913 research outputs found
Frequency Modulation of Spin-Transfer Oscillators
Spin-polarized dc electric current flowing into a magnetic layer can induce
precession of the magnetization at a frequency that depends on current. We show
that addition of an ac current to this dc bias current results in a frequency
modulated (FM) spectral output, generating sidebands spaced at the modulation
frequency. The sideband amplitudes and shift of the center frequency with drive
amplitude are in good agreement with a nonlinear FM model that takes into
account the nonlinear frequency-current relation generally induced by spin
transfer. Single-domain simulations show that ac current modulates the cone
angle of the magnetization precession, in turn modulating the frequency via the
demagnetizing field. These results are promising for communications and signal
processing applications of spin-transfer oscillators.Comment: 13 pages, 3 Figure
Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers
Ferromagnetic resonance and static magnetic properties of CoFe/Al2O3/CoFe/Py
and CoFe/Al2O3/CoFeB/Py magnetic tunnel junctions and of 25nm thick
single-layer Permalloy(Py) films have been studied as a function of temperature
down to 2K. The temperature dependence of the ferromagnetic resonance excited
in the Py layers in magnetic tunnel junctions shows knee-like enhancement of
the resonance frequency accompanied by an anomaly in the magnetization near
60K. We attribute the anomalous static and dynamic magnetic response at low
temperatures to interface stress induced magnetic reorientation transition at
the Py interface which could be influenced by dipolar soft-hard layer coupling
through the Al2O3 barrier
The disappearing hand: vestibular stimulation does not improve hand localisation.
Bodily self-consciousness depends on the coherent integration of sensory information. In addition to visual and somatosensory information processing, vestibular contributions have been proposed and investigated. Vestibular information seems especially important for self-location, but remains difficult to study.
This randomised controlled experiment used the MIRAGE multisensory illusion box to induce a conflict between the visually- and proprioceptively-encoded position of one hand. Over time, the perceived location of the hand slowly shifts, due to the fact that proprioceptive input is progressively weighted more heavily than the visual input. We hypothesised that left cold caloric vestibular stimulation (CVS) augments this shift in hand localisation.
The results from 24 healthy participants do not support our hypothesis: CVS had no effect on the estimations with which the perceived position of the hand shifted from the visually- to the proprioceptively-encoded position. Participants were more likely to report that their hand was 'no longer there' after CVS. Taken together, neither the physical nor the subjective data provide evidence for vestibular enhanced self-location
Laparoscopic Loop Ileostomy Reversal: Reducing Morbidity While Improving Functional Outcomes
The main advantage of a laparoscopic loop ileostomy reversal was found to be visualization of adhesions for lysis and mobilization of the stoma and small bowel
Current-Driven Microwave Dynamics in Magnetic Point Contacts as a Function of Applied Field Angle
We have measured microwave frequency, current-driven magnetization dynamics
in point contacts made to Co90Fe10/Cu/ Ni80Fe20 spin valves as a function of
applied field strength and angle relative to the film plane. As the field
direction is varied from parallel to nearly perpendicular, the device power
output increases by roughly two orders of magnitude while the frequencies of
the excitations decrease. For intermediate angles the excited frequency does
not monotonically vary with applied current and also exhibits abrupt,
current-dependent jumps. For certain ranges of current, and applied field
strength and direction, the excitation linewidths decrease to a few megahertz,
leading to quality factors over 18,000
Flux flow of Abrikosov-Josephson vortices along grain boundaries in high-temperature superconductors
We show that low-angle grain boundaries (GB) in high-temperature
superconductors exhibit intermediate Abrikosov vortices with Josephson cores,
whose length along GB is smaller that the London penetration depth, but
larger than the coherence length. We found an exact solution for a periodic
vortex structure moving along GB in a magnetic field and calculated the
flux flow resistivity , and the nonlinear voltage-current
characteristics. The predicted dependence describes well our
experimental data on unirradiated and irradiated
bicrystals, from which the core size , and the intrinsic depairing
density on nanoscales of few GB dislocations were measured for the
first time. The observed temperature dependence of
indicates a significant order parameter suppression in current channels between
GB dislocation cores.Comment: 5 pages 5 figures. Phys. Rev. Lett. (accepted
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Predictive representations can link model-based reinforcement learning to model-free mechanisms
Humans and animals are capable of evaluating actions by considering their long-run future rewards through a process described using model-based reinforcement learning (RL) algorithms. The mechanisms by which neural circuits perform the computations prescribed by model-based RL remain largely unknown; however, multiple lines of evidence suggest that neural circuits supporting model-based behavior are structurally homologous to and overlapping with those thought to carry out model-free temporal difference (TD) learning. Here, we lay out a family of approaches by which model-based computation may be built upon a core of TD learning. The foundation of this framework is the successor representation, a predictive state representation that, when combined with TD learning of value predictions, can produce a subset of the behaviors associated with model-based learning, while requiring less decision-time computation than dynamic programming. Using simulations, we delineate the precise behavioral capabilities enabled by evaluating actions using this approach, and compare them to those demonstrated by biological organisms. We then introduce two new algorithms that build upon the successor representation while progressively mitigating its limitations. Because this framework can account for the full range of observed putatively model-based behaviors while still utilizing a core TD framework, we suggest that it represents a neurally plausible family of mechanisms for model-based evaluation
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