Mobility of Bloch Walls via the Collective Coordinate Method

We have studied the problem of the dissipative motion of Bloch walls considering a totally anisotropic one dimensional spin chain in the presence of a magnetic field. Using the so-called "collective coordinate method" we construct an effective Hamiltonian for the Bloch wall coupled to the magnetic excitations of the system. It allows us to analyze the Brownian motion of the wall in terms of the reflection coefficient of the effective potential felt by the excitations due to the existence of the wall. We find that for finite values of the external field the wall mobility is also finite. The spectrum of the potential at large fields is investigated and the dependence of the damping constant on temperature is evaluated. As a result we find the temperature and magnetic field dependence of the wall mobility.Comment: 20 pages, 5 figure

Nitrogen and phosphorus in Ulva sp. in the Galician Rias Bajas (northwest Spain): Seasonal fluctuations and influence on growth

Coastal eutrophication has given cause for increasing concern. The Galician rias are ecosystems very sensitive to this phenomenon. In the present paper, we evaluate the possible nutrient limitation in Ulva sp., a green-tide alga. Variations in tissue nitrogen (N) and phosphorus (P), as well as growth rate, were determined over a complete seasonal cycle for Ulva sp. collected in the Rias Bajas (northwest Spain). Minimum levels of both nutrients were reached in spring and summer and maximum in winter. The low tissue P concentrations were striking. There was a parallelism between the evolution of the growth rate and nutrient contents. The most significant correlation found between growth rate and tissue P, and the equations established by stepwise multiple regression procedures, suggest that P may play a more important role than N in the limitation of productivity of Ulva sp. in the Galician Rias Bajas.La eutrofización costera es un problema de creciente preocupación y las rías gallegas son ecosistemas sensibles a este fenómeno. En este trabajo se intenta evaluar la posible limitación nutritiva en Ulva sp., un alga que forma parte de las denominadas mareas verdes. Se realizaron análisis de nitrógeno (N) y fósforo (P), así como bioensayos de crecimiento en Ulva sp. recogida mensualmente en cuatro rías de la costa gallega durante un año. Los niveles mínimos de ambos nutrientes se registraron en primavera y verano y los máximos en invierno, destacando las bajas concentraciones de P encontradas. Se observó un paralelismo entre la evolución de las tasas de crecimiento de Ulva y las concentraciones de nutrientes en el talo a lo largo del ciclo anual estudiado. La mayor correlación encontrada entre tasa de crecimiento y concentración de P, y las ecuaciones establecidas por el procedimiento de regresión múltiple paso a paso, indican que el fósforo parece jugar un papel más importante que el nitrógeno en la producción de Ulva sp. en las rías bajas gallegas.Instituto Español de Oceanografí

NMR linewidth and Skyrmion localization in quantum Hall ferromagnets

The non-monotonic behavior of the NMR signal linewidth in the 2D quantum Hall system is explained in terms of the interplay between skyrmions localization, due to the influence of disorder, and the non-trivial temperature dependent skyrmion dynamics.Comment: 5 pages, 2 figure

One-dimensional Gapless Magnons In A Single Anisotropic Ferromagnetic Nanolayer.

Gapless magnons in a plane ferromagnet with normal axis anisotropy are shown to exist besides the usual gapped modes that affect spin dependent transport properties only above a finite temperature. These magnons are one-dimensional objects, in the sense that they are localized inside the domain walls that form in the film. They may play an essential role in the spin dependent scattering processes even down to very low temperatures.9122680