| 1. |
- Dumas, Randy K., et al.
(författare)
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Accessing different spin-disordered states using first-order reversal curves
- 2014
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 90:10
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Tidskriftsartikel (refereegranskat)abstract
- Combined first-order reversal curve (FORC) analyses of the magnetization (M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the M-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin valve (PSV) and a [Co/Cu](8) multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and the FePt layers, the M and MR show a simple one-to-one relationship during reversal. In the [Co/Cu](8) multilayer, the correlation between the magnetization reversal and the MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MRs than those found along the conventional major loop field cycle. Unlike the M-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin disorder and the largest MR.
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| 2. |
- Dumas, Randy K., et al.
(författare)
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Recent Advances in Nanocontact Spin-Torque Oscillators
- 2014
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Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 50:6, s. 4100107-
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Tidskriftsartikel (refereegranskat)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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| 3. |
- 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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| 4. |
- Dumas, Randy K., et al.
(författare)
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Recent Advances in Nanocontact Spin-Torque Oscillators
- 2014
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Ingår i: Ieee Transactions on Magnetics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9464 .- 1941-0069. ; 50:6
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Tidskriftsartikel (refereegranskat)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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| 5. |
- Dumas, Randy K., et al.
(författare)
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Spintronics: Channeling spin waves
- 2014
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Ingår i: Nature Nanotechnology. - 1748-3387. ; 9:7, s. 503-504
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Tidskriftsartikel (refereegranskat)
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| 6. |
- Houshang, Afshin, 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. - : Institute of Electrical and Electronics Engineers (IEEE). - 1949-307X .- 1949-3088. ; 5
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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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| 7. |
- Iacocca, Ezio, 1986, et al.
(författare)
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Confined Dissipative Droplet Solitons in Spin-Valve Nanowires with Perpendicular Magnetic Anisotropy
- 2014
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 112:4
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic dissipative droplets are localized, strongly nonlinear dynamical modes excited in nanocontact spin valves with perpendicular magnetic anisotropy. These modes find potential application in nanoscale structures for magnetic storage and computation, but dissipative droplet studies have so far been limited to extended thin films. Here, numerical and asymptotic analyses are used to demonstrate the existence and properties of novel solitons in confined structures. As a nanowire's width is decreased with a nanocontact of fixed size at its center, the observed modes undergo transitions from a fully localized two-dimensional droplet into a two-dimensional droplet edge mode and then a pulsating one-dimensional droplet. These solitons are interpreted as dissipative versions of classical, conservative solitons, allowing for an analytical description of the modes and the mechanisms of bifurcation. The presented results open up new possibilities for the study of low-dimensional solitons and droplet applications in nanostructures.
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| 8. |
- Lodewijks, Kristof, 1984, et al.
(författare)
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Magnetoplasmonic Design Rules for Active Magneto-Optics
- 2014
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Ingår i: Nano letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 14:12, s. 7207-7214
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Tidskriftsartikel (refereegranskat)abstract
- Light polarization rotators and nonreciprocal optical isolators are essential building blocks in photonics technology. These macroscopic passive devices are commonly based on magneto-optical Faraday and Kerr polarization rotation. Magnetoplasmonics, the combination of magnetism and plasmonics, is a promising route to bring these devices to the nanoscale. We introduce design rules for highly tunable active magnetoplasmonic elements in which we can tailor the amplitude and sign of the Kerr response over a broad spectral range.
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| 9. |
- Mohseni, Seyed Majid, et al.
(författare)
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Magnetic droplet solitons in orthogonal nano-contact spin torque oscillators
- 2014
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 435, s. 84-87
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Tidskriftsartikel (refereegranskat)abstract
- We study microwave signal generation as a function of drive current and applied perpendicular magnetic field in nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has strong easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The orthogonal NC-STOs exhibit a dramatic transition from the conventional ferromagnetic resonance-like spin wave mode to a magnetic droplet soliton mode. In particular, the field and current dependence of the droplet soliton near threshold are discussed. Near threshold the droplet soliton undergoes complex dynamics that include mode hopping, as evident in the experimental frequency domain and magnetoresistance response. © 2013 Elsevier B.V.
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| 10. |
- Nguyen, T. N. Anh, et al.
(författare)
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Depth-Dependent Magnetization Profiles of Hybrid Exchange Springs
- 2014
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Ingår i: Physical Review Applied. - 2331-7019. ; 2:4
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Tidskriftsartikel (refereegranskat)abstract
- We report on the magnetization depth profile of a hybrid exchange-spring system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to a CoFeB thin film with in-plane anisotropy. The competition between these two orthogonal anisotropies promotes a strong depth dependence of the magnetization orientation. The angle of the magnetization vector is sensitive both to the strength of the individual anisotropies and to the local exchange constant and is thus tunable by changing the thickness of the CoFeB layer and by substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic depth profiles are directly probed by element-specific x-ray magnetic circular dichroism of the Fe and Ni layers located at different average depths. The experimental results are corroborated by micromagnetic simulations.
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