Ferromagnetic nanowires are likely to play an important role in future spintronic devices. Magnetic domain walls, which separate regions of opposing magnetization in a nanowire, can be manipulated and used to encode information for storage or to perform logic operations. Owing to their reduced size and dimensionality, the characterization of domain-wall motion is an important problem. To compete with other technologies, high-speed operation, and hence fast wall propagation, is essential. However, the domain-wall dynamics in nanowires has only been investigated in the last five years and some results indicate a drastic slowing down of wall motion in higher magnetic fields. Here we show that the velocity-field characteristic of a domain wall in a nanowire shows two linear regimes, with the wall mobility at high fields reduced tenfold from that at low fields. The transition is marked by a region of negative differential mobility and highly irregular wall motion. These results are in accord with theoretical predictions that, above a threshold field, uniform wall movement gives way to turbulent wall motion, leading to a substantial drop in wall mobility. Our results help resolve contradictory reports of wall propagation velocities in laterally confined geometries, and underscore the importance of understanding and enhancing the breakdown field for practical applications.

The electromotive force induced by a moving magnetic domain wall in a nanostrip has been calculated theoretically and detected experimentally. It is found that the emf depends only on the domain wall transformation frequency through a universal Josephson type relation, which is closely related to the topological nature of the domain wall. Our experimental measurements confirm the theoretical prediction.

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The interaction between a dc spin-polarized electric current and a magnetic domain wall in a Permalloy nanowire was studied by high-bandwidth scanning Kerr polarimetry. The full functional dependence of wall velocity on electric current and magnetic field is presented. With the pinning potential nulled by a field, current-induced velocity enhancements exceeded 35 m/s at a current density of approximately 6 x 10(11) A/m(2). This large enhancement, more than 10 times that found in pinning-dominated experiments, results in part from an interaction that is nonlinear in current and independent of current direction.

Hysteresis properties of ultrathin (1.5 -4 monolayer) epitaxial Fe films grown on flat and stepped W(110) surfaces are studied as a function of film thickness, temperature, and the strength and frequency of the applied sinusoidal magnetic field. Power law scaling of the hysteresis loop area is observed over five decades in frequency. Measured exponents depart significantly from those reported based on prior experiments and existing theoretical models. An abrupt transition from switching behavior to a stable magnetic state is observed at a critical frequency where the dynamic coercivity exceeds the applied field strength. [S0031-9007(97)03008-1] PACS numbers: 75.60. Ej, 75.40.Gb, 75.70.Ak The dynamics of magnetization reversal has recently attracted considerable scientific interest based on new opportunities to explore concepts of universality and scaling [1][2][3][4][5][6][7][8][9][10][11][12]. Theoretical efforts have explored hysteresis phenomena in model magnetic systems based on a variety of approaches [1][2][3][4][5][6][7][8][9][10]. A general objective of these efforts has been to discover scale-invariant descriptions of the energy loss per cycle (area of the hysteresis loop) as a function of external parameters (applied magnetic field strength H, frequency V, and temperature T ) and intrinsic system parameters (dimensionality and symmetry, i.e., magnetic anisotropy). For example, in the low frequency limit, invariant functions of the hysteresis loop area have been shown to reduce to a power law function of the form(1) where a, b, and g are exponents that depend on the dimensionality and symmetry of the system. Two basic types of dynamical models have been explored theoretically: Ising-like models [4][5][6]9] in which an energy barrier separates the two equivalent magnetized states, and continuous spin models [1][2][3][4]7,8] having no barrier. Specific values of the exponents in Eq. (1) for various models are summarized in Table I with corresponding references to the literature.Dynamical properties of magnetization reversal have also been investigated experimentally in ultrathin film systems [11][12][13][14]. These systems offer unique opportunities to explore dynamical effects in well-characterized structures in which relevant intrinsic parameters (dimensionality, anisotropy) can be controlled. Studies of Fe on Au(001) [11] and Co on Cu(001) [12] have apparently revealed dynamical scaling effects corroborating general theoretical predictions based on the continuous spin and Ising-like models: power law scaling of the loop area (exponents in Table I); constant loop area characteristic of adiabatic magnetization reversal at very low frequencies [12]; and threshold field effects associated with a doublewell barrier in Ising-type models.In this Letter, we report hysteresis loop measurements of well-characterized ultrathin Fe films grown on flat and stepped W(110) surfaces as a function of H, V, and T. Our results are consistent with universal behavior (thickness invariant exponents) and clearly exhibit uni...