| 1. |
- Awad, Ahmad, et al.
(author)
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Long-range mutual synchronization of spin Hall nano-oscillators
- 2017
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In: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481. ; 13, s. 292-299
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Journal article (peer-reviewed)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 300nm. Through further tailoring of the connection regions we can extend the synchronization range to 4μm. 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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| 2. |
- Chen, Tingsu, et al.
(author)
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Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part I : Analytical Model of the MTJ STO
- 2015
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In: IEEE Transactions on Electron Devices. - 0018-9383 .- 1557-9646. ; 62:3, s. 1037-1044
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Journal article (peer-reviewed)abstract
- Magnetic tunnel junction (MTJ) spin torque oscillators (STOs) have shown the potential to be used in a wide range of microwave and sensing applications. To evaluate the potential uses of MTJ STO technology in various applications, an analytical model that can capture MTJ STO's characteristics, while enabling system-and circuit-level designs, is of great importance. An analytical model based on macrospin approximation is necessary for these designs since it allows implementation in hardware description languages. This paper presents a new macrospin-based, comprehensive, and compact MTJ STO model, which can be used for various MTJ STOs to estimate the performance of MTJ STOs together with their application-specific integrated circuits. To adequately present the complete model, this paper is divided into two parts. In Part I, the analytical model is introduced and verified by comparing it against measured data of three different MTJ STOs, varying the angle and magnitude of the magnetic field, as well as the DC biasing current. The proposed analytical model is suitable for being implemented in Verilog-A and used for efficient simulations at device, circuit, and system levels. In Part II, the full Verilog-A implementation of the analytical model with accurate phase noise generation is presented and verified by simulations.
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| 3. |
- Chen, Tingsu, et al.
(author)
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Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part II : Verilog-A Model Implementation
- 2015
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In: IEEE Transactions on Electron Devices. - 0018-9383 .- 1557-9646. ; 62:3, s. 1045-1051
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Journal article (peer-reviewed)abstract
- The rapid development of the magnetic tunnel junction (MTJ) spin torque oscillator (STO) technology demands an analytical model to enable building MTJ STO-based circuits and systems so as to evaluate and utilize MTJ STOs in various applications. In Part I of this paper, an analytical model based on the macrospin approximation has been introduced and verified by comparing it with the measurements of three different MTJ STOs. In Part II, the full Verilog-A implementation of the proposed model is presented. To achieve a reliable model, an approach to reproducing the phase noise generated by the MTJ STO has been proposed and successfully employed. The implemented model yields a time domain signal, which retains the characteristics of operating frequency, linewidth, oscillation amplitude, and DC operating point, with respect to the magnetic field and applied DC current. The Verilog-A implementation is verified against the analytical model, providing equivalent device characteristics for the full range of biasing conditions. Furthermore, a system that includes an MTJ STO and CMOS RF circuits is simulated to validate the proposed model for system-and circuit-level designs. The simulation results demonstrate that the proposed model opens the possibility to explore STO technology in a wide range of applications.
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| 4. |
- Chung, Sunjae, et al.
(author)
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Effect of canted magnetic field on magnetic droplet nucleation boundaries
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Other publication (other academic/artistic)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.
(author)
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Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators
- 2016
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
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Journal article (peer-reviewed)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.
(author)
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Spin transfer torque generated magnetic droplet solitons (invited)
- 2014
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:17
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Journal article (peer-reviewed)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. |
- Dumas, Randy K., et al.
(author)
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Recent Advances in Nanocontact Spin-Torque Oscillators
- 2014
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In: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 50:6, s. 4100107-
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Journal article (peer-reviewed)abstract
- We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <100 nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/[Co/Ni](4) orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries.
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| 8. |
- Dumas, Randy K., et al.
(author)
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Recent Advances in Nanocontact Spin-Torque Oscillators
- 2014
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In: Ieee Transactions on Magnetics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9464 .- 1941-0069. ; 50:6
-
Journal article (peer-reviewed)abstract
- We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <100 nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/[Co/Ni](4) orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries.
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| 9. |
- Dumas, Randy K., et al.
(author)
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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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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110:25, s. 257202-
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Journal article (peer-reviewed)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. |
- Dürrenfeld, Philipp, et al.
(author)
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Modulation-mediated unlocking of a parametrically phase-locked spin torque oscillator
- 2014
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:25
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Journal article (peer-reviewed)abstract
- Modulation of an oscillator is crucial for its application in communication devices. While the output power and linewidth of single magnetic tunnel junction-based spin-torque oscillators (MTJ-STO) are not yet adequate for practical uses, the synchronization of such devices can overcome these limitations. Here, we investigate the modulation behavior of a parametrically synchronized MTJ-STO and show experimentally that modulation of the synchronized state preserves synchronization as long as the modulation frequency, integral(mod), is above a characteristic frequency, integral(unlock). The unlocking frequency increases with the modulation amplitude in agreement with analytical estimates and numerical simulations. These phenomena are described as a non-resonant unlocking mechanism, whose characteristics are directly related to inherent parameters of the oscillator. (C) 2014 AIP Publishing LLC.
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