  We study experimentally the effect of electric field on the magnetization of
Co/Al2O3 granular multilayers. We observe two distinct regimes: (a) low-field
regime when the net magnetization of the system changes in a reversible way
with the applied electric field and (b) high-field regime when the
magnetization decreases irreversibly. The former is attributed to the changes
in the relative 3d-orbital occupation of the minority and majority bands in the
Co granules. A theoretical model has been developed to explain the electric
field induced changes in the band structure of the granular system and hence
the magnetic moment. The latter result may be understood assuming the electric
field induces oxygen migration from Al2O3 to the Co granules, since an increase
in oxidation state of the Co granules is shown, through ab-initio calculations,
to give rise to a reduced magnetization of the system.
