| 1. |
- 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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| 2. |
- Banuazizi, Seyed Amir Hossein, et al.
(författare)
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Control of thermal budget in nanocontact spin-torque nano-oscillators
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the influence of the bottom Cu electrode thickness (tCu) in nanocontact spin-torque nano-oscillators (NC-STNOs) based on Si/SiO2/Pd(8)/Cu(tCu)/Co(8)/Cu(7)/NiFe(4.5)/Cu(3)/Pd(3) GMR stacks on the thermal budget of the magnetodynamically active region. Increasing tCu from 10 to 70 nm results in a ~50% reduction in Joule heating in both the Co and NiFe layers, which directly improves the microwave output stability and linewidth. Numerical simulations of the NC-STNO current distribution suggest that this improvement originates from a strongly reduced lateral current spread in the top ferromagnetic layer and a reduction in the device's resistance.
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| 3. |
- Burgos-Parra, E., et al.
(författare)
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Investigation of magnetic droplet solitons using x-ray holography with extended references
- 2018
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micromagnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [ Co/ Ni] x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at -33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (similar to 100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0-50 mT), where it is expected to observe regimes of the unstable droplet formation.
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| 4. |
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| 5. |
- Eklund, Anders, et al.
(författare)
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Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:9, s. 092405-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Fazlali, Masoumeh, et al.
(författare)
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Tuning exchange-dominated spin-waves using lateral current spread in nanocontact spin-torque nano-oscillators
- 2018
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Ingår i: Applied Physics Letters. - 0003-6951 .- 1077-3118.
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Tidskriftsartikel (refereegranskat)abstract
- We present an efficient method to tailor propagating spin waves in quasi-confined systems. We use nanocontact spin-torque nano-oscillators based on NiFe/Cu/Co spin-valves and study the ferromagnetic and spin-wave resonances (FMR and SWR) of both layers. We employ homodyne-detected ferromagnetic resonance spectroscopy, resembling spin-torque FMR, to detect the magnetodynamics. The external field is applied in-plane, giving a parallel configuration of the magnetic layers, which do not provide any spin-transfer torque. Instead, the excitation is caused by the Oersted-field. By varying the thickness of the bottom Cu electrode of the devices, we tune the current distribution in the samples, and thereby the Oersted field, which governs the spin wave characteristics.
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| 7. |
- 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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| 8. |
- 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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| 9. |
- 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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| 10. |
- 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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| 11. |
- Mohseni, Seyed Majid, et al.
(författare)
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High frequency operation of a spin-torque oscillator at low field
- 2011
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Ingår i: Physica Status Solidi. Rapid Research Letters. - : Wiley. - 1862-6254 .- 1862-6270. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a nano-contact based spin-torque oscillator (STO) combining a high operating frequency with low field operation. The STO is based on an orthogonal spin-valve architecture with an in-plane Co polarizer and an out-of-plane Co/Ni multilayer free layer. High frequency operation at low external fields is achieved by tailoring the Co/Ni layer properties to increase the strength (H(k)) of the perpendicular magnetic anisotropy, while simultaneously reducing the saturation magnetization (M(s)). Our approach emphasizes the importance of mu(0)(H(k)-M(s)) in determining the operating frequency in this system, and suggests that yet higher frequencies should be attainable through further optimization. [GRAPHICS]
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| 12. |
- Mohseni, Seyed Majid, et al.
(författare)
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Spin Torque-Generated Magnetic Droplet Solitons
- 2013
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 339:6125, s. 1295-1298
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Tidskriftsartikel (refereegranskat)abstract
- Dissipative solitons have been reported in a wide range of nonlinear systems, but the observation of their magnetic analog has been experimentally challenging. Using spin transfer torque underneath a nanocontact on a magnetic thin film with perpendicular magnetic anisotropy (PMA), we have observed the generation of dissipative magnetic droplet solitons and report on their rich dynamical properties. Micromagnetic simulations identify a wide range of automodulation frequencies, including droplet oscillatory motion, droplet "spinning," and droplet "breather" states. The droplet can be controlled by using both current and magnetic fields and is expected to have applications in spintronics, magnonics, and PMA-based domain-wall devices.
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| 13. |
- Sani, Sohrab R, et al.
(författare)
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Current induced vortices in multi-nanocontact spin-torque devices
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:7, s. 07C913-
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate spin transfer torque (STT) switching in multi-nanocontact STT devices fabricated using hole mask colloidal lithography. We also study the STT device resistance and switching properties as a function of applied magnetic field and nanocontact current. At low nanocontact current, magnetoresistance measurements show sharp, single-step switching at low switching fields. When the current is increased, the switching becomes multistep, and the switching field increases dramatically. We explain these results as arising from a transition from a predominantly single domain like switching to switching involving a vortex state. Micromagnetic simulations corroborate this picture, indicating that a single magnetic vortex nucleates in between the nanocontacts through the influence from the total Oersted field generated by the nanocontact ensemble.
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| 14. |
- Sani, Sohrab R., et al.
(författare)
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Hole mask colloidal lithography on magnetic multilayers for spin torque applications
- 2010
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Ingår i: Journal of Physics Conference Series. International Conference on Magnetism (ICM 2009). Karlsruhe, GERMANY. JUL 26-31, 2009. - BRISTOL : IOP Publishing. - 1742-6588 .- 1742-6596. ; 200, s. UNSP 072078-
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Konferensbidrag (refereegranskat)abstract
- We demonstrate the fabrication of metallic nano-contacts on magnetic multilayers using a Hole Mask Colloidal Lithography technique (HCL) based on Polystyrene spheres. The method applies PMMA as a sacrificial layer upon which a hole pattern is formed after lift- off of the spheres. An Au layer functions as a hard mask for the PMMA and the PMMA subsequently masks the SiO2 during its etching. The resulting pattern is a dense collection of randomly located nano-holes through a SiO2 film. Final devices are made using traditional photolithography to define a 600 nm circular mesa with about 3 to 4 nano-holes per device, and patterning of a metallic top contact.
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