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1.	Awad, Ahmad, et al. (author) Long-range mutual synchronization of spin Hall nano-oscillators 2017 In: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481. ; 13, s. 292-299 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.
2.	Balinsky, Michael, et al. (author) Modulation of the Spectral Characteristics of a Nano-Contact Spin-Torque Oscillator via Spin Waves in an Adjacent Yttrium-Iron Garnet Film 2016 In: IEEE Magnetics Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1949-307X .- 1949-3088. ; 7 Journal article (peer-reviewed)abstract NiFe-Cu-Co trilayer nano-contact spin-torque oscillators (NC-STOs) fabricated on an yttrium-iron garnet (YIG) film were studied in two different modes. In passive mode, i.e. without any NC-STO auto-oscillations, a microwave current through the nano-contact can excite spin waves (SW) in the YIG film, and, vice versa, antenna generated SWs in the YIG film can be detected by the nano-contact. In active mode, i.e., in the presence of auto-oscillations, significant changes appear in the NC-STO spectrum when its frequency approaches that of the SWs excited in the YIG. These results demonstrate strong coupling between NC-STOs and SWs in YIG and open new possibilities of 1) pure spin-current generation in YIG by NC-STOs; 2) mutual locking of a number of NC-STOs through SWs in YIG; and 3) improvement of NC-STO spectra through SW feedback in YIG.
3.	Balinsky, Michael, et al. (author) Spin Pumping and the Inverse Spin-Hall Effect via Magnetostatic Surface Spin-Wave Modes in Yttrium-Iron Garnet/Platinum Bilayers 2015 In: IEEE Magnetics Letters. - 1949-307X .- 1949-3088. ; 6:3000604 Journal article (peer-reviewed)abstract Spin pumping at a boundary between a yttrium-iron garnet (YIG) film and a thin platinum (Pt) layer is studied under conditions in which a magnetostatic surface spin wave (MSSW, or Damon-Eshbach mode) is excited in YIG by a narrow strip-line antenna. It is shown that the voltage created by the inverse spin-Hall effect (ISHE) in Pt is strongly dependent on the wavevector of the excited MSSW. For YIG film thicknesses of 41 and 0.9 mu m, the maximum ISHE voltage corresponds to the maximum of efficiently excited MSSW wavevectors and does not coincide with the maximum of absorbed microwave power. For a thinner (0.175 mu m) YIG film, the maximum of the ISHE voltage moves closer to the ferromagnetic resonance and almost coincides with the region of the maximum microwave absorption. We show that the effect is related to the change in the thickness profile and the wavenumber spectrum of the excited MSSW taking place when the YIG film thickness is increased.
4.	Balinsky, Michael, et al. (author) Spin Pumping and the Inverse Spin-Hall Effect via Magnetostatic Surface Spin-Wave Modes in Yttrium-Iron Garnet/Platinum Bilayers 2015 In: Ieee Magnetics Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1949-307X .- 1949-3088. ; 6 Journal article (peer-reviewed)abstract Spin pumping at a boundary between a yttrium-iron garnet (YIG) film and a thin platinum (Pt) layer is studied under conditions in which a magnetostatic surface spin wave (MSSW, or Damon-Eshbach mode) is excited in YIG by a narrow strip-line antenna. It is shown that the voltage created by the inverse spin-Hall effect (ISHE) in Pt is strongly dependent on the wavevector of the excited MSSW. For YIG film thicknesses of 41 and 0.9 mu m, the maximum ISHE voltage corresponds to the maximum of efficiently excited MSSW wavevectors and does not coincide with the maximum of absorbed microwave power. For a thinner (0.175 mu m) YIG film, the maximum of the ISHE voltage moves closer to the ferromagnetic resonance and almost coincides with the region of the maximum microwave absorption. We show that the effect is related to the change in the thickness profile and the wavenumber spectrum of the excited MSSW taking place when the YIG film thickness is increased.
5.	Banuazizi, Seyed Amir Hossein, et al. (author) Current, temperature, and magnetic field profiles in nanogap spin Hall nano-oscillators Other publication (other academic/artistic)abstract We carry out a detailed experimental and numerical study of nanogap spin Hall nano-oscillators (SHNOs) to determine the current distribution, the associated Oersted field, and the possible effects of the local temperature rise. We find substantial heating in the center of the SHNOs, leading to a nonuniform device resistance which redistributes the current in a nontrivial way. As a consequence, both the Oe field magnitude and its spatial profile are nonlinear functions of the current magnitude. Our results have direct consequences for spin-wave generation in these devices.
6.	Banuazizi, Seyed Amir Hossein, et al. (author) Order of magnitude improvement of nano-contact spin torque nano-oscillator performance 2017 In: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 9:5, s. 1896-1900 Journal article (peer-reviewed)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.
7.	Burgos-Parra, E., et al. (author) Investigation of magnetic droplet solitons using x-ray holography with extended references 2018 In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 8 Journal article (peer-reviewed)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.
8.	Dumas, Randy K., et al. (author) Recent Advances in Nanocontact Spin-Torque Oscillators 2014 In: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 50:6, s. 4100107- 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.
9.	Dumas, Randy K., et al. (author) Recent Advances in Nanocontact Spin-Torque Oscillators 2014 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.
10.	Dürrenfeld, Philipp, et al. (author) Controlled skyrmion nucleation in extended magnetic layers using a nanocontact geometry 2017 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 96:5 Journal article (peer-reviewed)abstract We propose and numerically simulate a spintronic device layout consisting of a nanocontact on top of an extended Co/Pt bilayer. The interfacial Dzyaloshinskii-Moriya interaction in such bilayer systems can lead to the possible existence of metastable skyrmions. A small dc current injected through the nanocontact enables the manipulation of the size as well as the annihilation of an initially present skyrmion, while ps-long current pulses allow for the controlled nucleation of single skyrmions underneath the nanocontact. The results are obtained from micromagnetic simulations and can be potentially used for future magnetic storage implementations.
11.	Dürrenfeld, Philipp, et al. (author) Low-current, narrow-linewidth microwave signal generation in NiMnSb based single-layer nanocontact spin-torque oscillators 2016 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 109:22 Journal article (peer-reviewed)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.
12.	Dürrenfeld, Philipp, et al. (author) Modulation-mediated unlocking of a parametrically phase-locked spin torque oscillator 2014 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:25 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.
13.	Dürrenfeld, Philipp, et al. (author) Parametric excitation in a magnetic tunnel junction-based spin torque oscillator 2014 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:5 Journal article (peer-reviewed)abstract Using microwave current injection at room temperature, we demonstrate parametric excitation of a magnetic tunnel junction (MTJ)-based spin-torque oscillator (STO). Parametric excitation is observed for currents below the auto-oscillation threshold, when the microwave current frequency f(e) is twice the STO free-running frequency f(0). Above threshold, the MTJ becomes parametrically synchronized. In the synchronized state, the STO exhibits an integrated power up to 5 times higher and a linewidth reduction of two orders of magnitude, compared to free-running conditions. We also show that the parametric synchronization favors single mode oscillations in the case of multimode excitation. (C) 2014 AIP Publishing LLC.
14.	Dürrenfeld, Philipp (author) Spin Torque and Spin Hall Nano-Oscillators with Single Magnetic Layers 2015 Doctoral thesis (other academic/artistic)abstract Spin-torque oscillators (STOs) are capable of producing tunable broadband high-frequency signals within a nanometer-sized region. In this thesis, novel designs for STOs will be discussed; the common theme of these designs will be their use of only one ferromagnetic (FM) material. This work was motivated by the need to understand the fundamental magnetization dynamics in such devices, improving signal quality, and reducing the complexities of device fabrication in comparison to conventional nanocontact (NC-)STOs based on a multilayered spin-valve structure. Single-layer NC-STOs: The multilayered spin-valve structure in NC-STOs may be replaced by a material stack with a single FM layer, reducing the number of metallic layers as well as the total thickness of the devices. Spin-transfer torque (STT) can be efficiently created at the interface to the FM layer underneath the nanocontact, leading to the production of microwave signals in zero applied field. Spin Hall effect (SHE) driven dynamics: A charge current flowing through platinum establishes spin accumulation at the edges of the material due to the SHE. This can be used for STT-driven magnetization dynamics in an adjacent FM layer. These so-called spin Hall nano-oscillators (SHNOs) operate with the current flow being directed in-plane, simplifying nanofabrication and allowing for direct optical access to the active area. SHNO devices are designed in two device geometries: One uses pointed gold electrodes to inject current into an extended active area, while the other focuses the current at a nanoconstriction. Implementation of Heusler alloys: The magnetodynamics of STOs are governed by the properties of the FM free layer. The ease of fabrication of single-layer NC-STOs and SHNOs opens up the possibility of implementing nonconventional ferromagnets with unique magnetic properties. One such material is the epitaxially grown half-metallic half-Heusler alloy NiMnSb, which has a fundamentally lower magnetic damping than permalloy, leading to microwave emission spectra with ultralow linewidths. Mutual synchronization of SHNOs: The mutual synchronization of STOs is accompanied by an increase in the output power and a decrease in the linewidth, thereby drastically improving the overall quality of the microwave signal. Here, the mutual synchronization of up to nine serially connected nanoconstriction SHNOs is demonstrated, which potentially opens up the possibility of implementing these devices in, for example, spin-wave-based computing.
15.	Dürrenfeld, Philipp, et al. (author) Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films 2015 In: Physical Review B. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:21 Journal article (peer-reviewed)abstract The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization M-S, Gilbert damping alpha, and exchange stiffness A. M-S and A are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for M-S and alpha. The calculated A shows the same trend as the experimental data but has a larger magnitude. In addition to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices.
16.	Fazlali, Masoumeh, et al. (author) Homodyne-detected ferromagnetic resonance of in-plane magnetized nanocontacts : Composite spin-wave resonances and their excitation mechanism 2016 In: PHYSICAL REVIEW B. - 2469-9950 .- 2469-9969. ; 93 Journal article (peer-reviewed)abstract This work provides a detailed investigation of the measured in-plane field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an extended Co/Cu/NiFe pseudo-spin-valve stack using a nanocontact (NC) geometry. The magnetodynamics are generated by a pulse-modulated microwave current, and the resulting rectified dc mixing voltage, which appears across the NC at resonance, is detected using a lock-in amplifier. Most notably, we find that the measured spectra of the NiFe layer are composite in nature and highly asymmetric, consistent with the broadband excitation of multiple modes. Additionally, the data must be fit with two Lorentzian functions in order to extract a reasonable value for the Gilbert damping of the NiFe. Aided by micromagnetic simulations, we conclude that (i) for in-plane fields the rf Oersted field in the vicinity of the NC plays the dominant role in generating the observed spectra, (ii) in addition to the FMR mode, exchange-dominated spin waves are also generated, and (iii) the NC diameter sets the mean wave vector of the exchange-dominated spin wave, in good agreement with the dispersion relation.
17.	Haidar, M., et al. (author) Compositional effect on auto-oscillation behavior of Ni 100 -Fe-x(x)/Pt spin Hall nano-oscillators 2021 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:1 Journal article (peer-reviewed)abstract We demonstrate the compositional effect on the magnetodynamic and auto-oscillation properties of Ni 100 - xFe(x)/Pt (x=10-40) nanoconstriction-based spin Hall nano-oscillators. Using spin-torque ferromagnetic resonance performed on microstrips, we measure a significant reduction in both damping and spin Hall efficiency with the increasing Fe content, which lowers the spin pumping contribution. The strong compositional effect on spin Hall efficiency is primarily attributed to the increased saturation magnetization in Fe-rich devices. As a direct consequence, higher current densities are required to drive spin-wave auto-oscillations at higher microwave frequencies in Fe-rich nanoconstriction devices. Our results establish the critical role of the compositional effect in engineering the magnetodynamic and auto-oscillation properties of spin Hall devices for microwave and magnonic applications.
18.	Haidar, Mohammad, et al. (author) Controlling Gilbert damping in a YIG film using nonlocal spin currents 2016 In: Physical Review B - Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 94:18 Journal article (peer-reviewed)abstract We demonstrate the control of Gilbert damping in 65-nm-thick yttrium iron garnet (YIG) films using a spin-polarized current generated by a direct current through a nanocontact, spin filtered by a thin Co layer. The magnetodynamics of both the YIG and the Co layers can be excited by a pulse-modulated microwave current injected through the nanocontact and the response detected as a lock-in amplified voltage over the device. The spectra show three clear peaks, two associated with the ferromagnetic resonance (FMR) in each layer, and an additional Co mode with a higher wave vector proportional to the inverse of the nanocontact diameter. By varying the sign and magnitude of the direct nanocontact current, we can either increase or decrease the linewidth of the YIG FMR peak consistent with additional positive or negative damping being exerted by the nonlocal spin current injected into the YIG film. Our nanocontact approach thus offers an alternative route in the search for auto-oscillations in YIG films.
19.	Houshang, Afshin, et al. (author) Effect of Excitation Fatigue on the Synchronization of Multiple Nanocontact Spin-Torque Oscillators 2014 In: Ieee Magnetics Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1949-307X .- 1949-3088. ; 5 Journal article (peer-reviewed)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.
20.	Houshang, Afshin, et al. (author) Spin-wave-beam driven synchronization of nanocontact spin-torque oscillators 2016 In: Nature Nanotechnology. - 1748-3387 .- 1748-3395. ; 11, s. 280-286 Journal article (peer-reviewed)abstract The synchronization of multiple nanocontact spin-torque oscillators (NC-STOs) is mediated by propagating spin waves (SWs). Although it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs, its role in the synchronization behaviour of multiple NCs has not been considered so far. Here we investigate the synchronization behaviour in multiple NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field. Synchronization is promoted (impeded) by the Oersted field landscape when the NCs are oriented vertically (horizontally) due to the highly anisotropic SW propagation. Not only is robust synchronization between two oscillators observed for separations larger than 1,000nm, but synchronization of up to five oscillators, a new record, has been observed in the vertical array geometry. Furthermore, the synchronization can no longer be considered mutual in nature.
21.	Iacocca, Ezio, 1986, et al. (author) Mode-coupling mechanisms in nanocontact spin-torque oscillators 2015 In: Physical Review B. - 1098-0121 .- 1550-235X. ; 91:10, s. artikel 104405 - Journal article (peer-reviewed)abstract Spin-torque oscillators (STOs) are devices that allow for the excitation of a variety of magnetodynamical modes at the nanoscale. Depending on both external conditions and intrinsic magnetic properties, STOs can exhibit regimes of mode hopping and even mode coexistence. Whereas mode hopping has been extensively studied in STOs patterned as nanopillars, coexistence has been only recently observed for localized modes in nanocontact STOs (NC-STOs), where the current is confined to flow through a NC fabricated on an extended pseudo spin valve. By means of electrical characterization and a multimode STO theory, we investigate the physical origin of the mode-couplingmechanisms favoring coexistence. Two couplingmechanisms are identified: (i) magnon-mediated scattering and (ii) intermode interactions. These mechanisms can be physically disentangled by fabricating devices where the NCs have an elliptical cross section. The generation power and linewidth from such devices are found to be in good qualitative agreement with the theoretical predictions, as well as provide evidence of the dominant mode-coupling mechanisms.
22.	Keatley, P. S., et al. (author) Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting 2017 In: Journal of Physics D-Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 50:16 Journal article (peer-reviewed)abstract Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by the injected RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.
23.	Keatley, P. S., et al. (author) Superharmonic injection locking of nanocontact spin-torque vortex oscillators 2016 In: Physical Review B. - : american physical society. - 2469-9950 .- 2469-9969. ; 94:9 Journal article (peer-reviewed)abstract Superharmonic injection locking of single nanocontact (NC) spin-torque vortex oscillators (STVOs) subject to a small microwave current has been explored. Frequency locking was observed up to the fourth harmonic of the STVO fundamental frequency f(0) in microwave magnetoelectronic measurements. The large frequency tunability of the STVO with respect to f(0) allowed the device to be locked to multiple subharmonics of the microwave frequency f(RF), or to the same subharmonic over a wide range of fRF by tuning the dc current. In general, analysis of the locking range, linewidth, and amplitude showed that the locking efficiency decreased as the harmonic number increased, as expected for harmonic synchronization of a nonlinear oscillator. Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences in the spatial character of the magnetization dynamics of states locked to the fundamental and harmonic frequencies, suggesting significant differences in the vortex core trajectories within the same device. Superharmonic injection locking of a NC-STVO may open up possibilities for devices such as nanoscale frequency dividers, while differences in the core trajectory may allow mutual synchronization to be achieved in multioscillator networks by tuning the spatial character of the dynamics within shared magnetic layers.
24.	Ranjbar, Mojtaba, et al. (author) CoFeB-Based Spin Hall Nano-Oscillators 2014 In: Ieee Magnetics Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1949-307X .- 1949-3088. ; 5 Journal article (peer-reviewed)abstract We demonstrate magnetization auto-oscillations driven by pure spin currents in spin Hall nano-oscillators based on CoFeB/Pt bilayers. Despite the very low anisotropic magnetoresistance of CoFeB, a substantial microwave signal power can be detected, even at room temperature, indicating that a sizable spin wave amplitude is generated. Spin torque ferromagnetic resonance measurements reveal that the generated auto-oscillation frequency lies below the ferromagnetic resonance frequency of CoFeB and is therefore well described by a self-localized spin wave bullet mode.
25.	Redjai Sani, Sohrab, et al. (author) Microwave Signal Generation in Single-Layer Nano-Contact Spin Torque Oscillators 2013 In: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 49:7, s. 4331-4334 Journal article (peer-reviewed)abstract We demonstrate spin transfer torque (STT) driven microwave signal generation, from about 250 MHz to above 3 GHz, in single perm alloy layers underneath a nano-contact with diameter of 100 nm. The threshold current for signal generation is found to be strongly hysteretic, the microwave signal shows a number of harmonics, zero-field operation is straightforward, and the microwave frequency increases quasi-linearly with drive current. All observations are consistent with STT driven motion of a vortex-antivortex pair nucleated by the Oersted field underneath the nano-contact. While the generated power is about 10 dB smaller than the best GMR based nano-contact spin torque oscillators, the linewidth of 6-100 MHz is of the same order.
26.	Sani, S. R., et al. (author) Microwave Signal Generation in Single-Layer Nano-Contact Spin Torque Oscillators 2013 In: Ieee Transactions on Magnetics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9464 .- 1941-0069. ; 49:7, s. 4331-4334 Journal article (peer-reviewed)abstract We demonstrate spin transfer torque (STT) driven microwave signal generation, from about 250 MHz to above 3 GHz, in single perm alloy layers underneath a nano-contact with diameter of 100 nm. The threshold current for signal generation is found to be strongly hysteretic, the microwave signal shows a number of harmonics, zero-field operation is straightforward, and the microwave frequency increases quasi-linearly with drive current. All observations are consistent with STT driven motion of a vortex-antivortex pair nucleated by the Oersted field underneath the nano-contact. While the generated power is about 10 dB smaller than the best GMR based nano-contact spin torque oscillators, the linewidth of 6-100 MHz is of the same order.
27.	Sharma, R., et al. (author) Modulation Rate Study in a Spin-Torque Oscillator-Based Wireless Communication System 2015 In: Ieee Transactions on Magnetics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9464 .- 1941-0069. ; 51:11 Journal article (peer-reviewed)abstract We study a wireless communication system based on a magnetic tunnel junction spin-torque nano-oscillator (STNO) by employing amplitude-shift-keying modulation. By varying the pulse modulation frequency (f(m)) from 1 kHz to 2 MHz and distance (D) between the antenna from 25 to 150 cm, we show a maximum data rate of 6 Mb/s (at D = 25 cm and fm = 1 MHz), a limit imposed by our setup and noise generated by the STNO itself. We also report the average amplitude noise (S-delta a) and average white frequency noise (S-wh) of the wireless communication system and discuss their dependence on the distance between the antennas.
28.	Sharma, R., et al. (author) Time-domain stability of parametric synchronization in a spin-torque nano-oscillator based on a magnetic tunnel junction 2017 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:2 Journal article (peer-reviewed)abstract We report on time-domain stability of the parametric synchronization in a spin-torque nano-oscillator (STNO) based on a magnetic tunnel junction. Time-domain measurements of the instantaneous frequency (f(i)) of a parametrically synchronized STNO showrandom short-term unlocking of the STNO signal for low injected radio-frequency (RF) power, which cannot be revealed in time-averaged frequency domain measurements. Macrospin simulations reproduce the experimental results and reveal that the random unlocking during synchronization is driven by thermal fluctuations. We show that by using a high injected RF power, random unlocking of the STNO can be avoided. However, a perfect synchronization characterized by complete suppression of phase noise, so-called phase noise squeezing, can be obtained only at a significantly higher RF power. Our macrospin simulations suggest that a lower temperature and a higher positive ratio of the fieldlike torque to the spin transfer torque reduce the threshold RF power required for phase noise squeezing under parametric synchronization.
29.	Spicer, T. M., et al. (author) Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator 2018 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:19 Journal article (peer-reviewed)abstract Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nanooscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a nonlinear "bullet" mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal. Published by AIP Publishing.
30.	Tiwari, D., et al. (author) Antidamping spin-orbit torques in epitaxial-Py(100)/beta-Ta 2017 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 111:23 Journal article (peer-reviewed)abstract We perform spin torque ferromagnetic resonance measurements on the Si(100)/TiN(100)/epi-Py(100)/beta-Ta system. We demonstrate current induced modulation of the Gilbert damping constant, which is about 30% for a current density of 6.25 x 10(9) A/m(2). We show that the observed modulation of the Gilbert damping constant cannot be explained by spin transfer torques arising from the spin Hall effect of the beta-Ta layer. An additional mechanism such as antidamping spinorbit torque resulting from the interface or the crystalline structure of Py thin films needs to be considered. Published by AIP Publishing.
31.	Tiwari, Dhananjay, et al. (author) Enhancement of spin-torque diode sensitivity in a magnetic tunnel junction by parametric synchronization 2016 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 108 Journal article (peer-reviewed)abstract We demonstrate enhanced radio frequency (RF) detection sensitivity in a magnetic tunnel junction based spin torqueoscillator (STO) using parametric synchronization. The parametric synchronization is observed above the auto-oscillation threshold of the STO and enhances the RF sensitivity at 2f0, where f0 is the auto-oscillation frequency. The maximum RF sensitivity for parametric synchronization at 2f0≃9GHz2f0≃9GHz is 16mV/mW, while the maximum RF sensitivity at f0≃4.5GHzf0≃4.5GHz without parametric synchronization is 12mV/mW. Macrospin simulations corroborate the experimental results.
32.	Yin, Yuli, et al. (author) Ferromagnetic and Spin-Wave Resonance on Heavy-Metal-Doped Permalloy Films : Temperature Effects 2017 In: IEEE Magnetics Letters. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1949-307X .- 1949-3088. ; 8 Journal article (peer-reviewed)abstract Broadband ferromagnetic resonance (FMR)spectroscopy is used to study the temperature (T)and dopantconcentration dependence of the magnetodynamic properties of Permalloy (Py = Ni80Fe20)and Py-100- M-x(x) films, where the dopant M = Pt, Au, and Ag. The saturation magnetization (MS)and Gilbert damping constant (a)are determined from the uniform FMR mode, while the spin wave stiffness (D)is extracted using the first perpendicular standing spinwave mode. The temperature dependence of D is best described by a T (2) law, which suggests a noticeable effect of the itinerant character of the electrons. The spin wave stiffness is also estimated using Bloch's law and the two methods are compared. The results strongly imply that not only spin wave and Stoner excitations, but also other mechanisms contribute to the reduction of MS. The damping increases with T for all samples, but the enhancement is most pronounced for Py doped with 30 at.% Au.
33.	Yin, Y. L., et al. (author) Ferromagnetic and Spin-Wave Resonance on Heavy-Metal-Doped Permalloy Films: Temperature Effects 2017 In: Ieee Magnetics Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1949-307X .- 1949-3088. ; 8 Journal article (peer-reviewed)abstract Broadband ferromagnetic resonance (FMR)spectroscopy is used to study the temperature (T)and dopantconcentration dependence of the magnetodynamic properties of Permalloy (Py = Ni80Fe20)and Py-100- M-x(x) films, where the dopant M = Pt, Au, and Ag. The saturation magnetization (MS)and Gilbert damping constant (a)are determined from the uniform FMR mode, while the spin wave stiffness (D)is extracted using the first perpendicular standing spinwave mode. The temperature dependence of D is best described by a T (2) law, which suggests a noticeable effect of the itinerant character of the electrons. The spin wave stiffness is also estimated using Bloch's law and the two methods are compared. The results strongly imply that not only spin wave and Stoner excitations, but also other mechanisms contribute to the reduction of MS. The damping increases with T for all samples, but the enhancement is most pronounced for Py doped with 30 at.% Au.
34.	Yin, Yuli, et al. (author) Tunable permalloy-based films for magnonic devices 2015 In: Physical Review B. - 1098-0121 .- 1550-235X. ; 92:2 Journal article (peer-reviewed)abstract Using both broadband ferromagnetic resonance (FMR) spectroscopy and ab initio calculations, we study the magnetodynamic properties of permalloy (Py, Ni80Fe20) and Py100-x M-x films with M as platinum (Pt), gold (Au), or silver (Ag). From the uniform FMR mode, we extract the saturation magnetization (M-S), damping (alpha), and inhomogeneous broadening (Delta H-0); from the first perpendicular standing spin-wave (PSSW) mode, we extract the exchange stiffness (A). M-S and A are found to decrease with increasing alloying, most strongly for Au and less so for Pt. On the other hand, alpha increases rapidly with both Pt and Au content, while being virtually independent of Ag content. The physical origins of the observed trends in alpha, M-S, and A are analyzed and explained using density functional theory calculations in the coherent potential approximation. The calculated trends quantitatively agree with the experimental observations. The drastically different impacts of Pt, Au, and Ag on the various fundamental magnetodynamic properties will allow for significant design freedom, where different properties can be varied independently of others through careful combinations of the Pt, Au, and Ag contents of Py100-x M-x films. By empirical approximations of each property's concentration dependence, we can dial in any desired combination of magnetodynamic properties within this parameter space. As a proof-of-principle demonstration we design a set of Py100-x-yPtxAgy films, where the saturation magnetization stays constant throughout the set and the damping can be tuned by a factor of 4.

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