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ABSTRACT: In this paper, we employ micromagnetic simulations to study non-adiabatic
stochastic resonance (NASR) excited by spin-transfer torque in a
super-paramagnetic free layer nanomagnet of a nanoscale spin valve. We find
that NASR dynamics involves thermally activated transitions among two static
states and a single dynamic state of the nanomagnet and can be well understood
in the framework of Markov chain rate theory. Our simulations show that a
direct voltage generated by the spin valve at the NASR frequency is at least
one order of magnitude greater than the dc voltage generated off the NASR
frequency. Our computations also reproduce the main experimentally observed
features of NASR such as the resonance frequency, the temperature dependence
and the current bias dependence of the resonance amplitude. We propose a simple
design of a microwave signal detector based on NASR driven by spin transfer
torque.
03/2011;
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ABSTRACT: This paper presents a micromagnetic study about the nonlinear properties exhibited by spin torque oscillators, implemented as nanoscale exchange biased spin-valves of elliptical cross-sectional area. The analysis is based on numerical simulations which also include the back-torque effect in the pinned layer. The external bias field is applied along the in-plane hard axis. Our results demonstrate that there exists a critical field at which the current dependence of the oscillation frequency changes passing from a ¿red shift¿ (typical of in-plane magnetization) to a ¿blue shift¿ (typical of out-of-plane magnetization). Such results are in qualitative agreement either with recent experiments and with an analytical non-linear theory which identifies the transition field as the field at which the non-linear frequency shift vanishes. In such condition, the generation linewidth exhibits a minimum, the phase noise is independent of the power noise, and the oscillation frequency is independent of both current and power.
IEEE Transactions on Magnetics 07/2010; · 1.36 Impact Factor
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ABSTRACT: This paper introduces an oscillator scheme based on the oscillations of magnetic domain walls due to spin-polarized currents, where the current is injected perpendicular to the sample plane in a localized part of a nanowire. Depending on the geometrical and physical characteristic of the system, we identify two different dynamical regimes (auto-oscillations) when an out-of-plane external field is applied. The first regime is characterized by nucleation of domain walls (DWs) below the current injection site and the propagation of those up to the end of the nanowire, we also found an oscillation frequency larger than 5 GHz with a linear dependence on the applied current density. This simple system can be used as a tuneable steady-state domain wall oscillator. In the second dynamical regime, we observe the nucleation of two DWs which propagate back and forth in the nanowire with a sub-GHz oscillation frequency. The micromagnetic spectral mapping technique shows the spatial distribution of the output power is localized symmetrically in the nanowire. We suggest that this configuration can be used as micromagnetic transformer to decouple electrically two different circuits.
IEEE Transactions on Magnetics 07/2010; · 1.36 Impact Factor
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ABSTRACT: This paper describes a numerical experiment, based on full micromagnetic simulations of current-driven magnetization dynamics in nanoscale spin valves, to identify the origins of spectral linewidth broadening in spin torque oscillators. Our numerical results show two qualitatively different regimes of magnetization dynamics at zero temperature: regular (single-mode precessional dynamics) and chaotic. In the regular regime, the dependence of the oscillator integrated power on frequency is linear, and consequently the dynamics is well described by the analytical theory of current-driven magnetization dynamics for moderate amplitudes of oscillations. We observe that for higher oscillator amplitudes, the functional dependence of the oscillator integrated power as a function of frequency is not a single-valued function and can be described numerically via introduction of nonlinear oscillator power. For a range of currents in the regular regime, the oscillator spectral linewidth is a linear function of temperature. In the chaotic regime found at large current values, the linewidth is not described by the analytical theory. In this regime we observe the oscillator linewidth broadening, which originates from sudden jumps of frequency of the oscillator arising from random domain wall nucleation and propagation through the sample. This intermittent behavior is revealed through a wavelet analysis that gives superior description of the frequency jumps compared to several other techniques. Comment: 11 pages, 4 figures to appear in PRB
04/2010;
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ABSTRACT: We have employed complete micromagnetic simulations to analyze dc current driven self-oscillations of a vortex core in a spin-valve nanopillar in a perpendicular field by including the coupled effect of the spin-torque and the magnetostatic field computed self-consistently for the entire spin-valve. The vortex in the thicker nanomagnet moves along a quasi-elliptical trajectory that expands with applied current, resulting in blue-shifting of the frequency, while the magnetization of the thinner nanomagnet is non-uniform due to the bias current. The simulations explain the experimental magnetoresistance-field hysteresis loop and yield good agreement with the measured frequency vs. current behavior of this spin-torque vortex oscillator. Comment: 10 pages, 3 figures, to be appear on APL
02/2010;
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ABSTRACT: The dependence of the linewidth on the temperature in a spin valve driven by spin-polarized currents is analyzed by means of full micromagnetic simulations. The results are compared to the recent analytical predictions by Tiberkevic and confirmed by the experiments of Boone In agreement with the Tiberkevic theory and experiments of Sankey , our micromagnetic results point out two regimes. The linewidth increases linearly with the temperature until a threshold value, above which a square root dependence is observed.
IEEE Transactions on Magnetics 11/2009; · 1.36 Impact Factor
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ABSTRACT: Spin-transfer-driven ferromagnetic resonance is theoretically analyzed in a Py/Cu/Py spin valve with elliptical cross-sectional area (90 nm times 30 nm) by means of macrospin and micromagnetic simulations. An additional damping contribution to the Gilbert parameter is obtained when the spatial dependence of the magnetization is taken into account. These results are used to quantitatively explain the large value of the damping parameter found in the experiments by Sankey et al. [Phys. Rev. Lett. 96, 227601 (2006)]. The dependence of this additional damping on the dc bias is also studied, and the strength of the nonuniformities is quantified by means of a phenomenological expression.
IEEE Transactions on Nanotechnology 08/2009; · 2.29 Impact Factor
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ABSTRACT: We perform three-dimensional micromagnetic simulations of current-driven magnetization dynamics in nanoscale exchange biased spin valves that take account of (i) back action of spin-transfer torque on the pinned layer, (ii) nonlinear damping, and (iii) random thermal torques. Our simulations demonstrate that all these factors significantly impact the current-driven dynamics and lead to a better agreement between theoretical predictions and experimental results. In particular, we observe that at a nonzero temperature and a subcritical current, the magnetization dynamics exhibits nonstationary behavior in which two independent persistent oscillatory modes are excited which compete for the angular momentum supplied by spin-polarized current. Our results show that this multimode behavior can be induced by combined action of thermal and spin transfer torques.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: Micromagnetic computations are used to describe spin-transfer driven ferromagnetic resonance in nanopillar spin valves with elliptical cross section. Analytical uniform magnetization models reproduce the resonance phenomenon adequately and these can be used to compute interface conductance. In this work, using the magnetic parameters extracted by fitting static magnetoresistance measurements, mixing conductances are obtained; these values are 25% and 20% lower than the ones previously reported. Nonuniform magnetization resonance is found.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: This paper shows that the presence of two dynamical regimes, characterized by different precessional axes, is the origin of the nonmonotonic behavior of the output integrated power for large-amplitude magnetization precession driven by spin-polarized current in nanoscale exchange-biased spin valves. In particular, an abrupt loss in the integrated output power exists at the transition current between those two regimes. After the introduction of a time-frequency analysis of magnetization dynamics based on the wavelet transform, we performed a numerical experiment by means of micromagnetic simulations. Our results predicted that, together with a modulation of the frequency of the main excited mode of the magnetization precession, at high nonlinear dynamical regime the instantaneous output power of the spin-torque oscillator can disappear and then reappear at nanosecond scale.
Phys. Rev. B. 03/2009; 79(10).
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ABSTRACT: This paper shows that the presence of two dynamical regimes, characterized by different precessional-axis, is the origin of the non-monotonic behavior of the output integrated power for large-amplitude magnetization precession driven by spin-polarized current in nanoscale exchange biased spin-valves. In particular, at the transition current between those two regimes exists an abruptly loss in the integrated output power. After the introduction of a time-frequency analysis of magnetization dynamics based on the wavelet transform, we performed a numerical experiment by means of micromagnetic simulations. Our results predicted that, together with a modulation of the frequency of the main excited mode of the magnetization precession, at high non-linear dynamical regime the instantaneous output power of the spin-torque oscillator can disappear and then reappear at nanosecond scale. Comment: 10 pages 4 figures, to be appear in Physical Review B
03/2009;
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ABSTRACT: In this paper, we present a numerical study to determine the feasibility of exciting and sustaining stable magnetization oscillations in magnetic nanocontact devices subjected to the combined action of a spin-polarized current and a perpendicular anisotropy field when no external field is applied. A systematic numerical analysis of the properties exhibited by such spintronic oscillators is carried out by means of a micromagnetic framework. The study reveals a nonlinear behavior of the excited frequency as the anisotropy field strength is varied. More noteworthy, full-scale investigations result in a hysteretic dependence of the excited frequency on the applied current together with the existence of two kinds of precessional spin-wave modes: an anisotropic radially propagating mode and a gyrotropic motion of a magnetic vortex-core.
IEEE Transactions on Magnetics 12/2008; · 1.36 Impact Factor
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ABSTRACT: We study, both experimentally and by numerical modeling, the magnetic dynamics that can be excited in a magnetic thin-film nanopillar device using the spin torque from a spatially localized current injected via a 10s-of-nm-diameter aperture. The current-driven magnetic dynamics can produce large amplitude microwave emission at zero magnetic field, with a frequency well below that of the uniform ferromagnetic resonance mode. Micromagnetic simulations indicate that the physical origin of this efficient microwave nano-oscillator is the nucleation and subsequent steady-state rotational dynamics of a magnetic vortex dipole driven by the localized spin torque. These results show this novel implementation of a spintronic nano-oscillator is a promising candidate for microwave technology applications. Comment: 19 pages, 4 figures,
07/2008;
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ABSTRACT: A full micromagnetic study of the spin-transfer-driven self-oscillations of individual ellipsoidal PyCu nanomagnets as small as 30×90×5.5 nm3 is presented. The magnetic parameters have been computed by fitting static magnetoresistance measurements. The main mode found in the experiments by
Sankey et al. [Phys. Rev. Lett. 96, 227601 (2006)]
is analyzed. The full width at half maximum is calculated without taking into account the effect of thermal activation. The full width is found to decrease from 6.5 to 3.3 MHz when increasing the current in the self-oscillation region. These narrow widths are mainly produced by the nonuniformities of the magnetization and since they are computed at zero temperature mark a limit for the spectral purity of the self-oscillations in those nanomagnets.
Journal of Applied Physics 02/2008; 103(7):07B107-07B107-3. · 2.17 Impact Factor
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ABSTRACT: Micromagnetic simulations are used to predict the behavior exhibited by spin-transfer oscillators when materials with perpendicular anisotropy are introduced in the “free” layer of nanocontact devices. Under a perpendicular-to-plane bias field, the frequency exhibits nonlinear dependence on the anisotropy field, mostly originated by the exchange-dominated propagating nature of spin-wave modes. The increase of frequency without using large bias fields makes it suitable for potential technological applications. A study of the feasibility of bias-field-free devices has been also performed deriving multiharmonic signals at gigahertz frequencies. Here, the magnetization describes a gyrotropic motion where both vortex-core polarization and rotation sense switch periodically.
Applied Physics Letters 10/2007; 91(16):162506-162506-3. · 3.84 Impact Factor
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ABSTRACT: This paper interprets and reproduces, by means of full micromagnetic simulations, the pioneering experimental data on magnetization dynamics driven by spin polarized current of the experiment by Kiselev The effect of the spatial dependence of the polarization function together with either nonuniform magnetostatic coupling from the fixed layer and classical Ampere field are shown to play a fundamental role in the magnetization dynamics. A detailed study of the stable magnetization self-oscillations shows that for high field and high current regimes, the dynamics is localized in the sides of the structure, where the energy dissipated by damping and the energy provided by the spin flow compensate exactly.
IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
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ABSTRACT: This paper deals with the influence of nonuniform distributions of both current density and spin-torque efficiency on the magnetization dynamics of point-contact devices. Results of micromagnetic calculations show how the inclusion of different spatial profiles could substantially affect both the magnetization dynamics in both time and frequency domain
IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
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ABSTRACT: This paper deals with micromagnetic model of magnetization reversal in nano scale-point contact devices driven by nonuniform injection of a spin-polarized current. A computational study of the magnetization reversal in the nanosecond regime will be presented considering the influence of the current density distribution below the aperture region on the reversal time. For high current values, a strong dependence of the reversal time on the current distribution has been observed. Finally, results of micromagnetic simulations show that the reversal time versus current behaviour (at T=0K) is monotonic, very different from the switching processes observed in standard spin valves and magnetic tunnel junctions with uniform current injection.
IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
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ABSTRACT: Finite-difference time-domain techniques applied to nonconfined geometries impose the numerical implementation of computational areas smaller than the physical ones, so that it is necessary to develop a method for the waves absorption at the artificial boundaries. Due to the impossibility to implement some sort of analytical Higdon-type operators within a micromagnetic framework for point-contact geometries, two different absorbing boundary conditions are proposed. They are based on different site-dependent damping functions, whose value rises either abruptly or smoothly close to the computational boundaries. In this paper, it is first tested their robustness and then pointed out their effectiveness in reducing the rate of wave reflection of about three orders of magnitude in some cases
IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
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ABSTRACT: In a recent investigation Sankey etal [Phys. Rev. Lett. 96, 227601 (2006)] demonstrated a technique for measuring spin-transfer-driven ferromagnetic resonance in individual ellipsoidal PyCu nanomagnets as small as 30×90×5.5 nm <sup>3</sup> . In the present work, these experiments are analyzed by means of full micromagnetic modeling finding quantitative agreement and enlightening the spatial distribution of the normal modes found in the experiment. The magnetic parameter set used in the computations is obtained by fitting static magnetoresistance measurements. The temperature effect is also included together with all the nonuniform contributions to the effective field as the magnetostatic coupling and the Ampere field. The polarization function of Slonczewski [J. Magn. Magn. Mater. 159, L1 (1996)] is used including its spatial and angular dependences. Experimental spin-transfer-driven ferromagnetic resonance spectra are reproduced using the same currents as in the experiment. The use of full micromagnetic modeling allows us to further investigate the spatial dependence of the modes. The dependence of the normal mode frequency on the dc and the external field together with a comparison to the normal modes induced by a microwave current is also addressed.
Journal of Applied Physics 06/2007; · 2.17 Impact Factor
