A. G. F. Garcia

Spin-Transfer-Driven Ferromagnetic Resonance of Individual Nanomagnets

We demonstrate a technique that enables ferromagnetic resonance measurements of the normal modes for magnetic excitations in individual nanoscale ferromagnets, smaller in volume by more than a factor of 50 compared to individual ferromagnetic samples measured by other resonance techniques. Studies of the resonance frequencies, amplitudes, linewidths, and line shapes as a function of microwave power, dc current, and magnetic field provide detailed new information about the exchange, damping, and spin-transfer torques that govern the dynamics in magnetic nanostructures.

Adjustable spin torque in magnetic tunnel junctions with two fixed layers

We have fabricated nanoscale magnetic tunnel junctions (MTJs) with an additional fixed magnetic layer added above the magnetic free layer of a standard MTJ structure. This acts as a second source of spin-polarized electrons that, depending on the relative alignment of the two fixed layers, either augments or diminishes the net spin torque exerted on the free layer. The compound structure allows a quantitative comparison of spin torque from tunneling electrons and from electrons passing through metallic spacer layers, as well as analysis of Joule self-heating effects. This has significance for current-switched magnetic random access memory, where spin torque is exploited and, for magnetic sensing, where it is detrimental.

Temperature Dependence of Spin-Transfer-Induced Switching of Nanomagnets

We measure the temperature, magnetic-field, and current dependence for the switching of nanomagnets by a spin-polarized current. Depending on current bias, switching can occur between either two static magnetic states or a static state and a current-driven precessional mode. In both cases, the switching is thermally activated and governed by the sample temperature, not a higher effective magnetic temperature. The activation barriers for switching between static states depend linearly on current, with a weaker dependence for dynamic to static switching.

Spin-torque ferromagnetic resonance measurements of damping in nanomagnets

The authors directly measure the magnetic damping parameter α in thin-film CoFeB and Permalloy (Py) nanomagnets at room temperature using a recently developed ferromagnetic resonance technique where the precessional mode of an individual nanomagnet can be excited by microwave-frequency spin-transfer torque and detected by the giant magnetoresistance effect. The authors obtain αCoFeB=0.014±0.003 and αPy=0.010±0.002, values comparable to measurements for extended thin films, establishing that patterned nanomagnets can exhibit magnetic damping that is consistent with that of unpatterned bulk material.

Reducing the critical current for short-pulse spin-transfer switching of nanomagnets

We have fabricated permalloy∕copper∕permalloy nanopillar spin valves designed to reduce the critical current for spin-transfer switching while maintaining thermal stability of the free layer. Pulsed current amplitudes necessary for switching a 4.5‐nm-thick permalloy free layer range from 0.4mA for a 100ns pulse to 2mA for a 1ns pulse, showing that the magnetization must be overdriven to achieve switching on short time scales. Comparisons to Landau–Lifshitz–Gilbert simulations indicate an effective damping parameter ≈0.03 and spin-torque efficiencies for parallel-to-antiparallel and antiparallel-to-parallel switching that are more symmetric than predicted by recent theoretical models.

Spin-transfer excitations of permalloy nanopillars for large applied currents

Using measurements of the spectra of microwave-frequency resistance oscillations, we determine the roomtemperature phase diagram of magnetic excitations caused by torques from dc spin-polarized currents in thin permalloy/copper/thick permalloy multilayer samples. We extend the measurements to larger values of current than have been reported previously. We find several additional modes that we are able to identify with motion of the thick magnetic layer as well as the thin one. Peaks in the microwave spectra at multiple frequencies suggest that spatially nonuniform dynamical states can be important in some circumstances. We compare the experimental phase diagram with simple theoretical models and achieve a good qualitative agreement.

Time-Resolved Spin-Torque Switching and Enhanced Damping in Permalloy/Cu/Permalloy Spin-Valve Nanopillars

We report time-resolved measurements of current-induced reversal of a free magnetic layer in Permalloy/Cu/Permalloy elliptical nanopillars at temperatures T=4.2 K to 160 K. Comparison of the data to Landau-Lifshitz-Gilbert macrospin simulations of the free layer switching yields numerical values for the spin torque and the Gilbert damping parameters as functions of T. The damping is strongly T dependent, which we attribute to the presence of an antiferromagnetic oxide layer around the perimeter of the Permalloy free layer. This adventitious antiferromagnetic oxide can have a major impact on spin-torque phenomena.

Spin transfer by nonuniform current injection into a nanomagnet

We have used nanofabrication techniques to incorporate an ∼20–30nm diameter nanoaperture within a 150×250nm2 elliptical magnetic multilayer to enable the localized injection of spin-polarized currents into a thin film nanomagnet. This results in very low spin transfer currents being required for at least partial nanomagnet reversal as well as for onset of dynamic precession. Micromagnetic simulations using Landau-Liftshitz-Gilbert equation with a spin-torque term indicate that reversal occurs via domain nucleation at the injection site followed by domain wall propagation away from the aperture, with the nanomagnet ending in one of several different states depending upon the current amplitude.

Enhancement in spin-torque efficiency by nonuniform spin current generated within a tapered nanopillar spin valve

We examine the effect a spatially non-uniform spin current with a component polarized partially out of the plane has on a low saturation magnetization nanomagnet free layer. Micromagnetic simulations indicate that the spin torque efficiency acting upon the reversing nanomagnet can be enhanced through this process, resulting in faster switching with smaller currents. In doing so, we determine that micromagnetic structure within the nanomagnets can be beneficial for reversal processes. We verify this enhancement experimentally in devices with a tapered nanopillar geometry that generates a spin current polarized partly out of plane. Finally, to take even better advantage of these effects, we examine micromagnetically the benefits of a tapered three-magnetic-layer structure that further reduces reversal times while maintaining the thermal stability of the free layer.