Jack C. Sankey

Microwave oscillations of a nanomagnet driven by a spin-polarized current

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.

Magnetic vortex oscillator driven by d.c. spin-polarized current

Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the magnetization dynamics of nanomagnets. A peculiar consequence of this spin torque, the ability to induce persistent oscillations in a nanomagnet by applying a d.c. current, has previously been reported only for spatially uniform nanomagnets. Here, we demonstrate that a quintessentially non-uniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin-valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized d.c. current. Comparison with micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz at ∼1.1 GHz, making these highly compact spin-torque vortex-oscillator devices potential candidates for microwave signal-processing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures.

Measurement of the spin-transfer-torque vector in magnetic tunnel junctions

The transfer of spin angular momentum from a spin-polarized current to a ferromagnet can generate sufficient torque to reorient the magnet’s moment. This torque could enable the development of efficient electrically actuated magnetic memories and nanoscale microwave oscillators. Yet difficulties in making quantitative measurements of the spin-torque vector have hampered understanding. Here we present direct measurements of both the magnitude and direction of the spin torque in magnetic tunnel junctions, the type of device of primary interest for applications. At low bias V, the differential torque dτ/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with recent predictions for near-perfect spin-polarized tunnelling. We find that the strength of the in-plane differential torque remains almost constant with increasing bias, despite a substantial decrease in the device magnetoresistance, and that with bias the torque vector also rotates out of the plane.

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.

Spin-transfer effects in nanoscale magnetic tunnel junctions

We report measurements of magnetic switching and steady-state magnetic precession driven by spin-polarized currents in nanoscale magnetic tunnel junctions with low-resistance, <5Ωμm2, barriers. The current densities required for magnetic switching are similar to values for all-metallic spin-valve devices. In the tunnel junctions, spin-transfer-driven switching can occur at voltages that are high enough to quench the tunnel magnetoresistance, demonstrating that the current remains spin polarized at these voltages.

Thermally-Activated Magnetic Reversal Induced by a Spin-Polarized Current

We have measured the statistical properties of magnetic reversal in nanomagnets driven by a spin-polarized current. Like reversal induced by a magnetic field, spin-transfer-driven reversal near room temperature exhibits the properties of thermally activated escape over an effective barrier. However, the spin-transfer effect produces qualitatively different behaviors than an applied magnetic field. We discuss an effective current vs field stability diagram. If the current and field are tuned so that their effects oppose one another, the magnet can exhibit telegraph-noise switching.

Strong and tunable nonlinear optomechanical coupling in a low-loss system

An optical cavity coupled to a micrometre-sized mechanical resonator offers the opportunity to see quantum effects in relatively large structures. It is now shown that a variety of coupling mechanisms enable investigation of these fascinating systems in a number of different ways.

Current-induced nanomagnet dynamics for magnetic fields perpendicular to the sample plane.

We present electrical measurements of high-frequency magnetic dynamics excited by spin-polarized currents in Co/Cu/Ni(80)Fe20 nanopillar devices, with a magnetic field applied perpendicular to the sample layers. As a function of current and magnetic field, the dynamical phase diagram contains several distinguishable precessional modes and also static magnetic states. Using detailed comparisons with numerical simulations, we provide rigorous tests of the theory of spin-transfer torques.

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.