E. B. Myers

Current-driven magnetization reversal and spin-wave excitations in Co /Cu /Co pillars

Using thin film pillars approximately 100 nm in diameter, containing two Co layers of different thicknesses separated by a Cu spacer, we examine the process by which the scattering from the ferromagnetic layers of spin-polarized currents flowing perpendicular to the layers causes controlled reversal of the moment direction in the thin Co layer. The well-defined geometry permits a quantitative analysis of this spin-transfer effect, allowing tests of competing theories for the mechanism and also new insight concerning magnetic damping. When large magnetic fields are applied, the spin-polarized current no longer fully reverses the magnetic moment, but instead stimulates spin-wave excitations.

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.

Role of spin-dependent interface scattering in generating current-induced torques in magnetic multilayers

We present a calculation of current-induced torques in metallic magnetic multilayers derived from the spin-dependent transmission and reflection properties of the magnetic layers. A scattering formalism is employed to calculate the torques in a magnetic-nonmagnetic-magnetic trilayer, for currents perpendicular to the layers, in both the ballistic and diffusive regimes.

Five Years of Pulsar Flux Density Monitoring: Refractive Scintillation and the Interstellar Medium

We have monitored the radio flux density of 21 pulsars on a daily basis for five years. The 610 MHz flux density time series for these pulsars range from nearly constant for the most distant and heavily scattered pulsars to rapidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations less than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the interstellar medium, is constant in strength on timescales of a few hours to several years. The modulation index of the flux density variations never exceeds 0.5, ruling out a density inhomogeneity spectrum with a steep power-law exponent (β > 4). The flux density variations for 15 of the pulsars are consistent with a Kolmogorov turbulence spectrum over a range of more than 5 orders of magnitude in scattering strength, with no detectable presence of an inner scale. For these lines of sight we constrain the inhomogeneity slope to be in the range 3.5 ≤ β ≤ 3.7, which brackets the Kolmogorov value of β = 3.67. The flux density variations are greater than predicted by this model for six pulsars—including the Crab and Vela—but this group is consistent with a Kolmogorov spectrum and an inner scale of ≈1010 cm. The lines of sight to three of the other pulsars in this group pass through H II regions around young, hot stars. For six pulsars we have found a change in the slope of the intensity structure function, which could be connected with a change in the slope of the inhomogeneity power spectrum at a scale of ≈1013 cm.

TUNNELING VIA INDIVIDUAL ELECTRONIC STATES IN FERROMAGNETIC NANOPARTICLES

We measure electron tunneling via discrete energy levels in ferromagnetic cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due to magnetic anisotropy, the energy of each tunneling resonance shifts as an applied magnetic field rotates the particles magnetic moment. We see both spin-increasing and decreasing tunneling transitions, but we do not observe the spin degeneracy at small magnetic fields seen previously in non-magnetic materials. The tunneling spectrum is denser than predicted for independent electrons, possibly due to spin-wave excitations.

Point-contact studies of current-controlled domain switching in magnetic multilayers

We present measurements demonstrating current-induced magnetic domain switching, and also other magnetic excitations, in point-contact devices containing alternating ferromagnetic (F) and noble metal (N) layers, for perpendicular currents ∼109 A/cm2. For F/N/F trilayers in which one F layer is much thinner than the other, we can controllably switch the magnetic moments in the two F layers parallel with a current bias of one sign, and switch them antiparallel with a reversed current. For thicker magnetic films, and for thin films in the presence of a saturating magnetic field, we observe nonhysteretic current-induced changes in resistance, which can be understood as current-induced spin-wave excitations. These observations are in agreement with a model of current-induced magnetic reorientations caused by local exchange forces between conduction electrons and the magnetic moments.