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Contracting vs. expanding spin wave bullets in spin Hall nano-oscillators
KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.ORCID iD: 0000-0003-3605-8872
University of Gothenburg.
University of Gothenburg.
KTH, School of Engineering Sciences (SCI), Applied Physics.
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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: urn:nbn:se:kth:diva-234467OAI: oai:DiVA.org:kth-234467DiVA, id: diva2:1246218
Note

QC 20180907

Available from: 2018-09-06 Created: 2018-09-06 Last updated: 2018-09-07Bibliographically approved
In thesis
1. 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)
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Note

QC 20180907

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

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Mazraati, HamidChung, SunjaeÅkerman, Johan

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