Dhritiman Bhattacharya

We report experimental manipulation of the magnetic states of elliptical cobalt magnetostrictive nanomagnets (with nominal dimensions of ∼340 nm × 270 nm × 12 nm) delineated on bulk 128° Y-cut lithium niobate with acoustic waves (AWs) launched from interdigitated electrodes. Isolated nanomagnets (no dipole interaction with any other nanomagnet) that are initially magnetized with a magnetic field to a single-domain state with the magnetization aligned along the major axis of the ellipse are driven into a vortex state by acoustic waves that modulate the stress anisotropy of these nanomagnets. The nanomagnets remain in the vortex state until they are reset by a strong magnetic field to the initial single-domain state, making the vortex state nonvolatile. This phenomenon is modeled and explained using a micromagnetic framework and could lead to the development of extremely energy efficient magnetization switching methodologies for low-power computing applications.

In the brain, the membrane potential of many neurons oscillates in a subthreshold damped fashion and fire when excited by an input frequency that nearly equals their eigen frequency. In this work, we investigate theoretically the artificial implementation of such “resonate-and-fire” neurons by utilizing the magnetization dynamics of a fixed magnetic skyrmion in the free layer of a magnetic tunnel junction (MTJ). To realize firing of this nanomagnetic implementation of an artificial neuron, we propose to employ voltage control of magnetic anisotropy or voltage generated strain as an input (spike or sinusoidal) signal, which modulates the perpendicular magnetic anisotropy. This results in continual expansion and shrinking (i.e., breathing) of a skyrmion core that mimics the subthreshold oscillation. Any subsequent input pulse having an interval close to the breathing period or a sinusoidal input close to the eigen frequency drives the magnetization dynamics of the fixed skyrmion in a resonant manner. The time varying electrical resistance of the MTJ layer due to this resonant oscillation of the skyrmion core is used to drive a Complementary Metal Oxide Semiconductor buffer circuit, which produces spike outputs. By rigorous micromagnetic simulation, we investigate the interspike timing dependence and response to different excitatory and inhibitory incoming input pulses. Finally, we show that such resonate and fire neurons have potential application in coupled nanomagnetic oscillator based associative memory arrays.

OBJECTIVE: Chronic alcoholism is associated with impaired cognitive abilities, with a more severe deficit in visual than in verbal functions. The visual processing deficits have classically been associated with impaired function of the visual cortex, located in the occipital lobe. The present study sought to increase current understanding of the impaired visual processing abilities in alcohol-dependent subjects and their correlation with metabolic aberrations in the occipital lobe using in vivo proton magnetic resonance spectroscopy (¹H MRS). METHOD: To that end, ¹H MRS was carried out in the primary visual cortex on 35 alcohol-dependent subjects and 35 healthy controls. Neuropsychological tests for visual processing skills were performed on all the subjects, and the deficits were reported as raw dysfunction rating scores. RESULTS: The alcohol-dependent subjects showed a significant increase in choline/creatine (Cho/Cr) and myo-Inositol/creatine (mI/Cr) ratios, whereas N-acetyl-aspartate/creatine (NAA/Cr) and glutamate-glutamine/creatine (Glu-Gln)/Cr ratios were significantly decreased. Reductions in NAA levels might be attributed to neuronal loss, whereas reductions in Glu-Gln levels might reflect perturbation of the Glu-Gln system in alcohol-dependent individuals, which could represent a neuroprotective adaptation. Elevations in mI levels may reflect astrocyte proliferation as well as an osmotic response to cell shrinkage, whereas a significant increase in Cho levels indicates altered cell membrane metabolism. Also, a significant inverse correlation between NAA/Cr and raw dysfunction scores (rDyS) on the Nahor-Benson (NB) test and Glu-Gln/Cr with rDyS of the NB and the Bender-Gestalt (BG) test was observed, whereas a positive correlation between rDyS of the BG and the NB test and Cho/Cr was observed. CONCLUSIONS: The results suggest that metabolic alterations in the primary visual cortex may contribute to the neuropsychological impairment in visual information processing observed in alcohol-dependent subjects.

We propose a two-terminal nanomagnetic memory element based on magnetization reversal of a perpendicularly magnetized nanomagnet employing a unipolar voltage pulse that modifies the perpendicular anisotropy of the system. Our work demonstrates that the presence of Dzyaloshinskii-Moriya interaction can create an alternative route for magnetization reversal that obviates the need for utilizing precessional magnetization dynamics as well as a bias magnetic field that are employed in traditional voltage control of magnetic anisotropy (VCMA)-based switching of perpendicular magnetization. We show with extensive micromagnetic simulation, in the presence of thermal noise, that the proposed skyrmion-mediated VCMA switching mechanism is robust at room temperature leading to extremely low error switching while also being potentially 1-2 orders of magnitude more energy efficient than state-of-the-art spin transfer torque-based switching.

We investigate surface-acoustic-wave- (SAW) induced ferromagnetic-resonance- (FMR) assisted spin-transfer-torque (STT) switching of perpendicular magnetic tunnel junctions (PMTJs) with inhomogeneities using micromagnetic simulations that include the effect of thermal noise. With suitable frequency excitation, the SAW can induce ferromagnetic resonance in magnetostrictive materials, and the magnetization can precess in a cone with high deflection from the perpendicular direction. With incorporation of inhomogeneity via lateral anisotropy variation as well as room-temperature thermal noise, the magnetization precession in different gains can be significantly incoherent. Interestingly, the precession in different grains are found to be in phase, even though the precession amplitude (angle of deflection from the perpendicular direction) varies across grains of different anisotropy. Nevertheless, the high mean deflection angle due to acoustically induced FMR can complement the STT switching by reducing the STT current significantly; even though the applied stress induced change in anisotropy is much lower than the total anisotropy barrier. This work indicates that SAW-induced FMR-assisted switching can improve energy efficiency while being scalable to very small dimensions, which is technologically important for STT random-access memory and elucidates the physical mechanism for the potential robustness of this paradigm in realistic scenarios with thermal noise and material inhomogeneity.