Exchange anisotropy in Co80Ni20/oxide nanoparticles

Monodispersed spherical Co 80 Ni 20 nanoparticles with diameters between 10 and 540 nm have been prepared by boiling liquid reduction of cobalt and nickel salts through a heterogeneous nucleation process (polyol process). All samples exhibit a core/shell structure with a ferromagnetic core surrounded by a surface layer consisting of antiferromagnetic oxides and organic matter. The ferromagnetic/antiferromagnetic interface, which arises from the core/shell structure, induces an exchange anisotropy at low temperature when the samples are subjected to a field cooling process or to a zero field cooling process starting from a remanent state. The shift of the hysteresis loops (exchange bias) and the increment of the coercive field are consequences of the presence of this magnetic anisotropy. The effect of the particle size, the temperature and the cooling process on the magnetic behaviour of the samples has been analysed.