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Synchronization of spin-torque driven nanooscillators for point contacts on a quasi-1D nanowire: Micromagnetic simulations

In this paper we present detailed numerical simulation studies on the synchronization of two spin-torque nanooscillators (STNO) in the quasi-1D geometry: magnetization oscillations are induced in a thin NiFe nanostripe by a spin polarized current injected via square-shaped CoFe nanomagnets on the top of this stripe. In a sufficiently large out-of-plane field, a propagating oscillation mode appears in such a system. Due to the absence of the geometrically caused wave decay in 1D systems, this mode is expected to enable a long-distance synchronization between STNOs. Indeed, our simulations predict that synchronization of two STNOs on a nanowire is possible up to the intercontact distance 3 mkm (for the nanowire width 50 nm). However, we have also found several qualitatively new features of the synchronization behaviour for this system, which make the achievement of a stable synchronization in this geometry to a highly non-trivial task. In particular, there exist a minimal distance between the nanocontacts, below which a synchronization of STNOs can not be achieved. Further, when the current value in the first contact is kept constant, the amplitude of synchronized oscillations depends non-monotonously on the current value in the second contact. Finally, for one and the same currents values through the contacts there might exist several synchronized states (with different frequencies), depending on the initial conditions.Comment: 13 pages with 4 figurews, recently submitted to PR

Effects of rf Current on Spin Transfer Torque Induced Dynamics

The impact of radiofrequency (rf) currents on the direct current (dc) driven switching dynamics in current-perpendicular-to-plane nanoscale spin valves is demonstrated. The rf currents dramatically alter the dc driven free layer magnetization reversal dynamics as well as the dc switching level. This occurs when the frequency of the rf current is tuned to a frequency range around the dc driven magnetization precession frequencies. For these frequencies, interactions between the dc driven precession and the injected rf induce frequency locking and frequency pulling effects that lead to a measurable dependence of the critical switching current on the frequency of the injected rf. Based on macrospin simulations, including dc as well as rf spin torque currents, we explain the origin of the observed effects.Comment: 5 pages, 4 figure

Origin of the spectral linewidth in non linear oscillators based on MgO tunnel junctions

We demonstrate the strong impact of the oscillator agility on the line broadening by studying spin transfer induced microwave emission in MgO-based tunnel junctions with current. The linewidth is almost not affected by decreasing the temperature. At very low currents, a strong enhancement of the linewidth at low temperature is attributed to an increase of the non linearity, probably due to the field-like torque. Finally we evidence that the noise is not dominated by thermal fluctuations but rather by the chaotization of the magnetization system induced by the spin transfer torque.Comment: 12 pages, 3 figures, published in Phys. Rev. B 80, 060404 (2009

Adiabatic Domain Wall Motion and Landau-Lifshitz Damping

Recent theory and measurements of the velocity of current-driven domain walls in magnetic nanowires have re-opened the unresolved question of whether Landau-Lifshitz damping or Gilbert damping provides the more natural description of dissipative magnetization dynamics. In this paper, we argue that (as in the past) experiment cannot distinguish the two, but that Landau-Lifshitz damping nevertheless provides the most physically sensible interpretation of the equation of motion. From this perspective, (i) adiabatic spin-transfer torque dominates the dynamics with small corrections from non-adiabatic effects; (ii) the damping always decreases the magnetic free energy, and (iii) microscopic calculations of damping become consistent with general statistical and thermodynamic considerations

Rate perception adapts across the senses: evidence for a unified timing mechanism

The brain constructs a representation of temporal properties of events, such as duration and frequency, but the underlying neural mechanisms are under debate. One open question is whether these mechanisms are unisensory or multisensory. Duration perception studies provide some evidence for a dissociation between auditory and visual timing mechanisms; however, we found active crossmodal interaction between audition and vision for rate perception, even when vision and audition were never stimulated together. After exposure to 5 Hz adaptors, people perceived subsequent test stimuli centered around 4 Hz to be slower, and the reverse after exposure to 3 Hz adaptors. This aftereffect occurred even when the adaptor and test were different modalities that were never presented together. When the discrepancy in rate between adaptor and test increased, the aftereffect was attenuated, indicating that the brain uses narrowly-tuned channels to process rate information. Our results indicate that human timing mechanisms for rate perception are not entirely segregated between modalities and have substantial implications for models of how the brain encodes temporal features. We propose a model of multisensory channels for rate perception, and consider the broader implications of such a model for how the brain encodes timing

Impact of COVID-19 on Arkansas Field Crop Farms

The coronavirus (COVID-19) pandemic is disrupting the global economy in unprecedented ways, and the agricultural sector is no exception. Prices of most commodities decreased since the pandemic began, which are affecting farm revenues and production costs. We assess the impact of COVID-19 on Arkansas field crop farms using the Representative Farm models, calibrated to the latest baseline published by the Food and Agricultural Policy Research Institute. The results show that COVID-19 decreases the net cash farm incomes of all five farms, increases the probability of having a negative net cash farm income, and increases the cost of commodity programs. Payments under the Coronavirus Food Assistance Program (CFAP) effectively compensate for the revenue losses due to COVID-19 projected for the 2019 crop for the five farms

Gilbert Damping in Magnetic Multilayers

We study the enhancement of the ferromagnetic relaxation rate in thin films due to the adjacent normal metal layers. Using linear response theory, we derive the dissipative torque produced by the s-d exchange interaction at the ferromagnet-normal metal interface. For a slow precession, the enhancement of Gilbert damping constant is proportional to the square of the s-d exchange constant times the zero-frequency limit of the frequency derivative of the local dynamic spin susceptibility of the normal metal at the interface. Electron-electron interactions increase the relaxation rate by the Stoner factor squared. We attribute the large anisotropic enhancements of the relaxation rate observed recently in multilayers containing palladium to this mechanism. For free electrons, the present theory compares favorably with recent spin-pumping result of Tserkovnyak et al. [Phys. Rev. Lett. \textbf{88},117601 (2002)].Comment: 1 figure, 5page

Current-induced spin-wave excitations in a single ferromagnetic layer

A new current induced spin-torque transfer effect has been observed in a single ferromagnetic layer without resorting to multilayers. At a specific current density of one polarity injected from a point contact, abrupt resistance changes due to current-induced spin wave excitations have been observed. The critical current at the onset of spin-wave excitations depends linearly on the external field applied perpendicular to the layer. The observed effect is due to current-driven heterogeneity in an otherwise uniform ferromagnetic layer.Comment: 12 pages, 4 figure