Nonadiabatic spin torque investigated using thermally activated magnetic domain wall dynamics.

Using transmission electron microscopy, we investigate the thermally activated motion of domain walls (DWs) between two positions in Permalloy (Ni80Fe20) nanowires at room temperature. We show that this purely thermal motion is well described by an Arrhenius law, allowing for a description of the DW as a quasiparticle in a one-dimensional potential landscape. By injecting small currents, the potential is modified, allowing for the determination of the nonadiabatic spin torque: ßt=0.010±0.004 for a transverse DW and ßv=0.073±0.026 for a vortex DW. The larger value is attributed to the higher magnetization gradients present.

Domain-wall depinning assisted by pure spin currents.

We study the depinning of domain walls by pure diffusive spin currents in a nonlocal spin valve structure based on two ferromagnetic Permalloy elements with copper as the nonmagnetic spin conduit. The injected spin current is absorbed by the second Permalloy structure with a domain wall, and from the dependence of the wall depinning field on the spin current density we find an efficiency of 6×10{-14} T/(A/m{2}), which is more than an order of magnitude larger than for conventional current induced domain-wall motion. Theoretically we find that this high efficiency arises from the surface torques exerted by the absorbed spin current that lead to efficient depinning.

Imaging of domain wall inertia in Permalloy half-ring nanowires by time-resolved photoemission electron microscopy.

Using photoemission electron microscopy, we image the dynamics of a field pulse excited domain wall in a Permalloy nanowire. We find a delay in the onset of the wall motion with respect to the excitation and an oscillatory relaxation of the domain wall back to its equilibrium position, defined by an external magnetic field. The origin of both of these inertia effects is the transfer of energy between energy reservoirs. By imaging the distribution of the exchange energy in the wall spin structure, we determine these reservoirs, which are the basis of the domain wall mass concept.

Quantitative determination of the nonlinear pinning potential for a magnetic domain wall.

Using microwave currents, we excite resonances of geometrically confined pinned domain walls, detecting the resonance by the rectification of the microwave current. By applying magnetic fields, the resonance frequency of the domain wall oscillator can be tuned over a wide range. Increasing the power leads to a redshift due to the nonlinearity of the system. From this frequency shift, we directly deduce the quantitative shape of the potential, so that a complete characterization of the pinning potential is obtained.

Relationship between nonadiabaticity and damping in permalloy studied by current induced spin structure transformations.

By direct imaging we determine spin structure changes in Permalloy wires and disks due to spin transfer torque as well as the critical current densities for different domain wall types. Periodic domain wall transformations from transverse to vortex walls and vice versa are observed, and the transformation mechanism occurs by vortex core displacement perpendicular to the wire. The results imply that the nonadiabaticity parameter beta does not equal the damping alpha, in agreement with recent theoretical predictions. The vortex core motion perpendicular to the current is further studied in disks revealing that the displacement in opposite directions can be attributed to different polarities of the vortex core.

Detection of current-induced resonance of geometrically confined domain walls.

Magnetic domain walls are found to exhibit quasiparticle behavior when subjected to geometrical variations. Because of the spin torque effect such a quasiparticle in a potential well is excited by an ac current leading to a dip in the depinning field at resonance for current densities as low as 2 x 10(10) A/m2. Independently the resonance frequencies of transverse walls and vortex walls are determined from the dc voltage that develops due to a rectifying effect of the resonant domain wall oscillation. The dependence on the injected current density reveals a strongly nonharmonic oscillation.