Spin-transfer torque in nanoscale magnetic devices

We discuss recent highlights from research at Cornell University, Ithaca, New York, regarding the use of spin-transfer torques to control magnetic moments in nanoscale ferromagnetic devices. We highlight progress on reducing the critical currents necessary to produce spin-torque-driven magnetic switching, quantitative measurements of the magnitude and direction of the spin torque in magnetic tunnel junctions, and single-shot measurements of the magnetic dynamics generated during thermally assisted spin-torque switching.     

New magnetic structures for switching converters

In the past the majority of power processing applications have been centered around a very few standard switching converter topologies. Recently, a number of new converter topologies have been proposed in order to find the best possible electrical inter connection of power processing elements: switches, storage components, and transformers, that would yield the highest efficiency and best performance. However the equally important and complementary problem of their best magnetic interconnection has been completely overlooked. In some new converter structures, the nature of the switching process and existing waveforms allows integration of previously separate inductors and transformers into a single magnetic structure. Several such magnetic core structures are proposed and analyzed, which lead to further converter simplifications and performance improvements.     

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.     

Two-particle tunneling current in Josephson junctions

The contribution to the tunneling current of a Josephson junction from the Two-particle tunneling, to the 2nd order approximation in the barrier transparency, is investigated. Expressions for the current onset amplitudes corresponding to eV = _1,2 are given together with the full expressions for the voltage and the temperature dependencies of the two-particles current. The theory has ben developed to take also into account corrections due to depairing mechanisms, which lead to the smearing of the current singularities. Introducing a depairing parameter , which accounts for the probability of these processes, I–V curves in the voltage region of the onset of single and two particle current are obtained. It is shown that, though having the same functional dependence, spreading occurs over a voltage range of different widths. In particular, it is shown that the width of the single-particle structure is twice larger the one for the two-particle. A careful investigation of the I–V curves in the region 2 -eV is presented and some aspects of the interesting voltage region near ¦₁-₂¦ is discussed.     

Magnetic junction switching by joint action of current pulse and magnetic field: numerical simulation

The process of magnetic junction switching by a spin-polarized current pulse in the presence of an external magnetic field has been numerically simulated at the current densities and magnetic fields below the corresponding threshold values for separate effects. It is established that the switching can be performed with controlled delay relative to the current pulse.     

Grid-controlled current switching in cathode-spot discharge

The first results of an investigation of the complete current switching (initiation and quenching) using a fine-mesh grid in discharge with a cathode spot on liquid cesium are reported. Experimental data show the possibility of using this method of control at current densities within 5–25 A/cm² in the grid plane.     

Magnetization switching by spin-polarized current in low-resistance magnetic tunnel junction with MgO [001] barrier

Current-induced magnetization switching (CIMS) was demonstrated in low resistance magnetic tunnel junctions (MTJs) with thin MgO [001] barrier. The resistance change by CIMS was more than 100%, which is much larger than the previous report in Al-O based MTJs. The switching current density was about 2/spl times/10/sup 7/ A/cm/sup 2/, which was comparable with that reported values in metallic multilayers.     

Electric-field-assisted switching in magnetic tunnel junctions

The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.     

Pulse switching in thin magnetic films

The main results of the investigations of pulse switching in thin magnetic films with uniaxial anisotropy which have been obtained in Moscow State University are reviewed. Simultaneous investigation of integral switching properties, inner effective field, and dynamic domains produced during pulse switching has increased our understanding of bi-directional incoherent rotation mechanism and found new peculiarities of the pulse switching by domain boundary propagation. A new variety of incoherent rotation which manifests itself at strong fields has been found. It has been also found that the curve representing the pulse field dependence of an inverse switching time in the general case consists of five distinct regions. The relation between these regions and the switching mechanisms are discussed.     

Complexity reduction in a telephone switching system

This paper reviews some of the recent developments in complexity theory as applied to telephone-switching. Some of these techniques are suitable for practical implementation in India.     

Trap-assisted tunneling in p-Si/SiO2 structures

Conduction in p-Si/SiO₂ structures in which a 65.3-nm SiO₂ layer has been subjected to hydrogen-plasma treatment at 20 °C does not depend on temperature in the range 77–300 K, in the accumulation regime under an electric field of 0.7–4.5×10⁶ V cm^?1 in the SiO₂ layer. A trap-assisted tunneling mechanism in the SiO₂ layer has been proposed as an explanation for this tunneling-type conduction in the p-Si/SiO₂ structures.     

Remanence and switching sensitivity in nanodot magnetic arrays

New results are reported of the computer simulations on the magnetic behaviour of magnetic arrays of nanoscopic dots, placed in cells of the square lattice. We show that the remanence magnetization M(r) decreases with the array size. For arrays 50 x 50, we investigate also the stability of the magnetic structure of an array in an oscillating magnetic field. The damage spreading technique reveals that this stability increases with the standard deviation sigma of the switching field of individual elements of the array. On the other hand, M(r) decreases with sigma. An optimalization of the system (large M(r) and large stability) can then be reached at some intermediate value of sigma.     

Resistive switching effects in oxide sandwiched structures

Resistive switching (RS) behaviors have attracted great interest due to their promising potential for the data storage. Among various materials, oxide-based devices appear to be more advantageous considering their handy fabrication and compatibility with CMOS technology, though the underlying mechanism is still controversial due to the diversity of RS behaviors. In this review, we focus on the oxide-based RS memories, in which the working mechanism can be understood basically according to a so-called filament model. The filaments formation/rupture processes, approaches developed to detect and characterize filaments, several effective attempts to improve the performances of RS and the quantum conductance behaviors in oxide-based resistive random access memory (RRAM) devices are addressed, respectively.     

Spin transport in spin filtering magnetic tunneling junctions

Taking into account spin-orbit coupling and s-d interaction, we investigate spin transport properties of the magnetic tunneling junctions with spin filtering barrier using Landauer-Büttiker formalism implemented with the recursive algorithm to calculate the real-space Green function. We predict completely different bias dependence of negative tunnel magnetoresistance (TMR) between the systems composed of nonmagnetic electrode (NM)/ferromagnetic barrier (FB)/ferromagnet (FM) and NM/FB/FM/NM spin filtering tunnel junctions (SFTJs). Analyses of the results provide us possible ways of designing the systems which modulate the TMR in the negative magnetoresistance regime.     

Moment switching in nanotube magnetic force probes

Magnetic images of high density vertically recorded media using metal-coated carbon nanotube tips exhibit a doubling of the spatial frequency under some conditions (Deng et al 2004 Appl. Phys. Lett. 85 6263). Here we demonstrate that this spatial frequency doubling is due to the switching of the moment direction of the nanotube tip. This results in a signal which is proportional to the absolute value of the signal normally observed in MFM. Our modeling indicates that a significant fraction of the tip volume is involved in the observed switching, and that it should be possible to image high bit densities with nanotube magnetic force sensors.     

The switching criterion in perpendicular magnetic recording

This paper presents a theoretical analysis of the effect of the "switching criterion", the level at which self-consistency is assumed in calculations on the perpendicular magnetic recording process. It can be proven that in a perpendicular recording configuration with an ideal keeper layer and a recording layer with a rectangular hysteresis loop, the switching criterion in stand-still recording situations is immaterial, because self-consistency is reached at all depth levels simultaneously. If either the keeper layer is absent, or the recording layer's hysteresis loop is sheared, it is shown that the higher the level at which self-consistency is assumed, the sharper the stand-still recorded transitions will be.     

Influence of a transverse magnetic field on the current distribution in parallel superconducting wires

In a system of parallel superconducting wires, which are connected at their ends, any two conductors form a closed superconducting loop, in which, according to the law of induction, no variation of the enclosed magnetic flux can occur. If a transverse magnetic field is applied to the system, the closed superconducting loops oppose the penetration of this field. As a consequence currents are induced in the conductors, whose magnetic field generates a certain current distribution, resulting from the geometrical arrangement of the conductors.The theoretical treatment depends upon whether the conductors are in the mixed or in the Meissner state. The current distribution was investigated in planar wire arrangements as well as in systems with wires positioned on a circle.