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Microwave Frequency Stability and Spin Wave Mode Structure in Nano-Contact Spin Torque Oscillators
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.ORCID iD: 0000-0003-1271-1814
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The nano-contact spin torque oscillator (NC-STO) is an emerging device for highly tunable microwave frequency generation in the range from 0.1 GHz to above 65 GHz with an on-chip footprint on the scale of a few μm. The frequency is inherent to the magnetic material of the NC-STO and is excited by an electrical DC current by means of the spin torque transfer effect. Although the general operation is well understood, more detailed aspects such as a generally nonlinear frequency versus current relationship, mode-jumping and high device-to-device variability represent open questions. Further application-oriented questions are related to increasing the electrical output power through synchronization of multiple NC-STOs and integration with CMOS integrated circuits.

This thesis consists of an experimental part and a simulation part. Experimentally, for the frequency stability it is found that the slow but strong 1/f-type frequency fluctuations are related to the degree of nonlinearity and the presence of perturbing, unexcited modes. It is also found that the NC-STO can exhibit up to three propagating spin wave oscillation modes with different frequencies and can randomly jump between them. These findings were made possible through the development of a specialized microwave time-domain measurement circuit. Another instrumental achievement was made with synchrotron X-rays, where we image dynamically the magnetic internals of an operating NC-STO device and reveal a spin wave mode structure with a complexity significantly higher than the one predicted by the present theory.

In the simulations, we are able to reproduce the nonlinear current dependence by including spin wave-reflecting barriers in the nm-thick metallic, magnetic free layer. A physical model for the barriers is introduced in the form of metal grain boundaries with reduced magnetic exchange coupling. Using the experimentally measured average grain size of 30 nm, the spin wave mode structure resulting from the grain model is able to reproduce the experimentally found device nonlinearity and high device-to-device variability.

In conclusion, the results point out microscopic material grains in the metallic free layer as the reason behind the nonlinear frequency versus current behavior and multiple propagating spin wave modes and thereby as a source of device-to-device variability and frequency instability.

Abstract [sv]

Dagens snabba utveckling inom informationsteknik drivs på av ständigt växande informationsmängder och deras samhällsanvändning inom allt från resursoptimering till underhållning. Utvecklingen möjliggörs till stor del hårdvarumässigt av miniatyrisering och integrering av elektroniska komponenter samt trådlös kommunikation med allt större bandbredd och högre överföringshastighet. Det senare uppnås främst genom utnyttjande av högre radiofrekvenser i teknologiskt tidigare oåtkomliga delar av spektrumet. Frekvensutnyttjandet har det senaste årtiondet ökat markant i mikrovågsområdet med typiska frekvenser runt 2.4 GHz och 5.2-5.8 GHz.

I den spinntroniska oscillatorn (STO:n) möjliggörs frekvensgenerering i det breda området från 0.1 GHz upp till över 65 GHz av en komponent med mikrometerstorlek som kan integreras direkt i CMOS-mikrochip. Till skillnad från i konventionella radiokretsar med oscillatorer konstruerade av integrerade transistorer och spolar, genereras mikrovågsfrekvensen direkt i STO:ns magnetiska material och omvandlas därefter till en elektrisk signal genom komponentens magnetoresistans. Dessa materialegenskaper möjliggör ett tillgängligt frekvensband med extrem bredd i en och samma STO, som därtill kan frekvensmoduleras direkt genom sin styrström och på så sätt förenklar konstruktionen av sändarsystem. STO:ns icke-linjära egenskaper kan potentiellt också användas för att i en och samma komponent blanda ned mottagna mikrovågssignaler och på så sätt förenkla konstruktionen även av mikrovågsmottagare.

STO:ns signalegenskaper bestäms av det magnetiska materialets fysik i form av magnetiseringsdynamik driven av elektriskt genererade spinnströmmar. I denna avhandling studeras denna dynamik experimentellt med särskilt fokus på frekvensstabiliteten i den hittills mest stabila STO-typen; nanokontakts-STO:n. Genom mätningar i tidsdomän av STO:ns elektriska signaler runt 25 GHz har frekvensstabiliteten funnits hänga samman med den typ av icke-linjärt beteende som också funnits vara utmärkande för tillverkningsvariationen i komponenterna. Mikroskopiska undersökningar av materialet visar att en trolig källa till denna variation är den magnetiska metallens uppbyggnad i form av korn i storleksordningen 30 nm, och datorsimuleringar av en sådan materialstruktur har visats kunna reproducera de experimentella resultaten. Därtill har en metod utvecklats för att med röntgenstrålning direkt mäta de små, magnetiska mikrovågsrörelserna i materialet. Denna röntgenteknik möjliggör detaljerade experimentella studier av magnetiseringsdynamiken och kan användas för att verifiera och vidareutveckla den existerande teorin för mikrovågsspinntronik.

Sammantaget förs STO-teknologin genom denna studie ett steg närmare sina tänkbara samhällsbreda tillämpningar inom snabb, trådlös kommunikation för massproducerade produkter med integrerad sensor- och datorfunktionalitet.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 91 p.
Series
TRITA-ICT, 2016:18
Keyword [en]
spintronics, microwave oscillators, magnetization dynamics, spin waves, phase noise, device modelling, electrical characterization, X-ray microscopy, STXM, XMCD
National Category
Condensed Matter Physics Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Physics; Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-188546ISBN: 978-91-7729-045-2 (print)OAI: oai:DiVA.org:kth-188546DiVA: diva2:936764
Public defence
2016-09-02, Sal C, Electrum, Isafjordsgatan 22, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2009-4190Swedish Research Council, 2012-5372
Note

QC 20160620

Available from: 2016-06-20 Created: 2016-06-13 Last updated: 2016-06-20Bibliographically approved
List of papers
1. Triple mode-jumping in a spin torque oscillator
Open this publication in new window or tab >>Triple mode-jumping in a spin torque oscillator
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2013 (English)In: 2013 22nd International Conference on Noise and Fluctuations, ICNF 2013, New York: IEEE conference proceedings, 2013, 6578965- p.Conference paper, Published paper (Refereed)
Abstract [en]

In a nano-contact Co/Cu/NiFe spin torque oscillator, mode-jumping between up to three frequencies within 22.5-24.0 GHz is electrically observed in the time domain. The measurements reveal toggling between two states with differing oscillation amplitude, of which the low-amplitude state is further divided into two rapidly alternating modes. Analysis of the mode dwell time statistics and the total time spent in each mode is carried out, and it is found that in both aspects the balance between the modes is greatly altered with the DC drive current.

Place, publisher, year, edition, pages
New York: IEEE conference proceedings, 2013
Keyword
DC drives, Dwell time, Low-amplitude, Nano contacts, Oscillation amplitude, Spin-torque oscillators, Time domain, Time spent
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-133279 (URN)10.1109/ICNF.2013.6578965 (DOI)000332005700092 ()2-s2.0-84883710260 (Scopus ID)978-1-4799-0668-0 (ISBN)
Conference
2013 22nd International Conference on Noise and Fluctuations, ICNF 2013, Montpellier, France, 24 June 2013 through 28 June 2013
Funder
Swedish Research Council, 2009-4190Swedish Research Council, 2012-5372
Note

QC 20131030

Available from: 2013-10-30 Created: 2013-10-29 Last updated: 2016-06-20Bibliographically approved
2. Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field
Open this publication in new window or tab >>Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field
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2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, no 9, 092405- p.Article in journal (Refereed) Published
Abstract [en]

The nano-scale spin torque oscillator (STO) is a compelling device for on-chip, highly tunable microwave frequency signal generation. Currently, one of the most important challenges for the STO is to increase its longer-time frequency stability by decreasing the 1/f frequency noise, but its high level makes even its measurement impossible using the phase noise mode of spectrum analyzers. Here, we present a custom made time-domain measurement system with 150MHz measurement bandwidth making possible the investigation of the variation of the 1/f as well as the white frequency noise in a STO over a large set of operating points covering 18-25GHz. The 1/f level is found to be highly dependent on the oscillation amplitude-frequency non-linearity and the vicinity of unexcited oscillation modes. These findings elucidate the need for a quantitative theoretical treatment of the low-frequency, colored frequency noise in STOs. Based on the results, we suggest that the 1/f frequency noise possibly can be decreased by improving the microstructural quality of the metallic thin films.

Keyword
Amplitude-frequency, Measurement bandwidth, Metallic thin films, Spin-torque oscillator (STO), Spin-torque oscillators, Theoretical treatments, Time domain measurement, Tunable microwave
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-144370 (URN)10.1063/1.4867257 (DOI)000332729200057 ()2-s2.0-84896799716 (Scopus ID)
Funder
Swedish Foundation for Strategic Research Swedish Research Council, 2009-4190Knut and Alice Wallenberg Foundation
Note

QC 20140422

Available from: 2014-04-22 Created: 2014-04-22 Last updated: 2017-12-05Bibliographically approved
3. Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part I: Analytical Model of the MTJ STO
Open this publication in new window or tab >>Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part I: Analytical Model of the MTJ STO
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2015 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 62, no 3, 1037-1044 p.Article in journal (Refereed) Published
Abstract [en]

Magnetic tunnel junction (MTJ) spin torque oscillators (STOs) have shown the potential to be used in a wide range of microwave and sensing applications. To evaluate the potential uses of MTJ STO technology in various applications, an analytical model that can capture MTJ STO's characteristics, while enabling system-and circuit-level designs, is of great importance. An analytical model based on macrospin approximation is necessary for these designs since it allows implementation in hardware description languages. This paper presents a new macrospin-based, comprehensive, and compact MTJ STO model, which can be used for various MTJ STOs to estimate the performance of MTJ STOs together with their application-specific integrated circuits. To adequately present the complete model, this paper is divided into two parts. In Part I, the analytical model is introduced and verified by comparing it against measured data of three different MTJ STOs, varying the angle and magnitude of the magnetic field, as well as the DC biasing current. The proposed analytical model is suitable for being implemented in Verilog-A and used for efficient simulations at device, circuit, and system levels. In Part II, the full Verilog-A implementation of the analytical model with accurate phase noise generation is presented and verified by simulations.

Keyword
Analytical model, macrospin, magnetic tunnel junction (MTJ), spin torque oscillator (STO)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-163459 (URN)10.1109/TED.2015.2390411 (DOI)000350332000051 ()2-s2.0-84923687907 (Scopus ID)
Funder
Swedish Research Council, 2009-4190
Note

QC 20150408

Available from: 2015-04-08 Created: 2015-04-07 Last updated: 2017-12-04Bibliographically approved
4. Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part II: Verilog-A Model Implementation
Open this publication in new window or tab >>Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part II: Verilog-A Model Implementation
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2015 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 62, no 3, 1045-1051 p.Article in journal (Refereed) Published
Abstract [en]

The rapid development of the magnetic tunnel junction (MTJ) spin torque oscillator (STO) technology demands an analytical model to enable building MTJ STO-based circuits and systems so as to evaluate and utilize MTJ STOs in various applications. In Part I of this paper, an analytical model based on the macrospin approximation has been introduced and verified by comparing it with the measurements of three different MTJ STOs. In Part II, the full Verilog-A implementation of the proposed model is presented. To achieve a reliable model, an approach to reproducing the phase noise generated by the MTJ STO has been proposed and successfully employed. The implemented model yields a time domain signal, which retains the characteristics of operating frequency, linewidth, oscillation amplitude, and DC operating point, with respect to the magnetic field and applied DC current. The Verilog-A implementation is verified against the analytical model, providing equivalent device characteristics for the full range of biasing conditions. Furthermore, a system that includes an MTJ STO and CMOS RF circuits is simulated to validate the proposed model for system-and circuit-level designs. The simulation results demonstrate that the proposed model opens the possibility to explore STO technology in a wide range of applications.

Keyword
Analytical model, macrospin, magnetic tunnel junction (MTJ), spin torque oscillator (STO)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-163460 (URN)10.1109/TED.2015.2390676 (DOI)000350332000052 ()2-s2.0-84923688220 (Scopus ID)
Funder
Swedish Research Council, 2009-4190
Note

QC 20150408

Available from: 2015-04-08 Created: 2015-04-07 Last updated: 2017-12-04Bibliographically approved
5. Direct observation and imaging of a spin-wave soliton with p−like symmetry
Open this publication in new window or tab >>Direct observation and imaging of a spin-wave soliton with p−like symmetry
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2015 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, 8889Article in journal (Refereed) Published
Abstract [en]

The prediction and realization of magnetic excitations driven by electrical currents via the spin transfer torque effect, enables novel magnetic nano-devices where spin-waves can be used to process and store information. The functional control of such devices relies on understanding the properties of non-linear spin-wave excitations. It has been demonstrated that spin waves can show both an itinerant character, but also appear as localized solitons. So far, it was assumed that localized solitons have essentially cylindrical, s−like symmetry. Using a newly developed high-sensitivity time-resolved magnetic x-ray microscopy, we instead observe the emergence of a novel localized soliton excitation with a nodal line, i.e. with p−like symmetry. Micromagnetic simulations identify the physical mechanism that controls the transition from s− to p−like solitons. Our results suggest a potential new pathway to design artificial atoms with tunable dynamical states using nanoscale magnetic devices.

Place, publisher, year, edition, pages
Nature Publishing Group, 2015
Keyword
spin-wave soliton, x-ray
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-163518 (URN)10.1038/ncomms9889 (DOI)000366296800003 ()2-s2.0-84947292475 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20150108

Available from: 2015-04-07 Created: 2015-04-07 Last updated: 2017-12-04Bibliographically approved
6. Nonlinearity, frequency stability and device-to-device variability in nano-contact spin torque oscillators with grainy thin films
Open this publication in new window or tab >>Nonlinearity, frequency stability and device-to-device variability in nano-contact spin torque oscillators with grainy thin films
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In nano-contact spin torque oscillators with a frequency range of 10-65 GHz, the propagating spin wave mode attracts interest due both to its high frequency stability and prospective use in magnonic devices. Its dependence of the frequency on the bias current however displays device-to-device variability on the order of several hundred MHz, with device specific nonlinearities that can be either continuous or discontinuous and have negative impact on the frequency stability. A model for this behavior is however still lacking. By using micromagnetic simulations, we investigate the impact of imperfections in the spin wave-carrying free magnetic layer and find that nonlinearities can be created when the propagating spin wave is reflected back to the active region. The oscillation then self-locks at the frequency of the resonant wavelength, resulting in a standing spin wave pattern. Simulations including nine randomly generated film structures with 30 nm-sized grains and exchange-reduced inter-grain boundaries give qualitative and partially quantitative agreement with experimental measurements. The results point out the spin wave-reflecting grain boundaries as a source of device nonlinearity, manufacturing variability and frequency destabilization.

Keyword
Spin torque, spin waves, magnetization dynamics, thin films, microstructure, microwaves, phase noise
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-188544 (URN)
Funder
Swedish Research Council
Note

Qc 20160616

Available from: 2016-06-13 Created: 2016-06-13 Last updated: 2016-06-20Bibliographically approved

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  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
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  • de-DE
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  • nn-NO
  • nn-NB
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Output format
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