin torque nano-oscillator performance
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2017 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 9, no 5, p. 1896-1900Article in journal (Refereed) Published
Abstract [en]

Spin torque nano-oscillators (STNO) represent a unique class of nano-scale microwave signal generators and offer a combination of intriguing properties, such as nano sized footprint, ultrafast modulation rates, and highly tunable microwave frequencies from 100 MHz to close to 100 GHz. However, their low output power and relatively high threshold current still limit their applicability and must be improved. In this study, we investigate the influence of the bottom Cu electrode thickness (t(Cu)) in nano-contact STNOs based on Co/Cu/NiFe GMR stacks and with nano-contact diameters ranging from 60 to 500 nm. Increasing t(Cu) from 10 to 70 nm results in a 40% reduction of the threshold current, an order of magnitude higher microwave output power, and close to two orders of magnitude better power conversion efficiency. Numerical simulations of the current distribution suggest that these dramatic improvements originate from a strongly reduced lateral current spread in the magneto-dynamically active region.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2017
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-204731 (URN)10.1039/c6nr07309c (DOI)000395594300017 ()28094381 (PubMedID)2-s2.0-85011392741 (Scopus ID)
Funder
EU, European Research Council, 307144Swedish Research CouncilSwedish Foundation for Strategic Research Knut and Alice Wallenberg Foundation
Note

QC 20170601

Available from: 2017-06-01 Created: 2017-06-01 Last updated: 2018-05-24Bibliographically approved
4. Tuning exchange-dominated spin-waves using lateral current spread in nanocontact spin-torque nano-oscillators
Open this publication in new window or tab >>Tuning exchange-dominated spin-waves using lateral current spread in nanocontact spin-torque nano-oscillators
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2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118Article in journal (Refereed) Submitted
Abstract [en]

We present an efficient method to tailor propagating spin waves in quasi-confined systems. We use nanocontact spin-torque nano-oscillators based on NiFe/Cu/Co spin-valves and study the ferromagnetic and spin-wave resonances (FMR and SWR) of both layers. We employ homodyne-detected ferromagnetic resonance spectroscopy, resembling spin-torque FMR, to detect the magnetodynamics. The external field is applied in-plane, giving a parallel configuration of the magnetic layers, which do not provide any spin-transfer torque. Instead, the excitation is caused by the Oersted-field. By varying the thickness of the bottom Cu electrode of the devices, we tune the current distribution in the samples, and thereby the Oersted field, which governs the spin wave characteristics.

Keywords
spin wave, nanocontact, spin torque nano-oscillator
National Category
Condensed Matter Physics Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228239 (URN)
Note

QC 20180524

Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2018-05-24Bibliographically approved
5. Control of thermal budget in nanocontact spin-torque nano-oscillators
Open this publication in new window or tab >>Control of thermal budget in nanocontact spin-torque nano-oscillators
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We investigate the influence of the bottom Cu electrode thickness (tCu) in nanocontact spin-torque nano-oscillators (NC-STNOs) based on Si/SiO2/Pd(8)/Cu(tCu)/Co(8)/Cu(7)/NiFe(4.5)/Cu(3)/Pd(3) GMR stacks on the thermal budget of the magnetodynamically active region. Increasing tCu from 10 to 70 nm results in a ~50% reduction in Joule heating in both the Co and NiFe layers, which directly improves the microwave output stability and linewidth. Numerical simulations of the NC-STNO current distribution suggest that this improvement originates from a strongly reduced lateral current spread in the top ferromagnetic layer and a reduction in the device's resistance.

Keywords
nanocontact, spin torque nano-oscillator, Joule heating
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228241 (URN)
Note

QC 20180524

Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2018-05-24Bibliographically approved
6. Magnetic droplet soliton nucleation in oblique fields
Open this publication in new window or tab >>Magnetic droplet soliton nucleation in oblique fields
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2018 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 97, no 184402Article in journal (Refereed) Published
Abstract [en]

We study the auto-oscillating magnetodynamics in orthogonal spin-torque nano-oscillators (STNOs) as a function of the out-of-plane (OOP) magnetic-field angle. In perpendicular fields and at OOP field angles down to approximately 50°, we observe the nucleation of a droplet. However, for field angles below 50°, experiments indicate that the droplet gives way to propagating spin waves, in agreement with our micromagnetic simulations. Theoretical calculations show that the physical mechanism behind these observations is the sign changing of spin-wave nonlinearity (SWN) by angle. In addition, we show that the presence of a strong perpendicular magnetic anisotropy free layer in the system reverses the angular dependence of the SWN and dynamics in STNOs with respect to the known behavior determined for the in-plane magnetic anisotropy free layer. Our results are of fundamental interest in understanding the rich dynamics of nanoscale solitons and spin-wave dynamics in STNOs.

Place, publisher, year, edition, pages
American Physical Society, 2018
Keywords
nanocontact, spin torque nano-oscillator, droplet, nucleation
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228245 (URN)10.1103/PhysRevB.97.184402 (DOI)000431986600004 ()2-s2.0-85047128171 (Scopus ID)
Note

QC 20180524

Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2018-06-05Bibliographically approved
7. Ultra-high frequency tunability in low-current and low-field spin-torque oscillators based on perpendicular magnetic tunnel junctions
Open this publication in new window or tab >>Ultra-high frequency tunability in low-current and low-field spin-torque oscillators based on perpendicular magnetic tunnel junctions
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(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

We demonstrate ultra-high frequency tunability of up to 4.4 GHz/mA, and low threshold currents of about -21 $\mu$A, in spin-torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions, in which both free and fixed layers have perpendicular magnetic anisotropy (PMA). By using different thicknesses of the two CoFeB layers, their individual PMA strengths can be tailored to achieve significant relative misalignment of their respective magnetizations in moderate in-plane fields. We observe a broad maximum in both the device resistance and the generated microwave power around maximum misalignment. Maximum frequency tunability is observed at low-to-moderate fields and decrease rapidly after maximum misalignment.

Keywords
spin-torque oscillator, magnetic tunnel junction, magnetic perpendicular anisotropy
National Category
Materials Engineering
Research subject
Materials Science and Engineering; Physics
Identifiers
urn:nbn:se:kth:diva-191278 (URN)
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20160829

Available from: 2016-08-26 Created: 2016-08-26 Last updated: 2018-05-24Bibliographically approved
8. Current, temperature, and magnetic field profiles in nanogap spin Hall nano-oscillators
Open this publication in new window or tab >>Current, temperature, and magnetic field profiles in nanogap spin Hall nano-oscillators
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We carry out a detailed experimental and numerical study of nanogap spin Hall nano-oscillators (SHNOs) to determine the current distribution, the associated Oersted field, and the possible effects of the local temperature rise. We find substantial heating in the center of the SHNOs, leading to a nonuniform device resistance which redistributes the current in a nontrivial way. As a consequence, both the Oe field magnitude and its spatial profile are nonlinear functions of the current magnitude. Our results have direct consequences for spin-wave generation in these devices.

National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228240 (URN)
Note

QC 20180524

Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2018-05-24Bibliographically approved
9. Mapping out the in-plane spin wave modes of constriction based spin Hall nano-oscillators
Open this publication in new window or tab >>Mapping out the in-plane spin wave modes of constriction based spin Hall nano-oscillators
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We experimentally study the auto-oscillating spin wave modes in NiFe/β-W constriction-based spin Hall nano-oscillators versus bias current, in-plane applied field strength, and azimuthal angle. We observe two different spin wave modes in weak in-plane fields: a linear-like mode confined in the minima of the internal field near the edges of the nano-constriction with weak frequency dependencies on the bias current and the applied field angle, and second, lower frequency mode that has significantly higher threshold current and stronger frequency dependencies on both bias current and the external field angle. Our micromagnetic modeling qualitatively reproduces experimental data and reveals that the second mode is the spin wave bullet. In contrast to the linear-like mode, the bullet is (a) localized in the middle of the constriction and (b) shrinks with the bias current. Our results are important for the development of the field-free spintronic oscillators for the applications in microwave signal generation and neuromorphic computing.

Keywords
spin wave, nano-oscillators, SHNOs, spin Hall, in-plane field
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228043 (URN)
Note

QC 20180524

Available from: 2018-05-16 Created: 2018-05-16 Last updated: 2018-05-24Bibliographically approved
10. In-plane field angle dependence of mutually synchronized constriction based spin Hall nano-oscillators
Open this publication in new window or tab >>In-plane field angle dependence of mutually synchronized constriction based spin Hall nano-oscillators
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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
Synchronization, in-plane field, spin Hall effect, nano-oscillators, SHNO
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228044 (URN)
Note

QC 20180524

Available from: 2018-05-16 Created: 2018-05-16 Last updated: 2018-10-03Bibliographically approved

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