MAGNETIZATION PROCESSES IN AMORPHOUS NANOPARTICLES OBTAINED BY HEAVY-IONS IRRADIATION OF NONMAGNETIC Y-CO FILMS

Ferromagnetic amorphous nanoparticles have been obtained by heavy ions (Pb, U) irradiation of paramagnetic YCo2 thin films. The precipitatio n of elongated particles formed of an amorphous ferromagnetic core of diameter similar to 20 Angstrom with an external core of recrystallize d YCo2 is observed. At low fluence the interactions between particles are weak and a clear perpendicular anisotropy, of shape in origin, is observed. The temperature dependence of the anisotropy field is well f itted in the chain of sphere approximation, revealing the irregular sh ape of the tracks. The coercive field, which amounts to 1/3 of the ani sotropy field at low temperature, is analyzed by considering either 1- D curling which implies continuous exchange interaction along the ion path or symmetrical fanning for a chain of decoupled spheres. When the temperature is increased, the coercivity vanishes as a result of the large thermal activation.