Magnetization reversal of thin ferromagnetic elements with surface anisotropy

The magnetization reversal process in thin-film ferromagnetic elements with surface anisotropy of various shapes and sizes is investigated by means of numerical simulation. The dependence of the perpendicular and in-plane hysteresis loops on the element thickness and the value of the surface anisotropy constant is obtained. For sufficiently large values of the surface anisotropy constant the magnetization reversal of thin-film elements is shown to occur due to the nucleation of the buckling mode. For an elongated rectangular element the nucleation field of the buckling mode is proportional to the absolute value of the surface anisotropy constant, and inversely proportional to the element thickness.Comment: 7 pages, 7 figure

Radiation from hot bare strange stars

We present the results of numerical simulations of stationary, spherically outflowing, pair winds, with total luminosities of L=10^{35}- 10^{42} ergs/s. These results have direct relevance to the emission from hot, bare, strange stars, which are thought to be powerful sources of electron-positron pairs created by the Coulomb barrier at the quark surface. The spectra of emergent photons and pairs are calculated. For L > 2x10^{35} erg/s, photons dominate the emerging emission. As L increases from 10^{35} to 10^{42} ergs/s, the mean photon energy decreases from ~ 400-500 keV to 40 keV, while the spectrum changes in shape from a wide annihilation line to being nearly blackbody with a high energy (> 100 keV) tail. Such a correlation of the photon spectrum with the luminosity, together with the fact that super-Eddington luminosities can be achieved, might be a good observational signature of hot, bare, strange stars.Comment: 4 pages, 4 figures, Accepted in MNRAS, includes minor correction