Feedback control of noise in spin valves by the spin-transfer torque

The miniaturisation of magnetic read heads and random access memory elements makes them vulnerable to thermal fluctuations. We demonstrate how current-induced spin-transfer torques can be used to suppress the effects of thermal fluctuations. This enhances the fidelity of perpendicular magnetic spin valves. The simplest realization is a dc current to stabilize the free magnetic layers. The power can be significantly reduced without losing fidelity by simple control schemes, in which the stabilizing current-induced spin-transfer torque is controlled by the instantaneous resistance.Comment: 4pages, 2 figure

Variability in the subtropical-tropical cells and its effect on near-surface temperature of the equatorial Pacific: a model study

A set of experiments utilizing different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-) tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Niño3-region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-) decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Niño episodes. The behaviour is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems

Spontaneous-Symmetry-Breaking Mechanism of Adiabatic Pumping

We consider heterostructures consisting of regions with a continuous symmetry in contact with regions wherein the symmetry is spontaneously broken. The low-frequency dynamics of the corresponding order parameter are shown to induce nonequilibrium transport, a ``pumping,'' out of the symmetry-broken regions, which is governed by the generator of the broken-symmetry operator. This pumping damps Goldstone-mode excitations and transfers them beyond traditional (static) proximity length scales. Our general conclusions are discussed for specific examples of order parameters in helimagnets, charge/spin-density waves, superconductors, and ferromagnets. We carry out a detailed calculation of such pumping for spiral magnetic orders in helimagnets possessing a duality in the representation of its symmetry-broken states.Comment: 5 pages, 2 figure

Spin-transfer in diffusive ferromagnet-normal metal systems with spin-flip scattering

The spin-transfer in biased disordered ferromagnet (F) - normal metal (N) systems is calculated by the diffusion equation. For F1-N2-F2 and N1-F1-N2-F2-N3 spin valves, the effect of spin-flip processes in the normal metal and ferromagnet parts are obtained analytically. Spin-flip in the center metal N2 reduces the spin-transfer, whereas spin-flip in the outer normal metals N1 and N3 can increase it by effectively enhancing the spin polarization of the device.Comment: 9 pages, 3 figure

Current-induced macrospin vs spin-wave excitations in spin valves

The mode dependence of current-induced magnetic excitations in spin valves is studied theoretically. The torque exerted on the magnetization by transverse spin currents as well as the Gilbert damping constant are found to depend strongly on the wave length of the excitation (spin wave). Analytic expressions are presented for the critical currents that excite a selected spin wave. The onset of macrospin (zero wavelength) vs finite wavelength instabilities depends on the device parameters and the current direction, in agreement with recent experimental findings.Comment: 8 pages, 3 figure

Non-collinear Magnetoelectronics

The electron transport properties of hybrid ferromagnetic|normal metal structures such as multilayers and spin valves depend on the relative orientation of the magnetization direction of the ferromagnetic elements. Whereas the contrast in the resistance for parallel and antiparallel magnetizations, the so-called Giant Magnetoresistance, is relatively well understood for quite some time, a coherent picture for non-collinear magnetoelectronic circuits and devices has evolved only recently. We review here such a theory for electron charge and spin transport with general magnetization directions that is based on the semiclassical concept of a vector spin accumulation. In conjunction with first-principles calculations of scattering matrices many phenomena, e.g. the current-induced spin-transfer torque, can be understood and predicted quantitatively for different material combinations.Comment: 163 pages, to be published in Physics Report

Spin-transport in multi-terminal normal metal - ferromagnet systems with non-collinear magnetizations

A theory of spin-transport in hybrid normal metal - ferromagnetic electronic circuits is developed, taking into account non-collinear spin-accumulation. Spin-transport through resistive elements is described by 4 conductance parameters. Microscopic expression for these conductances are derived in terms of scattering matrices and explicitly calculated for simple models. The circuit theory is applied to 2-terminal and 3-terminal devices attached to ferromagnetic reservoirs.Comment: 12 pages, 6 figures, to appear in Eur. Phys. J.

Magnetomechanical Torques in Small Magnetic Cantilevers

We study the dnamics of small magnetic cantilevers, either made from Si covered by a magnetic film or entirely ferromagnetic ones. The magnetomechanical torques are found to cause line splittings in ferromagnetic resonance spectra and magnetization reversal facilitated by mechanical degrees of freedom. We show that the magnetomechanical torques can extend the limits of detecting and exciting motion at the nanoscale. A "nanomotor" described here effectively transforms rf magnetic fields into mechanical oscillations. We furthermore propose to integrate mechanical oscillators into magnetoelectronic devices that make use of current-induced spin-transfer torques. This opens new possibilities for electric transducers of nanomechanical motion.Comment: 20 pages, 12 figures; submitted to a special issue of JJAP: Magnetization Dynamics in Spintronic Structures and Device

Magnetization noise in magnetoelectronic nanostructures

By scattering theory we show that spin current noise in normal electric conductors in contact with nanoscale ferromagnets increases the magnetization noise by means of a fluctuating spin-transfer torque. Johnson-Nyquist noise in the spin current is related to the increased Gilbert damping due to spin pumping, in accordance with the fluctuation-dissipation theorem. Spin current shot noise in the presence of an applied bias is the dominant contribution to the magnetization noise at low temperatures.Comment: 4 pages, 1 figur