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Linear, Non-Linear, and Synchronizing Spin Wave Modes in Spin Hall Nano-Oscillators
KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.ORCID iD: 0000-0003-3605-8872
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 [en]
spin Hall effect, spin Hall nano-oscillators, threshold current, microwave, spin wave, synchronization
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-234657ISBN: 978-91-7729-929-5 (print)OAI: oai:DiVA.org:kth-234657DiVA, id: diva2:1246423
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
List of papers
1. Low operational current spin Hall nano-oscillators based on NiFe/W bilayers
Open this publication in new window or tab >>Low operational current spin Hall nano-oscillators based on NiFe/W bilayers
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2016 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, no 24, article id 242402Article in journal (Refereed) Published
Abstract [en]

We demonstrate highly efficient spin Hall nano-oscillators (SHNOs) based on NiFe/beta-W bilayers. Thanks to the very high spin Hall angle of beta-W, we achieve more than a 60% reduction in the auto-oscillation threshold current compared to NiFe/Pt bilayers. The structural, electrical, and magnetic properties of the bilayers, as well as the microwave signal generation properties of the SHNOs, have been studied in detail. Our results provide a promising path for the realization of low-current SHNO microwave devices with highly efficient spin-orbit torque from beta-W. Published by AIP Publishing.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-201252 (URN)10.1063/1.4971828 (DOI)000391457500025 ()2-s2.0-85006791339 (Scopus ID)
Note

QC 20170215

Available from: 2017-02-15 Created: 2017-02-15 Last updated: 2018-09-07Bibliographically approved
2. CMOS compatible W/CoFeB/MgO spin Hall nano-oscillators with wide frequency tunability
Open this publication in new window or tab >>CMOS compatible W/CoFeB/MgO spin Hall nano-oscillators with wide frequency tunability
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2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 13, article id 132404Article in journal (Refereed) Published
Abstract [en]

We demonstrate low-operational-current W/Co20Fe60B20/MgO spin Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates. Thanks to a record high spin Hall angle of the beta-phase W (theta(SH) = -0.53), a very low threshold current density of 3.3 x 10(7) A/cm(2) can be achieved. Together with their very wide frequency tunability (7-28GHz), promoted by a moderate perpendicular magnetic anisotropy, HiR-Si/W/CoFeB based SHNOs are potential candidates for wide-band microwave signal generation. Their CMOS compatibility offers a promising route towards the integration of spintronic microwave devices with other on-chip semiconductor microwave components.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-226793 (URN)10.1063/1.5022049 (DOI)000429072800015 ()2-s2.0-85044750620 (Scopus ID)
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg FoundationEU, FP7, Seventh Framework Programme, 307144
Note

QC 20180504

Available from: 2018-05-04 Created: 2018-05-04 Last updated: 2018-09-07Bibliographically approved
3. Improving the magnetodynamical properties of NiFe/Pt bilayers through Hf dusting
Open this publication in new window or tab >>Improving the magnetodynamical properties of NiFe/Pt bilayers through Hf dusting
2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 113, no 9, p. 092401-Article in journal, Letter (Refereed) Published
Abstract [en]

We investigate the effect of hafnium (Hf) dusting on the magnetodynamical properties of NiFe/Pt bilayers using spin-torque-induced ferromagnetic resonance measurements on 6 μm wide microstrips on high-resistive Si substrates. Based on two series of NiFe(tNiFe)/Hf(tHf)/Pt(5) stacks, we first demonstrate that the zero-current magnetodynamic properties of the devices benefit from Hf dusting: (i) the effective magnetization of the NiFe layer increases by 4%–8% with Hf present and (ii) the damping α decreases linearly with tHf by up to 40%. The weaker anisotropic magnetoresistance (AMR≈0.3%–0.4%) of the 3 nm NiFe series is largely unaffected by the Hf, while the stronger AMR of the 5 nm NiFe series drops from 0.7% to 0.43% with increasing tHf. We find that the spin Hall efficiency ξSH is independent of the NiFe thickness, remaining unaffected (ξSH = 0.115) up to tHf = 0.4 nm and then decreasing linearly for higher tHf. The different trends of α and ξSH suggest that there is an optimum Hf thickness (≈0.4 nm) for which the threshold current for auto-oscillation should have a minimum, while the much lower damping should improve mutual synchronization. Our results also indicate that the spin-orbit torque is entirely damping-like with no field-like torque component. Finally, the internal spin Hall angle of Pt is estimated to be θSH = 0.22 by calculating the transparency of the interface.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2018
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-234311 (URN)10.1063/1.5026232 (DOI)2-s2.0-85052729304 (Scopus ID)
Note

QC 20180911

Available from: 2018-09-06 Created: 2018-09-06 Last updated: 2018-09-11Bibliographically approved
4. Auto-oscillating spin-wave modes of constriction-based spin Hall nano-oscillators in weak in-plane fields
Open this publication in new window or tab >>Auto-oscillating spin-wave modes of constriction-based spin Hall nano-oscillators in weak in-plane fields
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We experimentally study the auto-oscillating spin-wave modes in Ni80Fe20/β-W constriction-based spin Hall nano-oscillators as a function of bias current, in-plane applied field strength, and azimuthal field angle, in the low-field range of 40-80 mT. We observe two different spin-wave modes: i) a linear-like mode confined to the minima of the internal field near the edges of the nanoconstriction, with weak frequency dependencies on the bias current and the applied field angle, and ii) a second, lower frequency mode that has significantly higher threshold current and stronger frequency dependencies on both bias current and the external eld angle. Our micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the SHNO mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linear-like mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Utilizing intrinsic frequency doubling at zero eld angle we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-eld spintronic oscillators with applications in microwave signal generation and neuromorphic computing.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-234466 (URN)
Available from: 2018-09-06 Created: 2018-09-06 Last updated: 2018-09-07Bibliographically approved
5. Contracting vs. expanding spin wave bullets in spin Hall nano-oscillators
Open this publication in new window or tab >>Contracting vs. expanding spin wave bullets in spin Hall nano-oscillators
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We employ electrical measurements complemented by systematic micromagnetic simulations to reveal the complex dynamics of nanoconstriction-based spin Hall nano-oscillators. In particular, depending on the strength and out-of-plane angle of the applied magnetic field, we observe three distinct types of magnetization auto-oscillation: (a) a linear-like mode localized in the vicinity of the nanoconstriction by the demagnetizing field, (b) a further localized “regular” spin wave bullet, and(c) a “large” bullet that fills the entire area of the nanoconstriction. Although it has been assumed for some time that bullets only emerge if the nonlinearity of the system is negative (corresponding to the attraction of magnons), our results demonstrate that, in patterned films, they could be sustained even if the nonlinearity of the system is positive (corresponding to the repulsion of magnons). So, in contrast to the regular spin wave bullet, the auto-oscillation volume of its novel large counterpart enlarges, with the amplitude enhancing their drift stability and, correspondingly, reducing their linewidth. We demonstrate that tuning can be achieved between the observed modes at a fixed external field by changing only the drive current, thanks to the amplitude-dependent nonlinearity of the auto-oscillations. This flexibility of nanopatterned spin Hall nano-oscillators is desirable to achieve synaptic functionality in oscillator-based neuromorphic computing devices.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-234467 (URN)
Note

QC 20180907

Available from: 2018-09-06 Created: 2018-09-06 Last updated: 2018-09-07Bibliographically approved
6. 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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