Micromagnetic understanding of stochastic resonance driven by spin-transfer-torque (original) (raw)
Related papers
Time-domain study of frequency-power correlation in spin-torque oscillators
Physical Review B, 2010
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.
Fluctuating magnetic field induced resonant activation
The Journal of Chemical Physics, 2014
In this paper, we have studied the properties of a Brownian particle at stationary state in the presence of a fluctuating magnetic field. Time dependence of the field makes the system thermodynamically open. As a signature of that the steady state distribution function becomes function of damping strength, intensity of fluctuations and constant parts of the applied magnetic field. It also depends on the correlation time of the fluctuating magnetic field. Our another observation is that the random magnetic field can induce the resonant activation phenomenon. Here correlation time is increased under the fixed variance of the fluctuating field. But if the correlation time (τ) increases under the fixed field strength then the mean first passage time rapidly grows at low τ and it almost converges at other limit. This is sharp contrast to the usual colored noise driven open system case where the mean first passage time diverges exponentially. We have also observed that a giant enhancement of barrier crossing rate occurs particularly at large strength of constant parts of the applied magnetic field even for very weak fluctuating magnetic field. Finally, break down of the Arrhenius result and disappearance of the Kramers' turn over phenomenon may occur in the presence of a fluctuating magnetic field.
Finite-temperature modeling of nanoscale spin-transfer oscillators
Physical Review B, 2005
Magnetization dynamics induced by spin-polarized currents in magnetic nanodevices have been numerically simulated using a single-domain model proposed by Slonczewski extended to include temperature effects. For currents with a spin polarization antiparallel to the device easy axis and for fields above the magnetostatic anisotropy field, transfer of spin momentum from one layer to an adjacent layer can cause the layers to undergo sustained oscillations. Here we numerically calculate the expected excitation spectra and linewidths of spintransfer oscillators and explain observed variations in excitation linewidths. The linewidth arises from thermal excitations that give rise to disorder in the orbits and, in certain regimes, hopping between nearly degenerate orbits. The excitation spectra for a 2.5 nmϫ 50 nmϫ 100 nm device show transitions from thermally activated elliptical motion, at zero and low currents, to a bent elliptical motion at intermediate currents, and finally to tilted out-of-plane orbits. At the transition between in-plane and out-of-plane orbits, there is a region of low-frequency noise due to thermal hopping between degenerate orbits and a shift in the spectral behavior. The linewidth arising from thermal interactions is a sensitive function of the device volume and varies from 1 to 2 GHz for 2.5 nmϫ 50 nmϫ 100 nm devices to 20 to 40 MHz for 10 nmϫ 200 nmϫ 400 nm devices. The modeling explains the difference in linewidths observed for nanopillar and point-contact geometries as a natural consequence of inherent thermal fluctuations and the difference in excitation volumes.
Nonlinear magnetic stochastic resonance: Noise-strength-constant-force diagrams
1997
Signal-to-noise properties of a periodically driven noisy magnetic bistable system-superparamagnetic particles with uniaxial anisotropy-are investigated in the framework of the Fokker-Planck equation solved both analytically and numerically. The system is subject to a constant ͑bias͒ field that imparts even harmonics into its low-frequency magnetic spectrum. A comparative study of the linear and quadratic susceptibilities and stochastic resonances ͑SR͒ is carried out. We show that the quadratic SR is much sharper than the linear one and unlike the latter is essentially frequency dependent.
Journal of Applied Physics, 2011
Electrical detection of nonlinear ferromagnetic resonance in single elliptical permalloy thin film using a magnetic tunnel junction Appl. Phys. Lett. 99, 232506 (2011) Wide frequency range magnetoimpedance in tri-layered thin NiFe/Ag/NiFe films: Experiment and numerical calculation J. Appl. Phys. 110, 093914 (2011) Spin wave resonance detection using magnetic tunnel junction structure Appl. Phys. Lett. 99, 192501 (2011) Wide-range control of ferromagnetic resonance by spin Hall effect Appl. Phys. Lett. 99, 172501 (2011) Additional information on J. Appl. Phys. This paper describes a full micromagnetic characterization of the magnetization dynamics driven by spin-polarized current in anisotropic spin-torque oscillators (STOs). For field angles approaching the hard in-plane axis, the excited mode is uniform and a super-critical Hopf-bifurcation takes place at the critical current density J C . For field angles close to the easy axis of the free layer, the excited mode is localized (non-uniform) and a sub-critical Hopf-bifurcation occurs at J C . In this latter region, a hysteretic behaviour is, therefore, found. We demonstrate numerically that the non-linearities of the STO are strongly reduced when the oscillation frequency at the critical current is near the ferromagnetic resonance (FMR) frequency computed at zero bias current, and in particular, this condition corresponds to the field orientation at which a minimum in the FMR-frequency is achieved.
Magnetization self-oscillations induced by spin-polarized currents
Magnetics International Conference, 2005
Spin-polarized currents may exert significant torques on nanoscale ferromagnets. Experiments show that both switching and microwave oscillations of the magnetization are induced when the system is subject to the spin-polarized current and an external magnetic field. Under these conditions the nanomagnet behaves like a nonlinear system forced far from equilibrium by the current. For this reason, the correct framework for
Non-Markovian magnetization dynamics for uniaxial nanomagnets
A stochastic approach for the description of the time evolution of the magnetization of nanomagnets is proposed , that interpolates between the Landau-Lifshitz-Gilbert and the Landau-Lifshitz-Bloch approximations, by varying the strength of the noise. Its finite autocorrelation time, i.e. when it may be described as colored, rather than white, is, also, taken into account and the consequences, on the scale of the response of the magnetization are investigated. It is shown that the hierarchy for the moments of the magnetization can be closed, by introducing a suitable truncation scheme, whose validity is tested by direct numerical solution of the moment equations and compared to the averages obtained from a numerical solution of the corresponding colored stochastic Langevin equation. This comparison is performed on magnetic systems subject to both an external uniform magnetic field and an internal one-site uniaxial anisotropy.
Low-field current-hysteretic oscillations in spin-transfer nanocontacts
Physical Review B, 2007
We have measured spin-transfer-driven, large amplitude, current hysteretic, low frequency (< 500 MHz), narrowband oscillations in nanocontacts made to spin valve structures. The oscillations occur in zero field, persist up to 5 mT for in plane applied fields, and to beyond 400 mT for out of plane fields. Unlike previous measurements, the oscillation frequency is well below that for uniform-mode ferromagnetic resonance, is only a weak function of applied field, and is highly anharmonic. The oscillations are hysteretic with applied dc current, appearing at high currents but persisting to lower currents upon decrease of the current. We suggest that these observations are consistent with the dynamics of a nonuniform magnetic state in the vicinity of the contact nucleated by both the spin transfer torque and dc current-generated Oersted fields, with the dynamics driven by spin transfer. The electrical oscillation amplitudes are large and narrowband, with the largest amplitudes on the order of 1 mV and the narrowest linewidths below 1 MHz. Contribution of NIST, an agency of the U.S. government, not subject to copyright. 2-Pufall et al., Current-Hysteretic Low Frequency… Since the prediction that spin-polarized currents can exert significant torques in magnetic nanostructures, a wide variety of magnetization dynamics driven by spin transfer torques have been observed in a wide range of device geometries and experimental conditions.[1,2] The general characteristics of these observed dynamicsthe amplitude, the fundamental excitation frequency f 0 , the change of f 0 with current I and applied field µ 0 H app-are roughly understandable using theories that approximate the free layer dynamics as quasi-uniform large angle magnetization motion in the region of the device where current flows.[3,4] In the case of nanopillars, this region is the entirety of the free layer (possibly ignoring some region at the edge), and in nanocontacts consists of the region directly under the contact; in the latter case the mode remains centered (i.e., stationary) on the symmetry axis. Even this rough correspondence between theory and experiment is somewhat surprising, since one might expect excitations with nonuniform magnetization (on the scale of the contact) due to the large spatially-varying Oersted fields generated by the dc current itself. The effect of these fields, which approach 6.5 mT/mA (65 Oe/mA) at the edge of a 60 nm diameter contact, is an active area of computational magnetic research.[5] In this Communication we present measurements of large amplitude, narrowband signals from nanocontacts that are not easily explainable using such radially symmetric quasi-uniform mode approximations. The measurements were performed on nanocontacts nominally identical to those measured previously. The principal difference in the results reported here is that the in plane field magnitude is always less than 5 mT, whereas previously this magnitude was greater than 60 mT. We suggest that the observed dynamics may result from the generation and perturbation of a nonuniform magnetic state, such as a magnetic vortex, in the vicinity of the contact.
Sustained RF oscillations from thermally induced spin-transfer torque
We investigate the angular dependence of the spin torque generated when applying a temperature difference across a spin-valve. Our study shows the presence of a non-trivial fixed point in this angular dependence, i.e. the possibility for a temperature gradient to stabilize radio frequency oscillations without the need for an external magnetic field. This so called "wavy" behavior can already be found upon applying a voltage difference across a spin-valve but we find that this effect is much more pronounced with a temperature difference. Our semi-classical theory is parametrized with experimentally measured parameters and allows one to predict the amplitude of the torque with good precision. Although thermal spin torque is by nature less effective than its voltage counterpart, we find that in certain geometries, temperature differences as low as a few degrees should be sufficient to trigger the switching of the magnetization.
Stochastic resonance in nanoscale systems
2011
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