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 precipitation of elongated particles formed of an amorphous ferromagnetic core of diameter similar to 20 Angstrom with an external core of recrystallized 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 fitted in the chain of sphere approximation, revealing the irregular shape of the tracks. The coercive field, which amounts to 1/3 of the anisotropy 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.