The recent discovery that a spin-polarized electrical current can apply a large torque to a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. However, a central question remains unresolved: what type of magnetic motions can be generated by this torque? Theory predicts that spin transfer may be able to drive a nanomagnet into types of oscillatory magnetic modes not attainable with magnetic fields alone, but existing measurement techniques have provided only indirect evidence for dynamical states. The nature of the possible motions has not been determined. Here we demonstrate a technique that allows direct electrical measurements of microwave-frequency dynamics in individual nanomagnets, propelled by a d.c. spin-polarized current. We show that spin transfer can produce several different types of magnetic excitation. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a d.c. electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators.

Microwave Oscillations of a Nanomagnet Driven by a DC Spin-Polarized Current

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Non-collinear spin transfer in Co/Cu/Co multilayers | NIST

In the treatment of neurodegenerative, sensory and cardiovascular diseases, electrical probes and arrays have shown quite a promising success rate. However, despite the outstanding clinical outcomes, their operation is significantly hindered by non-selective control of electric fields. A promising alternative is micromagnetic stimulation (μMS) due to the high permeability of magnetic field through biological tissues. The induced electric field from the time-varying magnetic field generated by magnetic neurostimulators is used to remotely stimulate neighboring neurons. Due to the spatial asymmetry of the induced electric field, high spatial selectivity of neurostimulation has been realized. Herein, some popular choices of magnetic neurostimulators such as microcoils (μcoils) and spintronic nanodevices are reviewed. The neurostimulator features such as power consumption and resolution (aiming at cellular level) are discussed. In addition, the chronic stability and biocompatibility of these implantable neurostimulator are commented in favor of further translation to clinical settings. Furthermore, magnetic nanoparticles (MNPs), as another invaluable neurostimulation material, has emerged in recent years. Thus, in this review we have also included MNPs as a remote neurostimulation solution that overcomes physical limitations of invasive implants. Overall, this review provides peers with the recent development of ultra-low power, cellular-level, spatially selective magnetic neurostimulators of dimensions within micro- to nano-range for treating chronic neurological disorders. At the end of this review, some potential applications of next generation neuro-devices have also been discussed.

A review on magnetic and spintronic neurostimulation: challenges and prospects

In this paper, we investigated the dipolar magnetic coupling in ferromagnetic multi-layered structures. However, this kind of coupling has been extensively studied since the last few decades through the Neel model (orange peel coupling), but most of the analyses were based on mathematically modeling a simple sinusoidal rough interface that hides the details of such a coupling. Therefore, we add a generality to the Neel model via adapting the anisotropic morphological self-affine interfaces that can unravel the details of interesting effects that are technologically important to consider for future magnonic and spintronic devices. The tensorial coupling between the ferromagnetic (FM) layers has been obtained from the magnetostatic energy of a pseudo-spin valve structure (FM/NM/FM). Our findings show that the coupling strength is dependent not only on the roughness properties of the self-affine interfaces but also on the rotational angle between the patterned interfaces. The variation of this orientation angle along with the change of the interface correlation lengths can switch FM coupling to antiferromagnetic coupling and vice versa. These results are advantageous for the engineering and fabrication of magnonic waveguide circuits and spintronic devices specifically in spin valves, magnetoresistive elements, and magnetic tunneling junctions.

Morphological magnetostatic coupling in spin valves due to anisotropic self-affine interface roughness

Optical solitons in the generalized space–time fractional cubic-quintic nonlinear Schrödinger equation with a PT-symmetric potential

The impact of orange peel coupling on spin current induced magnetization switching in a Co/Cu/Ni-Fe nanopillar device is investigated by solving the switching dynamics of magnetization of the free layer governed by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. The value of the critical current required to initiate the magnetization switching is calculated analytically by solving the LLGS equation and verified the same through numerical analysis. Results of numerical simulation of the LLGS equation using Runge-Kutta fourth order procedure shows that the presence of orange peel coupling between the spacer and the ferromagnetic layers reduces the switching time of the nanopillar device from 67 ps to 48 ps for an applied current density of 4 × 1012Am−2. Also, the presence of orange peel coupling reduces the critical current required to initiate switching, and in this case, from 1.65 × 1012Am−2 to 1.39 × 1012Am−2.

Current induced magnetization switching in Co/Cu/Ni-Fe nanopillar with orange peel coupling

Reduction of switching time in pentalayer nanopillar device with different biasing configurations

Soliton and rogue wave solutions of the space–time fractional nonlinear Schrödinger equation with PT-symmetric and time-dependent potentials

The effect of biquadratic coupling on spin current induced magnetization switching in a Co/Cu/Ni-Fe nanopillar device is investigated by solving the free layer magnetization switching dynamics governed by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. The LLGS equation is numerically solved by using Runge-Kutta fourth order procedure for an applied current density of 5 × 1012 Am-2. Presence of biquadratic coupling in the ferromagnetic layers reduces the magnetization switching time of the nanopillar device from 61 ps to 49 ps.

D. Aravinthan

Papers

Current induced magnetization switching in Co/Cu/Ni-Fe nanopillar with orange peel coupling

Optical solitons in the generalized space–time fractional cubic-quintic nonlinear Schrödinger equation with a PT-symmetric potential

Reduction of switching time in pentalayer nanopillar device with different biasing configurations

Soliton and rogue wave solutions of the space–time fractional nonlinear Schrödinger equation with PT-symmetric and time-dependent potentials

Effect of biquadratic coupling on current induced magnetization switching in Co/Cu/Ni-Fe nanopillar