  We report on the occurrence of exchange bias on laser-ablated granular thin
films composed of Co nanoparticles embedded in amorphous zirconia matrix. The
deposition method allows controlling the degree of oxidation of the Co
particles by tuning the oxygen pressure at the vacuum chamber (from 2x10^{-5}
to 10^{-1} mbar). The nature of the nanoparticles embedded in the nonmagnetic
matrix is monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic
(AFM) CoOx, with a FM/AFM intermediate regime for which the percentage of the
AFM phase can be increased at the expense of the FM phase, leading to the
occurrence of exchange bias in particles of about 2 nm in size. For oxygen
pressure of about 10-3 mbar the ratio between the FM and AFM phases is optimum
with an exchange bias field about 900 Oe at 1.8 K. The mutual exchange coupling
between the AFM and FM is also at the origin of the induced exchange anisotropy
on the FM leading to high irreversible hysteresis loops, and the blocking of
the AFM clusters due to proximity to the FM phase.
