| 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. |
- Banuazizi, Seyed Amir Hossein, et al.
(författare)
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Order of magnitude improvement of nano-contact spin torque nano-oscillator performance
- 2017
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 9:5, s. 1896-1900
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Tidskriftsartikel (refereegranskat)abstract
- Spin torque nano-oscillators (STNO) represent a unique class of nano-scale microwave signal generators and offer a combination of intriguing properties, such as nano sized footprint, ultrafast modulation rates, and highly tunable microwave frequencies from 100 MHz to close to 100 GHz. However, their low output power and relatively high threshold current still limit their applicability and must be improved. In this study, we investigate the influence of the bottom Cu electrode thickness (t(Cu)) in nano-contact STNOs based on Co/Cu/NiFe GMR stacks and with nano-contact diameters ranging from 60 to 500 nm. Increasing t(Cu) from 10 to 70 nm results in a 40% reduction of the threshold current, an order of magnitude higher microwave output power, and close to two orders of magnitude better power conversion efficiency. Numerical simulations of the current distribution suggest that these dramatic improvements originate from a strongly reduced lateral current spread in the magneto-dynamically active region.
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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
-
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. |
- Chung, Sunjae, et al.
(författare)
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Comparison of Magnetic Droplet Nucleation Boundaries in Spin-torque Nano-oscillators with Different Fixed Layers
- 2023
-
Ingår i: New Physics: Sae Mulli. - 0374-4914 .- 2289-0041. ; 73:1, s. 7-11
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Tidskriftsartikel (refereegranskat)abstract
- We compared magnetic droplet nucleation boundaries in different spin{torque nano-oscillators with fixed layers of Co and [Co/Pd] multilayers. The nucleation conditions for magnetic droplets were determined as a functions of magnetic field and current and from the onset of a low-frequency noise. The spin{torque nano-oscillator with Co fixed layer showed an inverse proportional relation between magnetic fields and currents in a low magnetic field and a linear proportional relation in a high magnetic field. In the device constituting the Co/Pd multi-layer as a fixed layer, a linear proportional relation was observed in a whole range of magnetic field. Further details will be discussed in this study.
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| 5. |
- Chung, Sunjae, et al.
(författare)
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Direct Observation of Zhang-Li Torque Expansion of Magnetic Droplet Solitons
- 2018
-
Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 120:21
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic droplets are nontopological dynamical soli tons that can be nucleated in nanocontact based spin torque nano-oscillators (STNOs) with perpendicular magnetic anisotropy free layers. While theory predicts that the droplet should be of the same size as the nanocontact, its inherent drift instability has thwarted attempts at observing it directly using microscopy techniques. Here, we demonstrate highly stable magnetic droplets in all-perpendicular STNOs and present the first detailed droplet images using scanning transmission X-ray microscopy. In contrast to theoretical predictions, we find that the droplet diameter is about twice as large as the nanocontact. By extending the original droplet theory to properly account for the lateral current spread underneath the nanocontact, we show that the large discrepancy primarily arises from current-in-plane Zhang-Li torque adding an outward pressure on the droplet perimeter. Electrical measurements on droplets nucleated using a reversed current in the antiparallel state corroborate this picture.
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| 6. |
- 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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| 7. |
- Chung, Sunjae, et al.
(författare)
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Investigation of superlattices based on ferromagnetic semiconductor GaMnAs by planar Hall effect
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:7, s. 07D310-1-07D310-3
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Tidskriftsartikel (refereegranskat)abstract
- Two ferromagnetic semiconductor GaMnAs-based superlattices (SLs) were investigated by measuring the planar Hall effect (PHE) with the external magnetic field applied in the plane of the sample. The two GaMnAs/GaAs SLs differed only by the Be doping of the nonmagnetic GaAs spacer layers. Both SLs showed a typical two-step transition behavior in PHE field scans at 4.0 K, essentially the same as that normally observed on single GaMnAs ferromagnetic layers with two in-plane magnetic easy axes. As the temperature increased to 30 K, the behaviors of the PHE changed differently in the two SL samples. The PHE in the undoped SL can be described simply by the temperature dependence of the magnetic anisotropy within the film plane of a GaMnAs film. However, the Be-doped SL revealed a completely different behavior, showing a transition of magnetization with a negative coercive field. The observation of this feature in a ferromagnetic multilayer indicates the presence of spontaneous anti-parallel interlayer exchange coupling between the GaMnAs magnetic layers.
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| 8. |
- 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
-
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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| 9. |
- Chung, Sunjae, et al.
(författare)
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Magnetic droplet solitons in all-perpendicular nano-contact spin torque oscillators
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Annan publikation (populärvet., debatt m.m.)abstract
- Spin-torque-generated magnetic droplets are nontopological solitons that have both dynamic and static characteristics. Although theoretical studies of these droplets originally dealt with an all-perpendicular magnetic system, all experimental demonstrations have so far relied on orthogonal spin valve structures that require a rather strong magnetic field to nucleate the droplet. Here, for the first time, we show the nucleation and sustained operation of magnetic droplets under a low magnetic field using nanocontact spin-torque oscillators (NC-STO), both the free Co/Ni and fixed Co/Pd multilayers of which have strong perpendicular magnetic anisotropy. Droplet nucleation is observed as a change in the NC-STO resistance and the appearance of significant broadband microwave signal generation below 1 GHz. We also observe another important phenomenon in which the nucleated magnetic droplet can transform into a skyrmionic nanobubble in the low-field regime. Both magnetic droplet solitons and skyrmionic nanobubbles have been studied in detail using micromagnetic simulation.
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| 10. |
- Chung, Sunjae, et al.
(författare)
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Magnetic droplet solitons in orthogonal spin valves
- 2015
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Ingår i: Fizika Nizkih Temperatur. - : American Institute of Physics (AIP). - 0132-6414 .- 1816-0328. ; 41:10, s. 833-837
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Tidskriftsartikel (refereegranskat)abstract
- We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.
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| 11. |
- 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
-
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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| 12. |
- 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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| 13. |
- 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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| 14. |
- Dürrenfeld, Philipp, et al.
(författare)
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Low-current, narrow-linewidth microwave signal generation in NiMnSb based single-layer nanocontact spin-torque oscillators
- 2016
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 109:22
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Tidskriftsartikel (refereegranskat)abstract
- We report on the fabrication of nano-contact spin-torque oscillators based on single layers of the epitaxially grown half-metal NiMnSb with ultralow spin wave damping. We demonstrate magnetization auto-oscillations at microwave frequencies in the 1-3 GHz range in out-of-plane magnetic fields. Threshold current densities as low as 3 x 10(11) A m(-2) are observed as well as minimum oscillation linewidths of 200 kHz, both of which are much lower than the values achieved in conventional metallic spin-valve-based devices of comparable dimensions. These results enable the fabrication of spin transfer torque driven magnonic devices with low current density requirements, improved signal linewidths, and in a simplified single-layer geometry. Published by AIP Publishing.
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| 15. |
- 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-
-
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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| 16. |
- Eklund, Anders, 1986-, et al.
(författare)
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Nonlinearity, frequency stability and device-to-device variability in nano-contact spin torque oscillators with grainy thin films
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In nano-contact spin torque oscillators with a frequency range of 10-65 GHz, the propagating spin wave mode attracts interest due both to its high frequency stability and prospective use in magnonic devices. Its dependence of the frequency on the bias current however displays device-to-device variability on the order of several hundred MHz, with device specific nonlinearities that can be either continuous or discontinuous and have negative impact on the frequency stability. A model for this behavior is however still lacking. By using micromagnetic simulations, we investigate the impact of imperfections in the spin wave-carrying free magnetic layer and find that nonlinearities can be created when the propagating spin wave is reflected back to the active region. The oscillation then self-locks at the frequency of the resonant wavelength, resulting in a standing spin wave pattern. Simulations including nine randomly generated film structures with 30 nm-sized grains and exchange-reduced inter-grain boundaries give qualitative and partially quantitative agreement with experimental measurements. The results point out the spin wave-reflecting grain boundaries as a source of device nonlinearity, manufacturing variability and frequency destabilization.
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| 17. |
- Escobar, Roberto A., et al.
(författare)
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Monte Carlo Modeling of Mixed-Anisotropy [Co/Ni](2)/NiFe Multilayers
- 2016
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Ingår i: IEEE Magnetics Letters. - : IEEE. - 1949-307X .- 1949-3088. ; 7
-
Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of a thin film consisting of an exchange-coupled [Co/Ni](2)/NiFe multilayer have been studied as a function of the NiFe thickness by using Monte Carlo modeling and compared with experimental results of [Co/Ni](4)/Co/NiFe multilayers. Both modeling and experiment showed that the NiFe thickness controls the effective anisotropy. The direction of the easy axis is determined by a competition between the perpendicular crystalline anisotropy of the Co/Ni and the shape anisotropy of the multilayer. As the thickness of the NiFe layer increases, the reversal mechanism of the thin film changes from the nucleation of reverse domains to vortex propagation. Therefore, our results reveal the magnetic configurations and the easy axis reorientation of mixed-anisotropy multilayers.
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| 18. |
- Fang, Yeyu, 1984, et al.
(författare)
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A Nonvolatile Spintronic Memory Element with a Continuum of Resistance States
- 2013
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 23:15, s. 1919-1922
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Tidskriftsartikel (refereegranskat)abstract
- A continuum of stable remanent resistance states is reported in perpendicularly magnetized pseudo spin valves with a graded anisotropy free layer. The resistance states can be systematically set by an externally applied magnetic field. The gradual reversal of the free layer with applied field and the field-independent fixed layer leads to a range of stable and reproducible remanent resistance values, as determined by the giant magnetoresistance of the device. An analysis of first-order reversal curves combined with magnetic force microscopy shows that the origin of the effect is the field-dependent population of up and down domains in the free layer.
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| 19. |
- 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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| 20. |
- Jiang, Sheng, et al.
(författare)
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Experimental evidence of tunable nonlinearity in He+ irradiated spin-torque oscillators
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Spin-torque nano-oscillators (STNOs) are among the most promising candidates for nanoscale broadband microwave generators. Before this application can be realized, however, enormous efforts are required of researchers to meet the commercial requirements of high-frequency tunability, high power, and narrow linewidth. As the performance of STNOs is mainly attributed to the nonlinearity on the basis of nonlinear auto-oscillator theory, we here systematically study how this nonlinearity is affected by the free-layer's effective magnetization Meff (i.e., the perpendicular magnetic anisotropy, PMA) in STNOs. The PMA is gradually tuned by using different fluences of He+ irradiation. Consequently, the nonlinearity can be continuously tailored from positive to negative. In addition, due to the almost zero nonlinearity, the linewidth shows an improvement of more than two orders of magnitude. This experimental observation is in strong agreement with the theory of nonlinear auto-oscillators. Our study not only confirms the theoretical prediction of nonlinearity, but also indicates the route to be taken towards realizing commercial microwave generators.
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| 21. |
- Jiang, Sheng, et al.
(författare)
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Field-Free High-Frequency Exchange-Spring Spin-Torque Nano- Oscillators
- 2023
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 23:4, s. 1159-1166
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Tidskriftsartikel (refereegranskat)abstract
- Spin-torque nano-oscillators (STNOs) are a type of nanoscale microwave auto-oscillators utilizing spin-torque to generate magnetodynamics with great promise for applications in microwaves, magnetic memory, and neuromorphic computing. Here, we report the first demonstration of exchange-spring STNOs, with an exchange-spring ([Co/Pd]-Co) reference layer and a perpendicular ([Co/Ni]) free layer. This magnetic configuration results in high-frequency (>10 GHz) microwave emission at a zero magnetic field and exchange-spring dynamics in the reference layer and the observation of magnetic droplet solitons in the free layer at different current polarities. Our demonstration of bipolar and field-free exchange-spring-based STNOs operating over a 20 GHz frequency range greatly extends the design freedom and functionality of the current STNO technology for energy -efficient high-frequency spintronic and neuromorphic applications.
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| 22. |
- Jiang, Sheng, et al.
(författare)
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Impact of the Oersted Field on Droplet Nucleation Boundaries
- 2018
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Ingår i: IEEE Magnetics Letters. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1949-307X .- 1949-3088. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- We investigate how the Oersted field affects the magnetic droplet nucleation boundary in spin-torque nano-oscillators based on orthogonal spin-valve stacks with a perpendicular magnetic anisotropy Co/Ni free layer and an easy-plane anisotropy Ni80Fe20 fixed layer. The current-field nucleation boundary is determined experimentally using both microwave signal and dc resistance measurements. The Oersted field can, in principle, have an impact on droplet nucleation. This effect is considered approximately using an analytical equation for the nucleation boundary, which is extended to cover fields larger than the fixed-layer saturation field. We test the accuracy of this approach by comparing with micromagnetic simulations. Finally, we carry out a numerical fit to experimental data and find good agreement.
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| 23. |
- Jiang, Sheng, et al.
(författare)
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Magnetodynamics in orthogonal nanocontact spin-torque nano-oscillators based on magnetic tunnel junctions
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We demonstrate the magnetodynamics of nanocontact spin-torque nano-oscillators based on magnetic tunnel junctions, with a strong perpendicular magnetic anisotropy free layer and an easy-plane reference layer. First, the static magnetic properties are studied by conducting hysteresis loop and magnetoresistance measurements. Then, we characterize the generated microwave signals by applying dc currents. Field-sweep signals at xed current show a typical FMR-like frequency dependence. Interestingly, for current-sweep measurements, the plot of frequency versus dc current can be fitted well with a symmetric parabolic and an asymmetric linear term. The symmetric behavior is ascribed to the current-induced Joule heating and Oersted elds. The asymmetric dependence is due to the linear perpendicular spin torque (or eld-like torque) and the voltage-controlled magnetic anisotropy. Our observation hints at a route to improving the frequency tunability in future spintronic applications by selectively adding all these contributions.
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| 24. |
- Jiang, Sheng, et al.
(författare)
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Magnetodynamics in orthogonal nanocontact spin-torque nano-oscillators based on magnetic tunnel junctions
- 2019
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 115:15
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate field and current controlled magnetodynamics in nanocontact spin-torque nano-oscillators based on orthogonal magnetic tunnel junctions. We systematically analyze the microwave properties (frequency f, linewidth Delta f, power P, and frequency tunability df/dI) with their physical origins-perpendicular magnetic anisotropy, dampinglike and fieldlike spin transfer torque (STT), and voltage-controlled magnetic anisotropy (VCMA). These devices present several advantageous characteristics: high emission frequencies (f>20 GHz), high frequency tunability (df/dI=0.25 GHz/mA), and zero-field operation (f similar to 4 GHz). Furthermore, detailed investigation of f(H, I) reveals that df/dI is mostly governed by the large VCMA [287 fJ/(V m)], while STT plays a negligible role.
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| 25. |
- Jiang, Sheng, et al.
(författare)
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Reduced spin torque nano-oscillator linewidth using He+ irradiation
- 2020
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 116:7
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate an approach for improving the spectral linewidth of a spin torque nano-oscillator (STNO). Using He + ion irradiation, we tune the perpendicular magnetic anisotropy (PMA) of the STNO free layer such that its easy axis is gradually varied from strongly out-of-plane to moderate in-plane. As the PMA impacts the non-linearity N of the STNO, we can, in this way, control the threshold current, the current tunability of the frequency, and, in particular, the STNO linewidth, which dramatically improves by two orders of magnitude. Our results are in good agreement with the theory for nonlinear auto-oscillators, confirm theoretical predictions of the role of the nonlinearity, and demonstrate a straightforward path toward improving the microwave properties of STNOs.
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| 26. |
- Jiang, Sheng, et al.
(författare)
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Tuning the magnetodynamic properties of all-perpendicular spin valves using He+ irradiation
- 2018
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Ingår i: Aip Advances. - : AIP Publishing. - 2158-3226. ; 8:6
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Tidskriftsartikel (refereegranskat)abstract
- Using He+ ion irradiation, we demonstrate how the magnetodynamic properties of both ferromagnetic layers in all-perpendicular [Co/Pd]/Cu/[Co/Ni] spin valves can be tuned by varying the He+ ion fluence. As the perpendicular magnetic anisotropy of both layers is gradually reduced by the irradiation, different magnetic configurations can be achieved from all-perpendicular (up arrow up arrow), through orthogonal (->up arrow), to all in-plane (paired right arrows). In addition, both the magnetic damping (alpha) and the inhomogeneous broadening (Delta H-0) of the Co/Ni layer improve substantially with increasing fluence. While the GMR of the spin valve is negatively affected, decreasing linearly from an original value of 1.14% to 0.4% at the maximum fluence of 50x10(14) He+/cm(2), most of the Co/Ni layer improvement is achieved already at a fluence of 10x10(14) He+/cm(2), for which GMR only reduces to 0.9%. (C) 2018 Author(s).
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| 27. |
- 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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| 28. |
- 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
-
Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 10:5
-
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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| 29. |
- Le, Quang Tuan, 1976-, et al.
(författare)
-
Bias dependence of parallel spin torque in all-perpendicular magnetic tunnel junction nanodevices
-
Annan publikation (populärvet., debatt m.m.)abstract
- We performed spin-torque ferromagnetic resonance measurements on spin-torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs), with both free and fixed layers having perpendicular magnetic anisotropy, under the application of an in-plane field. A dc current between near-zero and -250 $\mu{A}$ was introduced to investigate the influence of spin-transfer torque effects on the asymmetric and symmetric components of the output voltage. Both components showed a minimum at a misalignment of about 30$^\circ$ between the magnetization of the two CoFeB layers, indicating a strong electric-field effect. We also found that the bias dependence of the spin-torque terms is in contrast to that of in-plane anisotropy CoFeB MTJs.
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| 30. |
- Le, Quang Tuan, 1976-, et al.
(författare)
-
Ultra-high frequency tunability in low-current and low-field spin-torque oscillators based on perpendicular magnetic tunnel junctions
-
Annan publikation (populärvet., debatt m.m.)abstract
- We demonstrate ultra-high frequency tunability of up to 4.4 GHz/mA, and low threshold currents of about -21 $\mu$A, in spin-torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions, in which both free and fixed layers have perpendicular magnetic anisotropy (PMA). By using different thicknesses of the two CoFeB layers, their individual PMA strengths can be tailored to achieve significant relative misalignment of their respective magnetizations in moderate in-plane fields. We observe a broad maximum in both the device resistance and the generated microwave power around maximum misalignment. Maximum frequency tunability is observed at low-to-moderate fields and decrease rapidly after maximum misalignment.
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| 31. |
- Lee, Sanghoon, et al.
(författare)
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Magnetotransport properties of ferromagnetic semiconductor GaMnAs-based superlattices
- 2012
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Ingår i: Current applied physics. - : Elsevier BV. - 1567-1739 .- 1878-1675. ; 12:SUPPL. 2, s. S31-S36
-
Tidskriftsartikel (refereegranskat)abstract
- Two series of GaMnAs/GaAs superlattices (SSs) comprised of ferromagnetic semiconductor GaMnAs layers and non-magnetic GaAs spacers were investigated by the electronic transport measurements with an external magnetic field applied in the plane of the sample. The two SL series consisted of specimens that were structurally the same, but the GaAs spacers layers of one series were doped by Be, while in the second series the spacers were undoped. Although in field scans taken at 4 K all SLs showed a typical anisotropic magnetoresistance (MR) behavior dominated by magnetic anisotropy, similar to that normally observed in single GaMnAs ferromagnetic layers grown on GaAs (001) substrates, some of the SLs showed signatures of interaction between the GaMnAs layers in the form of broadening of the MR hysteresis. The effect of inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers became clear in the MR data taken at 30 K, where the strength of the magnetic anisotropy was reduced to the level of the inter-layer interaction. Specifically, MR measurements on two of the Be-doped SLs (BD2 and BD3) exhibited conspicuously large values of resistance at zero field, along with transitions of magnetization with negative coercive fields. The observation of these features in GaMnAs/GaAs SLs indicates the presence of spontaneous antiferromagnetic (AFM) inter-layer exchange coupling (IEC) between the GaMnAs magnetic layers. The study further revealed that the IEC in the GaMnAs multilayers strongly depended on the properties of the non-magnetic GaAs spacers, such as their thickness and the density of carriers in the layers. Importantly, these IEC effects occurred on a longer range than that expected from current theoretical studies.
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| 32. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
-
Auto-oscillating spin-wave modes of constriction-based spin Hall nano-oscillators in weak in-plane fields
-
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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| 33. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
-
Auto-oscillating Spin-Wave Modes of Constriction-Based Spin Hall Nano-oscillators in Weak In-Plane Fields
- 2018
-
Ingår i: Physical Review Applied. - : American Physical Society. - 2331-7019. ; 10:5
-
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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| 34. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
-
Contracting vs. expanding spin wave bullets in spin Hall nano-oscillators
-
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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| 35. |
- Mazraati, Hamid, et al.
(författare)
-
Free- and reference-layer magnetization modes versus in-plane magnetic field in a magnetic tunnel junction with perpendicular magnetic easy axis
- 2016
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 94:10
-
Tidskriftsartikel (refereegranskat)abstract
- We study the magnetodynamic modes of a magnetic tunnel junction with perpendicular magnetic easy axis (p-MTJ) in in-plane magnetic fields using device-level ferromagnetic resonance spectroscopy. We compare our experimental results to those of micromagnetic simulations of the entire p-MTJ. Using an iterative approach to determine the material parameters that best fit our experiment, we find excellent agreement between experiments and simulations in both the static magnetoresistance and magnetodynamics in the free and reference layers. From the micromagnetic simulations, we determine the spatial mode profiles, the localization of the modes and, as a consequence, their distribution in the frequency domain due to the inhomogeneous internal field distribution inside the p-MTJ under different applied field regimes. We also conclude that the excitation mechanism is a combination of the microwave voltage modulated perpendicular magnetic anisotropy, the microwave Oersted field, and the spin-transfer torque generated by the microwave current.
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| 36. |
- Mazraati, Hamid, et al.
(författare)
-
Free- and reference-layer magnetization modes vs.~in-plane magnetic field in a magnetic tunnel junction with perpendicular magnetic easy axis
- 2016
-
Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795.
-
Tidskriftsartikel (populärvet., debatt m.m.)abstract
- We study the magnetodynamic modes of a magnetic tunnel junction with perpendicular magnetic easy axis (p-MTJ) in in-plane magnetic fields using device-level ferromagnetic resonance spectroscopy. We compare our experimental results to those of micromagnetic simulations of the entire p-MTJ. Using an iterative approach to determine the material parameters that best fit our experiment, we find excellent agreement between experiments and simulations in both the static magnetoresistance and magnetodynamics in the free and reference layers. From the micromagnetic simulations, we determine the spatial mode profiles, the localization of the modes and, as a consequence, their distribution in the frequency domain due to the inhomogeneous internal field distribution inside the p-MTJ under different applied field regimes. We also conclude that the excitation mechanism is a combination of the microwave voltage modulated perpendicular magnetic anisotropy, the microwave Oersted field, and the spin-transfer torque generated by the microwave current.
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| 37. |
- Mazraati, Hamid, et al.
(författare)
-
In-plane field angle dependence of mutually synchronized constriction based spin Hall nano-oscillators
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We study mutual synchronization phenomena in multiple nanoconstriction-based SHNOs under weak in-plane fields down to μ0H = 30 mT and investigate the angular dependence of the synchronization condition. We compare double nanoconstriction and multiple nanoconstrictions with different spacings of 300 and 900 nm between the constrictions. For all the tested devices, we observe clear evidence of mutual synchronization of individual nanoconstrictions (NCs) only for angles smaller than a critical angle. This critical angle is higher for the 300 nm spacing than for the 900 nm spacing as a result of the stronger synchronization arising from the shorter distance. Direct inspection of the spin waves using μ-BLS maps confirms synchronization of the double nanoconstrictions. Alongside the synchronization, we observe a strong second harmonic that could be interpreted as a sign that the synchronization is mediated by the propagation of the second harmonic of the spin waves. Micromagnetic simulation explains the synchronization at the lower angles by the direction of the spatial profile of the modes and confirms the role of exchange coupling in the synchronization of nanoconstriction-based SHNOs.
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| 38. |
- Mazraati, Hamid, Industrial PhD Student, 1989-, et al.
(författare)
-
Low operational current spin Hall nano-oscillators based on NiFe/W bilayers
- 2016
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 109
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate highly efficient spin Hall nano-oscillators (SHNOs) based on NiFe/beta-W bilayers. Thanks to the very high spin Hall angle of beta-W, we achieve more than a 60% reduction in the auto-oscillation threshold current compared to NiFe/Pt bilayers. The structural, electrical, and magnetic properties of the bilayers, as well as the microwave signal generation properties of the SHNOs, have been studied in detail. Our results provide a promising path for the realization of low-current SHNO microwave devices with highly efficient spin-orbit torque from beta-W. Published by AIP Publishing.
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| 39. |
- Mazraati, Hamid, et al.
(författare)
-
Mapping out the in-plane spin wave modes of constriction based spin Hall nano-oscillators
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We experimentally study the auto-oscillating spin wave modes in NiFe/β-W constriction-based spin Hall nano-oscillators versus bias current, in-plane applied field strength, and azimuthal angle. We observe two different spin wave modes in weak in-plane fields: a linear-like mode confined in the minima of the internal field near the edges of the nano-constriction with weak frequency dependencies on the bias current and the applied field angle, and second, lower frequency mode that has significantly higher threshold current and stronger frequency dependencies on both bias current and the external field angle. Our micromagnetic modeling qualitatively reproduces experimental data and reveals that the second mode is the spin wave bullet. In contrast to the linear-like mode, the bullet is (a) localized in the middle of the constriction and (b) shrinks with the bias current. Our results are important for the development of the field-free spintronic oscillators for the applications in microwave signal generation and neuromorphic computing.
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| 40. |
- 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
-
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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| 41. |
- Mohseni, Seyed Morteza, et al.
(författare)
-
Magnetic droplet soliton nucleation in oblique fields
- 2018
-
Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 97:18
-
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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| 42. |
- 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
-
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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| 43. |
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| 44. |
- Mohseni, Seyed Majid, et al.
(författare)
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Spin Torque-Generated Magnetic Droplet Solitons
- 2013
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 339:6125, s. 1295-1298
-
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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| 45. |
- Moradi, M., et al.
(författare)
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Au/NiFe Magnetoplasmonics: Large Enhancement of Magneto-Optical Kerr Effect for Magnetic Field Sensors and Memories
- 2015
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Ingår i: Electronic Materials Letters. - : Springer Science and Business Media LLC. - 1738-8090 .- 2093-6788. ; 11:3, s. 440-446
-
Tidskriftsartikel (refereegranskat)abstract
- Surface plasmon polariton resonance (SPPR) can be originated from the surface charge oscillation via light localization at the interface of a metal, for example, Au or Ag and a dielectric. Such localization can be implemented to increase the magneto-optical (MO) activity of a magnetic medium while SPPR is fulfilled, which is known as magnetoplasmonics. In this paper, a magnetoplasmonic bilayer of Au/NiFe (Py) sputter deposited on glass is demonstrated. Large enhancement in MO-Ken effedt (MOKE) response due to SPPR effect is observed at different light incident angles. By measuring and analyzing the MO signals from the sample with different thicknesses of Au and Py layers, the optimal thicknesses' range is obtained with the largest MOKE. The large MOKE intensity from ultra-soft magnetic Py layer with low coercivity and small saturation field suggests a weak magnetic field-sensitive MO-based element. Finally, different applications of such structures, for example, weak magnetic field sensors and magnetic multilevel memory elements are demonstrated.
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| 46. |
- Mukherjee, S., et al.
(författare)
-
Role of boron diffusion in CoFeB/MgO magnetic tunnel junctions
- 2015
-
Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 91:8
-
Tidskriftsartikel (refereegranskat)abstract
- Several scientific issues concerning the latest generation read heads for magnetic storage devices, based on CoFeB/MgO/CoFeBmagnetic tunnel junctions (MTJs) are known to be controversial, including such fundamental questions as to the behavior and the role of B in optimizing the physical properties of these devices. Quantitatively establishing the internal structures of several such devices with different annealing conditions using hard x-ray photoelectron spectroscopy, we resolve these controversies and establish that the B diffusion is controlled by the capping Ta layer, though Ta is physically separated from the layer with B by several nanometers. While explaining this unusual phenomenon, we also provide insight into why the tunneling magnetoresistance (TMR) is optimized at an intermediate annealing temperature, relating it to B diffusion, coupled with our studies based on x-ray diffraction and magnetic studies.
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| 47. |
- Nguyen, T. N. Anh, et al.
(författare)
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Depth-Dependent Magnetization Profiles of Hybrid Exchange Springs
- 2014
-
Ingår i: Physical Review Applied. - 2331-7019. ; 2:4
-
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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| 48. |
- Nguyen, Thi Ngoc Anh, et al.
(författare)
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Effect of flattened surface morphology of anodized aluminum oxide templates on the magnetic properties of nanoporous Co/Pt and Co/Pd thin multilayered films
- 2018
-
Ingår i: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 427, s. 649-655
-
Tidskriftsartikel (refereegranskat)abstract
- For the first time, nanoporous Al2O3 templates with smoothed surface relief characterized by flattened interpore areas were used in the fabrication of Co/Pd and Co/Pt multilayers (MLs) with strong perpendicular magnetic anisotropy (PMA). Alternating gradient magnetometry (AGM) revealed perfectly conserved PMA in the Co/Pd and Co/Pt porous MLs (antidot arrays) with a ratio of remanent magnetization (Mr) to saturation magnetization (MS) of about 0.99, anisotropy fields (Ha) of up to 2.6 kOe, and a small deviation angle of 8° between the easy magnetization axis and the normal to the film surface. The sufficient magnetic hardening of the porous MLs with enhanced coercive field HC of up to ∼1.9 kOe for Co/Pd and ∼1.5 kOe for Co/Pt MLs, as compared to the continuous reference samples (∼1.5–2 times), is associated with the pinning of the magnetic moments on the nanopore edges. Application of the Stoner–Wohlfarth model for fitting the experimental M/MS(H) curves yielded clear evidence of the predominantly coherent rotation mechanism of magnetization reversal in the porous films.
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| 49. |
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| 50. |
- Qejvanaj, F., et al.
(författare)
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Planar Hall-Effect Bridge Sensor With NiFeX (X = Cu, Ag, and Au) Sensing Layer
- 2015
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Ingår i: Ieee Transactions on Magnetics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9464 .- 1941-0069. ; 51:11
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a new material alloy for planar Hall-effect bridge (PHEB) sensors and the accurate analysis of the resistance and sensitivity of these materials. The sensing layer is based on NiFeX (X = Cu, Ag, and Au). These alloys have a lower resistance without a significant loss of sensitivity. The presented PHEB sensors with NiFeX sensing layer show a coercivity of 1.7 Oe, lower than that of PHEB sensors with NiFe sensing layers, which have coercivities of 2.2 Oe.
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