  We synthesized pure and Co-doped (6.25 12.5 at.) ZrO$_2$ nanopowders in order
to study their magnetic properties.We analyzed magnetic behavior as a function
of the amount of Co and the oxygenation, which was controlled by low pressure
thermal treatments. As prepared pure and Co-doped samples are diamagnetic and
paramagnetic respectively. Ferromagnetism can be induced by performing low
pressure thermal treatments, which becomes stronger as the dwell time of the
thermal treatment is increased. This behavior can be reversed, recovering the
initial diamagnetic or paramagnetic behavior, by performing reoxidizing thermal
treatments. Also, a cumulative increase can be observed in the saturation of
the magnetization with the number of low pressure thermal treatments performed.
We believe that this phenomenon indicates that cobalt segregation induced by
the thermal treatments is the responsible for the magnetic properties of the
ZrO$_2$ Co system.
