Oscillations of the field-driven domain-wall ͑DW͒ velocity in permalloy nanowires are observed above the Walker breakdown condition using high-speed magneto-optic polarimetry. A one-dimensional analytical model and numerical simulations of DW motion and spin dynamics are used to interpret the experimental data. Velocity oscillations are shown to be much more sensitive to properties of the DW guide structure ͑which also affect DW mobility͒ than the DW spin precessional frequency, which is a local property of the material. The results demonstrate the feasibility of experimentally probing the complex field-driven DW dynamics in magnetic nanowires, thereby testing the validity of theoretical models and numerical simulations that describe the dynamics.

Spin dynamics of field-driven domain walls (DWs) guided by Permalloy nanowires are studied by high-speed magneto-optic polarimetry and numerical simulations. DW velocities and spin configurations are determined as functions of longitudinal drive field, transverse bias field, and nanowire width. Nanowires having cross-sectional dimensions large enough to support vortex wall structures exhibit regions of drive-field strength (at zero bias field) that have enhanced DW velocity resulting from coupled vortex structures that suppress oscillatory motion. Factor of ten enhancements of the DW velocity are observed above the critical longitudinal drive-field (that marks the onset of oscillatory DW motion) when a transverse bias field is applied. Nanowires having smaller cross-sectional dimensions that support transverse wall structures also exhibit a region of higher mobility above the critical field, and similar transverse-field induced velocity enhancement but with a smaller enhancement factor. The bias-field enhancement of DW velocity is explained by numerical simulations of the spin distribution and dynamics within the propagating DW that reveal dynamic stabilization of coupled vortex structures and suppression of oscillatory motion in the nanowire conduit resulting in uniform DW motion at high speed.

Low-energy-electron-diffraction ͑LEED͒ intensity measurements and multiple-scattering analysis for V͑100͒, supported by accurate characterization of surface impurity concentrations based on Auger-electron spectroscopy, are used to obtain a meaningful extrapolation of the first-layer relaxation to the clean surface value: d 12 = 1.36± 0.05 Å, corresponding to ⌬ 12 = −10% ± 3% relative to the bulk value d 0 = 1.514 Å. A highsensitivity probe for surface magnetism based on magneto-optic Kerr effect polarimetry using the cleanest surfaces achieved in the LEED experiment ͑ϳ5% C͒ yields a ͑sensitivity limited͒ null result with an estimated upper limit of 0.05 B /surface atom. These results are discussed within the framework of related experiments and in relation to the predictive accuracy of ab initio calculations that explore the surface structure and magnetism of V͑100͒ both of which are sensitive to different approximations for the exchange-correlation potential in density-functional theory.

We demonstrate that in-plane charge current can effectively control the spin precession resonance in an Al2O3/CoFeB/Ta heterostructure. Brillouin Light Scattering (BLS) was used to detect the ferromagnetic resonance field under microwave excitation of spin waves at fixed frequencies. The current control of spin precession resonance originates from modification of the in-plane uniaxial magnetic anisotropy field k , which changes symmetrically with respect to the current direction. Numerical simulation suggests that the anisotropic stress introduced by Joule heating plays an important role in controlling k . These results provide new insights into current manipulation of magnetic properties and have broad implications for spintronic devices.

Numerical simulations are used to investigate static and dynamic properties of spin distributions within domain walls confined by rectangular cross section Permalloy nanowire conduits having widths up to 1000 nm and thickness up to 50 nm. Phase boundaries and critical regions associated with domain-wall spin distributions of various topologies [transverse(or asymmetric transverse), vortex, double-vortex, triple-vortex and cross-tie] are accurately determined using high-performance computing resources. Mobility curves are calculated that characterize domain-wall propagation for an interesting region of the spin texture phase diagram: 20 nm thick nanowires with widths of 60-700 nm at axial drive fields extending to 150 Oe. The simulations (and corresponding experiments, which are discussed), reveal new propagating fixed configuration domain-wall topologies with enhanced velocity. Effects of temperature on the spin distributions and dynamics are explored, by conducting simulations that include separately varying temperaturedependent parameter (saturation magnetization and exchange constant) and simulating effects of temperature-dependent fluctuations using the Langevin dynamics feature of the simulation code. Related temperature-dependent experiments are discussed. The simulation studies demonstrate a close connection between static and (field-driven) dynamic spin configurations in nanowire-confined domain walls and demonstrate the importance of exploring model-system parameter space at high numerical precision.