| 1. |
- Keatley, P. S., et al.
(författare)
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Direct observation of magnetization dynamics generated by nanocontact spin-torque vortex oscillators
- 2016
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 94:6
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved scanning Kerr microscopy has been used to directly image the magnetization dynamics of nanocontact (NC) spin-torque vortex oscillators (STVOs) when phase locked to an injected microwave (rf) current. The Kerr images reveal free-layer magnetization dynamics that extend outside the NC footprint, where they cannot be detected electrically, but which are crucial to phase-lock STVOs that share common magnetic layers. For a single NC, dynamics were observed not only when the STVO frequency was fully locked to that of the rf current, but also for a partially locked state characterized by periodic changes in the core trajectory at the rf frequency. For a pair of NCs, we explore the correlation between the spatial character of injection-locked dynamics and the free-running spectra. Insight gained from these images may improve understanding of the conditions required for mutual phase locking of multiple STVOs, and hence enhanced microwave power emission.
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| 2. |
- Keatley, P. S., et al.
(författare)
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Magneto-optical observation of mutual phase-locking in a pair of spin-torque vortex oscillators
- 2015
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Ingår i: 2015 IEEE International Magnetics Conference, INTERMAG 2015. - : IEEE conference proceedings. - 9781479973224
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Konferensbidrag (refereegranskat)abstract
- Nano-contact (NC) spin-torque vortex oscillators (STVOs) that share constituent ferromagnetic layers are anticipated to overcome the technological bottlenecks of low microwave power and phase instability exhibited by individual STVOs.1 Mutual synchronisation of multiple NC-STVOs can be achieved by dynamic coupling within the common magnetic layers.1,2 Alternatively, injection of a small microwave (RF) current can lock the phase of multiple STVOs.3 Direct observation of the coupled magnetization dynamics is challenging and has so far been inferred from micromagnetic simulations.4 In this work a combination of RF electrical characterization and time-resolved scanning Kerr microscopy (TRSKM) has been used to image the magnetization dynamics excited in the continuous film NiFe free layer of a pair of NC-STVOs.
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| 3. |
- Sharma, R., et al.
(författare)
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Modulation Rate Study in a Spin-Torque Oscillator-Based Wireless Communication System
- 2015
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Ingår i: IEEE transactions on magnetics. - : IEEE Press. - 0018-9464 .- 1941-0069. ; 51:11
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Tidskriftsartikel (refereegranskat)abstract
- We study a wireless communication system based on a magnetic tunnel junction spin-torque nano-oscillator (STNO) by employing amplitude-shift-keying modulation. By varying the pulse modulation frequency (f(m)) from 1 kHz to 2 MHz and distance (D) between the antenna from 25 to 150 cm, we show a maximum data rate of 6 Mb/s (at D = 25 cm and fm = 1 MHz), a limit imposed by our setup and noise generated by the STNO itself. We also report the average amplitude noise (S-delta a) and average white frequency noise (S-wh) of the wireless communication system and discuss their dependence on the distance between the antennas.
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| 4. |
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| 5. |
- Spicer, T. M., et al.
(författare)
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Spatial mapping of torques within a spin Hall nano-oscillator
- 2018
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:21
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved scanning Kerr microscopy (TRSKM) was used to study precessional magnetization dynamics induced by a radio frequency (RF) current within a Al2O3/Py(5 nm)/Pt(6 nm)/Au(150 nm) spin Hall nano-oscillator structure. The Au layer was formed into two needle-shaped electrical contacts that concentrated the current in the center of a Py/Pt mesa of 4 mu m diameter. Due to the spin Hall effect, current within the Pt layer drives a spin current into the Py layer, exerting a spin transfer torque (STT). By injecting RF current and exploiting the phase sensitivity of TRSKM and the symmetry of the device structure, the STT and Oersted field torques have been separated and spatially mapped. The STT and torque due to the in-plane Oersted field are observed to exhibit minima at the device center that is ascribed to spreading of RF current that is not observed for DC current. Torques associated with the RF current may destabilize the position of the self-localized bullet mode excited by the DC current and inhibit injection locking. The present study demonstrates the need to characterize both DC and RF current distributions carefully.
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| 6. |
- Awad, A. A., et al.
(författare)
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Long-range mutual synchronization of spin Hall nano-oscillators
- 2017
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Ingår i: Nature Physics. - : Nature Publishing Group. - 1745-2473 .- 1745-2481. ; 13:3, s. 292-
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Tidskriftsartikel (refereegranskat)abstract
- The spin Hall effect in a non-magnetic metal with spin-orbit coupling injects transverse spin currents into adjacent magnetic layers, where the resulting spin transfer torque can drive spin wave auto-oscillations. Such spin Hall nano-oscillators (SHNOs) hold great promise as extremely compact and broadband microwave signal generators and magnonic spin wave injectors. Here we show that SHNOs can also be mutually synchronized with unprecedented efficiency. We demonstrate mutual synchronization of up to nine individual SHNOs, each separated by 300 nm. Through further tailoring of the connection regions we can extend the synchronization range to 4 mu m. The mutual synchronization is observed electrically as an increase in the power and coherence of the microwave signal, and confirmed optically using micro-Brillouin light scattering microscopy as two spin wave regions sharing the same spectral content, in agreement with our micromagnetic simulations.
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| 7. |
- Banuazizi, Seyed, et al.
(författare)
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Order of magnitude improvement of nano-contact spin torque nano-oscillator performance
- 2017
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Ingår i: 2017 IEEE International Magnetics Conference, INTERMAG 2017. - : Institute of Electrical and Electronics Engineers (IEEE). - 9781538610862
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Konferensbidrag (refereegranskat)abstract
- Spin torque nano-oscillators [1,2] (STNO) represent a unique class of nano-scale microwave signal generators where spin transfer torque [3-5] (STT) from a direct spin-polarized current drives and controls the auto-oscillation of the local free layer magnetization, which through its oscillating magnetoresistance transforms the direct current into a tunable microwave voltage.
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| 8. |
- Chen, Tingsu, et al.
(författare)
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A highly tunable microwave oscillator based on MTJ STO technology
- 2014
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Ingår i: Microwave and optical technology letters (Print). - : Wiley. - 0895-2477 .- 1098-2760. ; 56:9, s. 2092-2095
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Tidskriftsartikel (refereegranskat)abstract
- This article presents a fully ESD-protected, highly tunable microwave oscillator based on magnetic tunnel junction (MTJ) spin torque oscillator (STO) technology. The oscillator consists of a compact MTJ STO and a 65 nm CMOS wideband amplifier, which amplifies the RF signal of the MTJ STO to a level that can be used to drive a PLL. The (MTJ STO+amplifier IC) pair shows a measured quality factor (Q) of 170 and a wide tunability range from 3 to 7 GHz, which demonstrate its potential to be used as a microwave oscillator in multiband, multistandard radios.
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| 9. |
- Chen, Tingsu, et al.
(författare)
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Spin-Torque and Spin-Hall Nano-Oscillators
- 2016
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Ingår i: Proceedings of the IEEE. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9219 .- 1558-2256. ; 104:10, s. 1919-1945
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Tidskriftsartikel (refereegranskat)abstract
- This paper reviews the state of the art in spin-torque and spin-Hall-effect-driven nano-oscillators. After a brief introduction to the underlying physics, the authors discuss different implementations of these oscillators, their functional properties in terms of frequency range, output power, phase noise, and modulation rates, and their inherent propensity for mutual synchronization. Finally, the potential for these oscillators in a wide range of applications, from microwave signal sources and detectors to neuromorphic computation elements, is discussed together with the specific electronic circuitry that has so far been designed to harness this potential.
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| 10. |
- Chung, Sunjae, et al.
(författare)
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Magnetic droplet solitons in orthogonal spin valves
- 2015
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Ingår i: Fizika Nizkih Temperatur. - : American Institute of Physics (AIP). - 0132-6414 .- 1816-0328. ; 41:10, s. 833-837
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Tidskriftsartikel (refereegranskat)abstract
- We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.
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| 11. |
- Dürrenfeld, P., et al.
(författare)
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A 20 nm spin Hall nano-oscillator
- 2017
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Ingår i: Nanoscale. - : RSC Publishing. - 2040-3364 .- 2040-3372. ; 9:3, s. 1285-1291
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Tidskriftsartikel (refereegranskat)abstract
- Spin Hall nano-oscillators (SHNOs) are an emerging class of pure spin current driven microwave signal generators. Through the fabrication of 20 nm nano-constrictions in Pt/NiFe bilayers, we demonstrate that SHNOs can be scaled down to truly nanoscopic dimensions, with the added benefit of ultra-low operating currents and improved power conversion efficiency. The lateral confinement leads to a strong shape anisotropy field as well as an additional demagnetizing field whose reduction with increasing auto-oscillation amplitude can yield a positive current tunability contrary to the negative tunability commonly observed for localized excitations in extended magnetic layers. Micromagnetic simulations corroborate the experimental findings and suggest that the active magnetodynamic area resides up to 100 nm outside of the nano-constriction.
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| 12. |
- Dürrenfeld, P., et al.
(författare)
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Spin Hall effect-controlled magnetization dynamics in NiMnSb
- 2015
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 117:17
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the influence of a spin current generated from a platinum layer on the ferromagnetic resonance (FMR) properties of an adjacent ferromagnetic layer composed of the halfmetallic half-Heusler material NiMnSb. Spin Hall nano-oscillator devices are fabricated, and the technique of spin torque FMR is used to locally study the magnetic properties as in-plane anisotropies and resonance fields. A change in the FMR linewidth, in accordance with the additional spin torque produced by the spin Hall effect, is present for an applied dc current. For sufficiently large currents, this should yield auto-oscillations, which however are not achievable in the present device geometry.
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| 13. |
- Houshang, A., et al.
(författare)
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Effect of Excitation Fatigue on the Synchronization of Multiple Nanocontact Spin-Torque Oscillators
- 2014
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Ingår i: IEEE Magnetics Letters. - 1949-307X .- 1949-3088. ; 5, s. 3000404-
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Tidskriftsartikel (refereegranskat)abstract
- Nanocontact spin-torque oscillators (NC-STOs) act as intrinsically nanoscale and highly current and magnetic field tunable, ultrawide band microwave signal generators. However, their low output power and high phase noise remain critical obstacles toward actual applications. Mutual synchronization of multiple NCs is one possibility to overcome these shortcomings. This letter presents a detailed study of the mutual synchronization in a NC-STO with two NCs. In particular, the effect of repeated measurements on the synchronization behavior is explored. Repeated measurements at high drive currents are shown to significantly degrade the performance of the devices with the most striking consequence being that the devices can no longer be synchronized. Ferromagnetic resonance measurements reveal a decrease in the saturation magnetization and an increase in the damping coefficient in annealed NiFe films, consistent with Cu diffusion into the NiFe from the adjacent Cu layers. This increase in damping will act to sever the spin wave-mediated communication channel between the NCs necessary for synchronization. These results highlight an important consideration when studying the synchronization behavior of multi-NC devices where Joule heating is expected to scale unfavorably with the number of NCs.
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| 14. |
- Keatley, P. S., et al.
(författare)
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Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting
- 2017
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Ingår i: Journal of Physics D-Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 50:16
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Tidskriftsartikel (refereegranskat)abstract
- Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by the injected RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.
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| 15. |
- Keatley, P. S., et al.
(författare)
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Superharmonic injection locking of nanocontact spin-torque vortex oscillators
- 2016
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Ingår i: Physical Review B. - : american physical society. - 2469-9950 .- 2469-9969. ; 94:9
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Tidskriftsartikel (refereegranskat)abstract
- Superharmonic injection locking of single nanocontact (NC) spin-torque vortex oscillators (STVOs) subject to a small microwave current has been explored. Frequency locking was observed up to the fourth harmonic of the STVO fundamental frequency f(0) in microwave magnetoelectronic measurements. The large frequency tunability of the STVO with respect to f(0) allowed the device to be locked to multiple subharmonics of the microwave frequency f(RF), or to the same subharmonic over a wide range of fRF by tuning the dc current. In general, analysis of the locking range, linewidth, and amplitude showed that the locking efficiency decreased as the harmonic number increased, as expected for harmonic synchronization of a nonlinear oscillator. Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences in the spatial character of the magnetization dynamics of states locked to the fundamental and harmonic frequencies, suggesting significant differences in the vortex core trajectories within the same device. Superharmonic injection locking of a NC-STVO may open up possibilities for devices such as nanoscale frequency dividers, while differences in the core trajectory may allow mutual synchronization to be achieved in multioscillator networks by tuning the spatial character of the dynamics within shared magnetic layers.
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| 16. |
- Ranjbar, M., et al.
(författare)
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CoFeB-Based Spin Hall Nano-Oscillators
- 2014
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Ingår i: IEEE Magnetics Letters. - 1949-307X .- 1949-3088. ; 5, s. 3000504-
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate magnetization auto-oscillations driven by pure spin currents in spin Hall nano-oscillators based on CoFeB/Pt bilayers. Despite the very low anisotropic magnetoresistance of CoFeB, a substantial microwave signal power can be detected, even at room temperature, indicating that a sizable spin wave amplitude is generated. Spin torque ferromagnetic resonance measurements reveal that the generated auto-oscillation frequency lies below the ferromagnetic resonance frequency of CoFeB and is therefore well described by a self-localized spin wave bullet mode.
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| 17. |
- Sharma, Raghav, et al.
(författare)
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Mode-hopping mechanism generating colored noise in a magnetic tunnel junction based spin torque oscillator
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:13, s. 132404-
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Tidskriftsartikel (refereegranskat)abstract
- The frequency noise spectrum of a magnetic tunnel junction based spin torque oscillator is examined where multiple modes and mode-hopping events are observed. The frequency noise spectrum is found to consist of both white noise and 1/f frequency noise. We find a systematic and similar dependence of both white noise and 1/f frequency noise on bias current and the relative angle between the reference and free layers, which changes the effective damping and hence the mode-hopping behavior in this system. The frequency at which the 1/f frequency noise changes to white noise increases as the free layer is aligned away from the anti-parallel orientation w.r.t the reference layer. These results indicate that the origin of 1/f frequency noise is related to mode-hopping, which produces both white noise as well as 1/f frequency noise similar to the case of ring lasers.
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| 18. |
- Sharma, R., et al.
(författare)
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Time-domain stability of parametric synchronization in a spin-torque nano-oscillator based on a magnetic tunnel junction
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on time-domain stability of the parametric synchronization in a spin-torque nano-oscillator (STNO) based on a magnetic tunnel junction. Time-domain measurements of the instantaneous frequency (f(i)) of a parametrically synchronized STNO showrandom short-term unlocking of the STNO signal for low injected radio-frequency (RF) power, which cannot be revealed in time-averaged frequency domain measurements. Macrospin simulations reproduce the experimental results and reveal that the random unlocking during synchronization is driven by thermal fluctuations. We show that by using a high injected RF power, random unlocking of the STNO can be avoided. However, a perfect synchronization characterized by complete suppression of phase noise, so-called phase noise squeezing, can be obtained only at a significantly higher RF power. Our macrospin simulations suggest that a lower temperature and a higher positive ratio of the fieldlike torque to the spin transfer torque reduce the threshold RF power required for phase noise squeezing under parametric synchronization.
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| 19. |
- Spicer, T. M., et al.
(författare)
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Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator
- 2018
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:19
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nanooscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a nonlinear "bullet" mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal. Published by AIP Publishing.
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| 20. |
- Tiwari, D., et al.
(författare)
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Antidamping spin-orbit torques in epitaxial-Py(100)/beta-Ta
- 2017
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 111:23
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Tidskriftsartikel (refereegranskat)abstract
- We perform spin torque ferromagnetic resonance measurements on the Si(100)/TiN(100)/epi-Py(100)/beta-Ta system. We demonstrate current induced modulation of the Gilbert damping constant, which is about 30% for a current density of 6.25 x 10(9) A/m(2). We show that the observed modulation of the Gilbert damping constant cannot be explained by spin transfer torques arising from the spin Hall effect of the beta-Ta layer. An additional mechanism such as antidamping spinorbit torque resulting from the interface or the crystalline structure of Py thin films needs to be considered. Published by AIP Publishing.
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| 21. |
- Zahedinejad, Mohammad, 1986, et al.
(författare)
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Current Modulation of Nanoconstriction Spin-Hall Nano-Oscillators
- 2017
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Ingår i: IEEE Magnetics Letters. - : Institute of Electrical and Electronics Engineers Inc.. - 1949-307X .- 1949-3088. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- A single nanoconstriction spin-Hall nano-oscillator (NC-SHNO) in out-of-plane fields is presented as a nonlinear amplitude and frequency modulator operated by radio-frequency (RF) current modulation. The current modulation was carried out in different NC-SHNO nonlinearity regimes corresponding to negative, zero, and positive values of df/dI in order to investigate the device response to an 80 MHz modulating current. Our study shows that current modulation of SHNOs can be quantitatively predicted by a nonlinear frequency and amplitude modulation (NFAM) model using the values of df/dI and d2f/dI2 extracted from the free-running frequency f versus current I profile. The NFAM model reproduces the asymmetric sideband amplitude as well as the red and blue shift of the frequency in excellent agreement with the experimental results. The ability to predict the modulation process is a necessary benchmark in designing SHNO modulators for future integrated microwave circuits.
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