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Dynamics and Intrinsic Variability of Spintronic Devices
KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electronics and Embedded systems.ORCID iD: 0000-0002-7484-9694
2023 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Dynamik och Inneboende Variabilitet hos Spinntronik-enheter (Swedish)
Abstract [en]

Spintronics is a scientific domain focusing on utilizing electron spin for information processing. This is the element that distinguishes it from electronics, which only utilizes the charge of electrons. A common purpose of spintronic devices is to implement additional functionalities to state-of-the-art Complementary Metal-Oxide Semiconductor (CMOS) technology. The aim of this work was to assess the intrinsic variabilities of Nano-Constriction Spin Hall Nano-Oscillators (NC-SHNOs) and the dynamics of Perpendicular Magnetic Tunnel Junctions (pMTJs). 

The first part of the thesis focuses on NC-SHNO and two-dimensional arrays. They are nanometer-sized microwave oscillators, allowing for a wide frequency tuning range, and are compatible with CMOS Back End Of Line (BEOL). These devices are based on a heavy metal/ferromagnetic bilayer. Environmental conditions during processing, fabrication techniques, and temperature of operation can all create variabilities in the device's functioning. Crystallization grains naturally form during the sputtering of the metals. Atomic Force Microscope (AFM) characterization showed the grains being of different shapes, about 30 nm in size. Here, the aim was to develop a simulation technique based on importing the measured grain structure into micromagnetic simulations. Their results match the device-to-device variability and multi-modal behavior found in microwave measurements. Moreover, the presence of grains influences the synchronization of the arrays.

The second part of this work focuses on pMTJ. These non-volatile memory elements have two metastable states, parallel (P) and antiparallel (AP), separated by an energy barrier Eb. Here, the aim was to show their potential as True Random Number Generators (TRNGs). A pulse-activated measurement set-up was used to realize random bitstreams. The randomness was confirmed by the National Institute of Standards and Technology Statistical Testing Suite (NIST-STS). After one whitening Exclusive OR (XOR) stage, all tests were successfully passed.

The assessment was completed with the development of a model describing both macrospin and domain wall-mediated magnetization reversals, i.e. switching between P and AP. The analysis of the reversal dynamics was carried out with micromagnetic simulations and String Method calculations. As expected, Eb is lowered by the field and by decreasing the device size. This allows for faster fluctuations, marking the device as a potential TRNG. Both the switching attempt frequency and the energy barrier were explored by finite-temperature micromagnetic simulations.

This thesis shows the potential of realistic simulations combined with measurements to assess oscillators. It also shows the efficacy of spintronic devices as 10s-MHz TRNG.

Abstract [sv]

Spinntronik är ett vetenskapligt område som fokuserar på att utnyttja elektronens spin för informationsbehandling. Denna aspekt skiljer spinntronik från elektronik, som endast utnyttjar elektronens laddning. En vanlig syfte med spintronik-komponenter är att implementera ytterligare funktionalitet i toppmodern Complementary Metal-Oxide Semiconductor (CMOS) -teknologi. Syftet med detta arbete var att utvärdera de inneboende variabiliteterna hos Nano-Constriction Spin Hall Nano-Oscillators (NC-SHNOs) och dynamiken hos Perpendicular Magnetic Tunnel Junctions (pMTJs).

Den första delen av avhandlingen fokuserar på NC-SHNOs och tvådimensionella matriser. De är nanometerstora mikrovågsgeneratorer som möjliggör ett brett frekvensområde och är kompatibla med CMOS Back End Of Line (BEOL). Dessa komponenter är baserade på ett tungmetall/ferromagnetisk bilager. Miljöförhållanden under bearbetning, tillverkningstekniker och driftstemperatur kan alla skapa variabiliteter i komponentens funktion. Kristallisationskorn bildas naturligt under metallskiktens sputtring. Karaktärisering med Atomic Force Microscope (AFM) visade att kornen hade olika former och var ungefär 30 nm i storlek. Här var målet att utveckla en simuleringsmetod baserad på att importera den uppmätta kornstrukturen i mikromagnetiska simuleringar. Deras resultat matchar variabiliteten mellan enheter och det multimodala beteendet som återfinns i mikrovågsmätningar. Dessutom påverkar närvaron av korn synkroniseringen av matriserna.

Den andra delen av detta arbete fokuserar på pMTJs. Dessa icke-flyktiga minneselement har två metastabila tillstånd, parallella (P) och antiparallella (AP), separerade av en energibarriär Eb. Här var målet att visa deras potential som sanna slumptalsgeneratorer (TRNGs). En uppsättning pulsmätningar användes för att generera slumpmässiga bitströmmar. Slumpmässigheten bekräftades av National Institute of Standards and Technology Statistical Testing Suite (NIST-STS). Efter en XOR-steg för att göra strömmen mer vit passerades alla tester framgångsrikt.

Studien slutfördes med utvecklingen av en modell som beskriver både makrospinn och domänvägg-medierade magnetiseringsrevereseringar, det vill säga övergångar mellan P- och AP-tillstånd. Analysen av reverseringssdynamiken utfördes med mikromagnetiska simuleringar och String Method-beräkningar. Som förväntat minskas Eb av fältet och genom att minska komponentens storlek. Detta möjliggör snabbare fluktuationer och identifierar komponenten som en potentiell TRNG. Både försöksfrekvensen för reversering och energibarriären utforskades med hjälp av mikromagnetiska simuleringar vid ändlig temperatur.

Resultaten som presenteras här visar nödvändigheten av noggrann hänsyn till lagrens brister för att skapa övertygande neuromorfiska enheter. Dessutom bevisar detta arbete spinntronik-enheters effektivitet som TRNGs med en frekvens på 10s-MHz.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2023. , p. xxvi, 104
Series
TRITA-EECS-AVL ; 2023:65
Keywords [en]
Spintronics, micromagnetic simulations, SHNO, MTJ, microwave measurements, realistic simulations, low-temperature measurements, fabrication, grains
Keywords [sv]
Spinntronik, mikromagnetiska simuleringar, SHNO, MTJ, mikrovågsmätningar, realistiska simuleringar, lågtemperaturmätningar, nanotillverkning, korn
National Category
Nano Technology Condensed Matter Physics Probability Theory and Statistics
Research subject
Information and Communication Technology
Identifiers
URN: urn:nbn:se:kth:diva-336768ISBN: 978-91-8040-702-1 (print)OAI: oai:DiVA.org:kth-336768DiVA, id: diva2:1798755
Public defence
2023-10-13, Ka-Sal B, Electrum https://kth-se.zoom.us/j/64577443824, Kistagången 16, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2017-04196Available from: 2023-09-20 Created: 2023-09-20 Last updated: 2023-10-09Bibliographically approved
List of papers
1. Impact of Random Grain Structure on Spin-Hall Nano-Oscillator Modal Stability
Open this publication in new window or tab >>Impact of Random Grain Structure on Spin-Hall Nano-Oscillator Modal Stability
2022 (English)In: IEEE Electron Device Letters, ISSN 0741-3106, E-ISSN 1558-0563, Vol. 43, no 2, p. 312-315Article in journal (Refereed) Published
Abstract [en]

Spin-Hall nano-oscillators are a promising class of microwave spintronic devices with potential applications in RF/microwave communication and neuromorphic computing. The nano-constriction spin-Hall nano-oscillators (NC-SHNO) have relatively high power, narrow linewidth, and low drive current. Several synchronization schemes e.g. arrays of spin-wave coupled oscillators have been proposed for more stable operation and higher output power. For such arrays, it is crucial to have good oscillator stability and small device-to-device variability. Here, a micromagnetic simulation technique is proposed that includes realistic material properties and hence enables variability and modal stability to be investigated. It is demonstrated, using both measurements and simulation, that the presence of physical grains in the free magnetic layer can induce multiple oscillation modes or frequency sidebands. Our investigation could help in the development of more stable NC-SHNOs that would enable oscillator arrays with stronger synchronization.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2022
Keywords
Micromagnetic simulations, microwave measurements, modal stability, SHNO, spintronics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-309001 (URN)10.1109/LED.2021.3137952 (DOI)000748371400040 ()2-s2.0-85122071395 (Scopus ID)
Note

QC 20220329

Available from: 2022-02-24 Created: 2022-02-24 Last updated: 2023-09-20Bibliographically approved
2. Stochastic Magnetic Actuated Random Transducer Devices Based on Perpendicular Magnetic Tunnel Junctions
Open this publication in new window or tab >>Stochastic Magnetic Actuated Random Transducer Devices Based on Perpendicular Magnetic Tunnel Junctions
Show others...
2023 (English)In: Physical Review Applied, E-ISSN 2331-7019, Vol. 19, no 2, article id 024035Article in journal (Refereed) Published
Abstract [en]

True random number generators are of great interest in many computing applications, such as cryptography, neuromorphic systems, and Monte Carlo simulations. Here, we investigate perpendicular magnetic-tunnel-junction nanopillars (pMTJs) activated by short-duration (nanosecond) pulses in the ballistic limit for such applications. In this limit, a pulse can transform the Boltzmann distribution of initial free-layer magnetization states into randomly magnetized down or up states, i.e., a bit that is 0 or 1, easily determined by measurement of the tunnel resistance of the junction. It is demonstrated that bit streams with millions of events: (1) are very well approximated by a normal distribution; (2) pass multiple statistical tests for true randomness, including all the National Institute of Standards and Technology tests for random number generators with only one XOR operation; (3) can be used to create a uniform distribution of 8-bit random numbers; and (4) can have no drift in the bit probability with time. The results presented here show that pMTJs operated in the ballistic regime can generate true random numbers at around 50-MHz bit rates, while being more robust to environmental changes, such as their operating temperature, compared to other stochastic nanomagnetic devices.

Place, publisher, year, edition, pages
American Physical Society (APS), 2023
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-325041 (URN)10.1103/PhysRevApplied.19.024035 (DOI)000935714900003 ()2-s2.0-85148327666 (Scopus ID)
Note

QC 20230328

Available from: 2023-03-28 Created: 2023-03-28 Last updated: 2023-09-20Bibliographically approved
3. Grain structure influence on synchronized two-dimensional spin-Hall nano-oscillators
Open this publication in new window or tab >>Grain structure influence on synchronized two-dimensional spin-Hall nano-oscillators
2023 (English)In: AIP Advances, E-ISSN 2158-3226, Vol. 13, no 5, article id 055103Article in journal (Refereed) Published
Abstract [en]

Nanoconstriction spin-Hall nano-oscillators (NC-SHNOs) are excellent devices for a wide variety of applications, from RF communication to bio-inspired computing. NC-SHNOs are easy to fabricate in large arrays, are CMOS compatible, and feature a narrow linewidth and high output power. However, in order to take full advantage of the device capabilities, a systematic analysis of the array behavior with respect to the number and dimensions of oscillators, the temperature of operation, and the influence of layer quality is needed. Here, we focus on micromagnetic simulations of 2 x 2 and 4 x 4 NC-SHNO arrays with single oscillators separated by up to 300 nm. We observe a synchronization scheme that allows for column-wise selection of the oscillation frequency for a larger pitch. However, for smaller pitches, a coherent oscillation volume was observed, and this volume included both the constrictions and extended beyond that region. A local variation in the exchange coupling in the active oscillator region was investigated by placing physical grains in the free magnetic layer, and it was shown to influence both the stable current range and the resulting frequency and output power. De-coupling the oscillators along rows or columns could provide higher power due to more favorable phase shifts between oscillators. Our investigation helps in achieving a deeper understanding of the intrinsic working principles of NC-SHNO arrays and how they reach fully synchronized states, and this will help to expand non-conventional computing capabilities.

Place, publisher, year, edition, pages
AIP Publishing, 2023
National Category
Embedded Systems
Identifiers
urn:nbn:se:kth:diva-329372 (URN)10.1063/5.0147668 (DOI)000981460800009 ()2-s2.0-85158018709 (Scopus ID)
Note

QC 20230620

Available from: 2023-06-20 Created: 2023-06-20 Last updated: 2023-09-20Bibliographically approved
4. Energy Barriers for Thermally Activated Magnetization Reversal in Perpendicularly Magnetized Nanodisks in a Transverse Field
Open this publication in new window or tab >>Energy Barriers for Thermally Activated Magnetization Reversal in Perpendicularly Magnetized Nanodisks in a Transverse Field
2023 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Thermally-induced transitions between bistable magnetic states of magnetic tunnel junctions (MTJ) are of interest for generating random bitstreams and for applications in stochastic computing. An applied field transverse to the easy axis of a perpendicularly magnetized MTJ (pMTJ) can lower the energy barrier (Eb) to these transitions leading to faster fluctuations. In this study, we present analytical and numerical calculations of Eb considering both coherent (macrospin) reversal and non-uniform wall-mediated magnetization reversal for a selection of nanodisk diameters and applied fields. Non-uniform reversal processes dominate for larger diameters, and our numerical calculations of Eb using the String method show that the transition state has a sigmoidal magnetization profile. The latter can be described with an analytical expression that depends on only one spatial dimension, parallel to the applied field, which is also the preferred direction of profile motion during reversal. Our results provide nanodisk energy barriers as a function of the transverse field, nanodisk diameter, and material characteristics, which are useful for designing stochastic bitstreams.

Keywords
pMTJ, stochastic magnetization dynamics, micromagnetic modeling
National Category
Nano Technology Condensed Matter Physics
Research subject
Information and Communication Technology; Physics, Material and Nano Physics
Identifiers
urn:nbn:se:kth:diva-336767 (URN)10.48550/arXiv.2305.09558 (DOI)
Funder
Swedish Research Council, 2017-04196
Note

Under review process in Physical Review Applied by the American Physical Society

QC 20231102

Available from: 2023-09-19 Created: 2023-09-19 Last updated: 2023-11-02Bibliographically approved
5. Enhanced Stochastic Bit Rate for Perpendicular Magnetic Tunneling Junctions in a Transverse Field
Open this publication in new window or tab >>Enhanced Stochastic Bit Rate for Perpendicular Magnetic Tunneling Junctions in a Transverse Field
2023 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Perpendicular magnetic tunneling junctions(pMTJs) as true random number generators (TRNGs) have been investigated by means of high-temperature micromagnetic simulations using MuMax3. An in-plane applied field, which lowers the energy barrier for thermally activated reversal, can be used to control and increase the bitrates. We study the attempt rate and the energy barrier for 10 and 40 nm diameter devices in various applied magnetic fields. At room temperature, the presence of the field leads to orders of magnitude increase in the bitrate, up to ∼ 100 MHz.

Keywords
MTJ, TRNG, stochastic magnetization dynamics, micromagnetic modeling
National Category
Nano Technology Condensed Matter Physics
Research subject
Information and Communication Technology; Physics, Material and Nano Physics
Identifiers
urn:nbn:se:kth:diva-336761 (URN)
Conference
26th International Conference on Noise and Fluctuations (ICNF) in Grenoble, October 17-20,2023
Funder
Swedish Research Council, 2017-04196
Note

QC 20230920

Available from: 2023-09-19 Created: 2023-09-19 Last updated: 2023-09-20Bibliographically approved

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Capriata, Corrado Carlo Maria

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