| 1. |
- Mohseni, Seyed Morteza, et al.
(författare)
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Magnetic droplet soliton nucleation in oblique fields
- 2018
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 97:18
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Tidskriftsartikel (refereegranskat)abstract
- We study the auto-oscillating magnetodynamics in orthogonal spin-torque nano-oscillators (STNOs) as a function of the out-of-plane (OOP) magnetic-field angle. In perpendicular fields and at OOP field angles down to approximately 50°, we observe the nucleation of a droplet. However, for field angles below 50°, experiments indicate that the droplet gives way to propagating spin waves, in agreement with our micromagnetic simulations. Theoretical calculations show that the physical mechanism behind these observations is the sign changing of spin-wave nonlinearity (SWN) by angle. In addition, we show that the presence of a strong perpendicular magnetic anisotropy free layer in the system reverses the angular dependence of the SWN and dynamics in STNOs with respect to the known behavior determined for the in-plane magnetic anisotropy free layer. Our results are of fundamental interest in understanding the rich dynamics of nanoscale solitons and spin-wave dynamics in STNOs.
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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. |
- 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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| 4. |
- Chung, Sunjae, et al.
(författare)
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Effect of canted magnetic field on magnetic droplet nucleation boundaries
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The influence on magnetic droplet nucleation boundaries by canted magnetic elds are investigated and reported. The nucleation boundary condition, In = αAH + BH + C, is determined at different canted angles (0°< θH<20°) using magnetoresistance (MR) and microwave measurements in nanocontact spintorque oscillators (NC-STOs). As θH increased, the nucleation boundary shifts gradually towards higher In and H. The coefficient B of the nucleation boundary equation also nearly doubled as θH increases. On theother hand, the coefficient αA remained constant for all values of θH. These observations can be explained by considering the drift instability of magnetic droplets and the different tilt behaviour of the Co fixed layer induced by different θH.
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| 5. |
- Chung, Sunjae, et al.
(författare)
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Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators
- 2016
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current–field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.
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| 6. |
- Chung, Sunjae, et al.
(författare)
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Spin transfer torque generated magnetic droplet solitons (invited)
- 2014
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:17
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Tidskriftsartikel (refereegranskat)abstract
- We present recent experimental and numerical advancements in the understanding of spin transfer torque generated magnetic droplet solitons. The experimental work focuses on nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has an easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The NC-STO resistance and microwave signal generation are measured simultaneously as a function of drive current and applied perpendicular magnetic field. Both exhibit dramatic transitions at a certain current dependent critical field value, where the microwave frequency drops 10 GHz, modulation sidebands appear, and the resistance exhibits a jump, while the magnetoresistance changes sign. We interpret these observations as the nucleation of a magnetic droplet soliton with a large fraction of its magnetization processing with an angle greater than 90 degrees, i.e., around a direction opposite that of the applied field. This interpretation is corroborated by numerical simulations. When the field is further increased, we find that the droplet eventually collapses under the pressure from the Zeeman energy. (C) 2014 AIP Publishing LLC.
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| 7. |
- Chung, Sunjae, et al.
(författare)
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Tunable spin configuration in [Co/Ni]-NiFe spring magnets
- 2013
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 46:12, s. 125004-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate a tunable spin configuration in [Co/Ni](4)/Co-NiFe exchange spring magnets. The strong perpendicular magnetic anisotropy of the Co/Ni multilayer, which competes with the in-plane shape anisotropy of the Ni81Fe19 (Py) layer, allows for unique magnetic configurations. By varying the NiFe thickness (t(NiFe)) from 2.6 to 3.0 nm, we show that the magnetization tilt angle can be easily tuned from completely out-of-plane (0 degrees) to in-plane (90 degrees). This transition, which occurs for a small range of t(NiFe), can be estimated from the major loop remanence and one-dimensional micromagnetic calculations. These tunable magnetization tilt spring magnets are highly promising for future applications in spin-transfer torque-based devices.
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| 8. |
- Davies, J. E., et al.
(författare)
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Reversal mode instability and magnetoresistance in perpendicular (Co/Pd)/Cu/(Co/Ni) pseudo-spin-valves
- 2013
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:2, s. 022409-
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Tidskriftsartikel (refereegranskat)abstract
- We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)(4)/Co/Cu/(Co/Ni)(4)/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling and allow direct determination of the interlayer coupling strength as a function of temperature.
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| 9. |
- Dumas, Randy K., et al.
(författare)
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Spin-Wave-Mode Coexistence on the Nanoscale : A Consequence of the Oersted-Field-Induced Asymmetric Energy Landscape
- 2013
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110:25, s. 257202-
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Tidskriftsartikel (refereegranskat)abstract
- It has been argued that if multiple spin wave modes are competing for the same centrally located energy source, as in a nanocontact spin torque oscillator, that only one mode should survive in the steady state. Here, the experimental conditions necessary for mode coexistence are explored. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field, which leads to a physical separation of the modes, and is further promoted by spin wave localization at reduced applied field angles. Finally, both simulation and experiment reveal a low frequency signal consistent with the intermodulation of two coexistent modes.
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| 10. |
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