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: βt=0.010±0.004 for a transverse DW and βv=0.073±0.026 for a vortex DW. The larger value is attributed to the higher magnetization gradients present.

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 10 10 A=m 2 . 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.

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 does not equal the damping , 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.

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.

Using microwave CUtTents, 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.Laterally confined magnetic domain walls exhibit a range of novel physical effects and are also promising candidates for applications in memory devices [1] as well as in logic circuits [2].To use domain walls in devices, the walls have to be pinned controllably at well-defined pinning positions. Examples of artificial pinning sites include notches [3][4][5][6] and protrusions [7], both creating attractive potentials for domain walls. Domain walls can be moved between different pinning sites to implement, e.g., logic operations [2] or storage [1]. Another application was proposed by He and Zhang [8], who suggest to use a localized domain wall oscillator as a tunable microwave source. But so far it is unclear to what extent the frequency of such an oscillator can be tuned, which is one of the key requirements for applications.For reliable operation of devices based on domain wall motion or resonance, in addition to well-defined pinning potentials, also sufficiently low critical current densities and sufficiently fast switching are required. It was shown that resonant excitations [9] as well as resonant pulse trains [10] allow for a significantly lowered threshold current density. To further understand and control domain wall dynamics, the quantitative shape of the pinning potentials has to be determined. In addition to engineered pinning centers, pinning at defects intrinsic to the material or caused by the processing is one of the key problems and obstacles for device applications, and only quantitative information about the pinning potential will lead to a further understanding of the dynamics of such random pinning processes. So an in-depth understanding of the pinning potential landscape due to constrictions but, in particular, due to intrinsic defects is a key requisite to further development in the field of domain wall dynamics.Depending on the type of the domain wall, the dynamic behavior of pinned domain walls is determined by different contributions of the wall spin structure. While in the case of transverse walls the dynamic behavior is determined mainly by the motion of the entire wall, for vortex walls, the singularity in the center of a magnetic vortex, which points out of the plane and is one of the smallest confined magnetic structures present in domain walls, plays a key role.A force acting on such a vortex core will cause a movement of the vortex core in the direction perpendicular to the force [I 1,12]. Therefore, if excited by a continuous ac field or current, the vortex core will car...