| 1. |
- Bonetti, Stefano, et al.
(författare)
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Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:17, s. 174427-
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Tidskriftsartikel (refereegranskat)abstract
- The integrated power and linewidth of a propagating and a self-localized spin-wave mode excited by spin-polarized current in an obliquely magnetized magnetic nanocontact are studied experimentally as functions of the angle theta(e) between the external bias magnetic field and the nanocontact plane. It is found that the power of the propagating mode increases monotonically with theta(e), while the power of the self-localized mode has a broad maximum near theta(e) = 40 degrees and exponentially vanishes near the critical angle theta(e) = 58 degrees, at which the localized mode disappears. The linewidth of the propagating mode in the interval of angles 58 degrees < theta(e) < 90 degrees, where only this mode is excited, is adequately described by the existing theory, while in the angular interval where both modes can exist the observed linewidth of both modes is substantially broadened due to the telegraph switching between the modes. Numerical simulations and an approximate analytical model give a good semiquantitative description of the observed results.
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| 2. |
- Dumas, Randy K., et al.
(författare)
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Accessing different spin-disordered states using first-order reversal curves
- 2014
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 90:10
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Tidskriftsartikel (refereegranskat)abstract
- Combined first-order reversal curve (FORC) analyses of the magnetization (M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the M-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin valve (PSV) and a [Co/Cu](8) multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and the FePt layers, the M and MR show a simple one-to-one relationship during reversal. In the [Co/Cu](8) multilayer, the correlation between the magnetization reversal and the MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MRs than those found along the conventional major loop field cycle. Unlike the M-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin disorder and the largest MR.
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| 3. |
- Dumas, Randy K., et al.
(författare)
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Probing vertically graded anisotropy in FePtCu films
- 2011
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 84, s. 054434-
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Tidskriftsartikel (refereegranskat)abstract
- Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (TA). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300°C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low TA. A fundamentally different mechanism is found after annealing at 400°C, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500°C, a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated TA.
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| 4. |
- Dürrenfeld, Philipp, et al.
(författare)
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Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films
- 2015
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Ingår i: Physical Review B. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:21
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Tidskriftsartikel (refereegranskat)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.
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| 5. |
- Fazlali, Masoumeh, et al.
(författare)
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Homodyne-detected ferromagnetic resonance of in-plane magnetized nanocontacts : Composite spin-wave resonances and their excitation mechanism
- 2016
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Ingår i: PHYSICAL REVIEW B. - 2469-9950 .- 2469-9969. ; 93
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Tidskriftsartikel (refereegranskat)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.
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| 6. |
- Haidar, Mohammad, et al.
(författare)
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Controlling Gilbert damping in a YIG film using nonlocal spin currents
- 2016
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 94:18
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Tidskriftsartikel (refereegranskat)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.
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| 7. |
- Iacocca, Ezio, 1986, et al.
(författare)
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Analytical investigation of modulated spin-torque oscillators in the framework of coupled differential equations with variable coefficients
- 2012
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 85:18
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Tidskriftsartikel (refereegranskat)abstract
- Modulation of spin torque oscillators (STOs) is investigated by analytically solving the time-dependent coupled equations of an auto-oscillator. A Fourier-series solution is proposed, leading to the coefficients being determined with a linear set of equations, from which a nonlinear amplitude and frequency modulation (NFAM) scheme is obtained. In this framework, the NFAM features are related to the intrinsic STO parameters, revealing a frequency dependence of the harmonic-dependent modulation index that allows a modulation bandwidth to be defined for these devices. The presented results expose a rich parameter space, where the modulation and the STO's operation conditions define the observed modulation features. The Fourier-series representation of the time signal is suitable for studying periodic perturbations on the auto-oscillator equation.
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| 8. |
- Iacocca, Ezio, 1986, et al.
(författare)
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Generation linewidth of mode-hopping spin torque oscillators
- 2014
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 89:5
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Tidskriftsartikel (refereegranskat)abstract
- Experiments on spin torque oscillators commonly observe multimode signals. Recent theoretical works have ascribed the multimode signal generation to coupling between energy-separated spin wave modes. Here, we analyze in detail the dynamics generated by such mode coupling. We show analytically that the mode-hopping dynamics broaden the generation linewidth and makes it generally well described by a Voigt line shape. Furthermore, we show that the mode-hopping contribution to the linewidth can dominate, in which case it provides a direct measure of the mode-hopping rate. Due to the thermal drive of mode-hopping events, the mode-hopping rate also provides information on the energy barrier separating modes and temperature-dependent linewidth broadening. Our results are in good agreement with experiments, revealing the physical mechanism behind the linewidth broadening in multimode spin torque oscillators.
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| 9. |
- Iacocca, Ezio, 1986, et al.
(författare)
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Mode-coupling mechanisms in nanocontact spin-torque oscillators
- 2015
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 91:10, s. artikel 104405 -
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Tidskriftsartikel (refereegranskat)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.
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| 10. |
- Iacocca, Ezio, 1986, et al.
(författare)
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Resonant excitation of injection-locked spin-torque oscillators
- 2013
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Ingår i: Physical Review B. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:21, s. artikel nr 214428-
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Tidskriftsartikel (refereegranskat)abstract
- Theory predicts that a strongly injection-locked spin-torque oscillator (STO) should show a characteristic ringing frequency both on its approach to the locked state and under the influence of thermal noise. While experiments have so far failed to detect such ringing, we here show numerically and analytically how current modulation of injection-locked STOs can excite the ringing frequency in a resonant manner, and hence increase the experimental sensitivity. The complexity of such dynamics leads to a nonlinear resonance, which can even unlock the STO as a function of the modulation strength. The results presented here offer a plausible method for experimentally measuring the ringing frequency of STOs. Moreover, the onset of unlocking also provides a measure for the maximum modulation strength that can be applied to phase-locked STOs.
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