| 1. |
- Ahlberg, Martina, et al.
(författare)
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Freezing and thawing magnetic droplet solitons
- 2022
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic droplets are a type of non-topological magnetic soliton, which are stabilised and sustained by spin-transfer torques for instance. Without this, they would collapse. Here Ahlberg et al show that by decreasing the applied magnetic field, droplets can be frozen, forming a static nanobubble Magnetic droplets are non-topological magnetodynamical solitons displaying a wide range of complex dynamic phenomena with potential for microwave signal generation. Bubbles, on the other hand, are internally static cylindrical magnetic domains, stabilized by external fields and magnetostatic interactions. In its original theory, the droplet was described as an imminently collapsing bubble stabilized by spin transfer torque and, in its zero-frequency limit, as equivalent to a bubble. Without nanoscale lateral confinement, pinning, or an external applied field, such a nanobubble is unstable, and should collapse. Here, we show that we can freeze dynamic droplets into static nanobubbles by decreasing the magnetic field. While the bubble has virtually the same resistance as the droplet, all signs of low-frequency microwave noise disappear. The transition is fully reversible and the bubble can be thawed back into a droplet if the magnetic field is increased under current. Whereas the droplet collapses without a sustaining current, the bubble is highly stable and remains intact for days without external drive. Electrical measurements are complemented by direct observation using scanning transmission x-ray microscopy, which corroborates the analysis and confirms that the bubble is stabilized by pinning.
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| 2. |
- 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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| 3. |
- Haidar, M., et al.
(författare)
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Compositional effect on auto-oscillation behavior of Ni 100 -Fe-x(x)/Pt spin Hall nano-oscillators
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:1
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the compositional effect on the magnetodynamic and auto-oscillation properties of Ni 100 - xFe(x)/Pt (x=10-40) nanoconstriction-based spin Hall nano-oscillators. Using spin-torque ferromagnetic resonance performed on microstrips, we measure a significant reduction in both damping and spin Hall efficiency with the increasing Fe content, which lowers the spin pumping contribution. The strong compositional effect on spin Hall efficiency is primarily attributed to the increased saturation magnetization in Fe-rich devices. As a direct consequence, higher current densities are required to drive spin-wave auto-oscillations at higher microwave frequencies in Fe-rich nanoconstriction devices. Our results establish the critical role of the compositional effect in engineering the magnetodynamic and auto-oscillation properties of spin Hall devices for microwave and magnonic applications.
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| 4. |
- Jiang, S., et al.
(författare)
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Magnetic droplet soliton pairs
- 2024
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Ingår i: Nature Communications. - : Nature Research. - 2041-1723. ; 15:1
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate magnetic droplet soliton pairs in all-perpendicular spin-torque nano-oscillators (STNOs), where one droplet resides in the STNO free layer (FL) and the other in the reference layer (RL). Typically, theoretical, numerical, and experimental droplet studies have focused on the FL, with any additional dynamics in the RL entirely ignored. Here we show that there is not only significant magnetodynamics in the RL, but the RL itself can host a droplet driven by, and coexisting with, the FL droplet. Both single droplets and pairs are observed experimentally as stepwise changes and sharp peaks in the dc and differential resistance, respectively. While the single FL droplet is highly stable, the coexistence state exhibits high-power broadband microwave noise. Furthermore, micromagnetic simulations reveal that the pair dynamics display periodic, quasi-periodic, and chaotic signatures controlled by applied field and current. The strongly interacting and closely spaced droplet pair offers a unique platform for fundamental studies of highly non-linear soliton pair dynamics.
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| 5. |
- Jiang, Sheng, et al.
(författare)
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Using magnetic droplet nucleation to determine the spin torque effciency and asymmetry in Cox(NiFe)1-x thin films
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We demonstrate how to extract the material dependent spin torque efficiency (ε) and asymmetry(λ) from the eld{current nucleation boundaries of magnetic droplet solitons in orthogonal nanocontacts in-torque oscillators with Cox(Ni80Fe20)1-x, (x=0{1), fixed layers. As the perpendicular component of the xed layer magnetization plays a central role in governing droplet nucleation, the nucleation boundaries exhibit monotonic shifts towards higher perpendicular magnetic elds when the xed layer magnetization μ0Ms,p is tuned from 1.04 to 1.7 T. We then extract ε and λ from tsto the nucleation boundaries and nd that while ε does not vary with composition, λ increases from1.5 to 3 with increasing Co content. The analysis of droplet nucleation boundaries is hence a useful tool for the systematic study of both ε and λ as functions of material composition.
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| 6. |
- Jiang, Sheng, et al.
(författare)
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Using Magnetic Droplet Nucleation to Determine the Spin Torque Efficiency and Asymmetry in Co-x(Ni,Fe)(1-x) Thin Films
- 2018
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Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate how to extract the material-dependent spin-torque efficiency (epsilon) and asymmetry (lambda) from the field-current nucleation boundaries of magnetic droplet solitons in orthogonal nano-contact spintorque oscillators with Co-x(Ni80Fe20)(1-x), (x = 0 -1), fixed layers. As the perpendicular component of the fixed-layer magnetization plays a central role in governing droplet nucleation, the nucleation boundaries exhibit monotonic shifts towards higher perpendicular magnetic fields when the fixed-layer magnetization mu M-0(s, p) is tuned from 1.04 to 1.7 T. We then extract epsilon and lambda from fits to the nucleation boundaries and find that while epsilon does not vary with composition,lambda increases from 1.5 to 3 with increasing Co content. The analysis of droplet nucleation boundaries is hence a useful tool for the systematic study of both epsilon and lambda as functions of material composition.
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| 7. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
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Auto-oscillating spin-wave modes of constriction-based spin Hall nano-oscillators in weak in-plane fields
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
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Auto-oscillating Spin-Wave Modes of Constriction-Based Spin Hall Nano-oscillators in Weak In-Plane Fields
- 2018
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Ingår i: Physical Review Applied. - : American Physical Society. - 2331-7019. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally study the auto-oscillating spin-wave modes in Ni(80)Fc(20)/beta-W constriction-based spin Hall nano-oscillators as a function of bias current, strength of the in-plane applied field, and azimuthal field angle in the low-field range of 40-80 mT. We observe two different spin-wave modes: (i) a linearlike mode confined to the internal field minima near the edges of the nanoconstriction, and only weakly dependent on the bias current and the applied-field angle, and (ii) a second, lower-frequency mode with significantly higher threshold current and stronger dependence on both the bias current and the externalfield angle. Micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the spin Hall nano-oscillator mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linearlike mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Using intrinsic frequency doubling at zero field angle, we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-field spintronic oscillators with applications in microwave-signal generation and neuromorphic computing.
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| 9. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
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Contracting vs. expanding spin wave bullets in spin Hall nano-oscillators
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 10. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
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Improving the magnetodynamical properties of NiFe/Pt bilayers through Hf dusting
- 2018
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:9
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Tidskriftsartikel (refereegranskat)abstract
- 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 mu m wide microstrips on high-resistive Si substrates. Based on two series of NiFe(t(NiFe))/Hf(t(Hf))/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 a decreases linearly with tHf by up to 40%. The weaker anisotropic magnetoresistance (AMR similar or equal to 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 t(Hf). We find that the spin Hall efficiency zeta(SH) is independent of the NiFe thickness, remaining unaffected (zeta(SH) = 0.115) up to t(Hf) = 0.4 nm and then decreasing linearly for higher t(Hf). The different trends of alpha and zeta(SH) suggest that there is an optimum Hf thickness (similar or equal to 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 theta(SH) = 0.22 by calculating the transparency of the interface. Published by AIP Publishing.
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