Memory effects of exchange coupling in CoO/Ni81Fe19 bilayers

By changing the field-cooling procedure, the value and even the sign of the exchange field H-E in CoO/Ni81Fe19 bilayers can be greatly altered. The value of H-E can be made to vanish at any temperature below T-N. We also show that the magnetization of the Ni81Fe19 is the crucial parameter in the field-cooling procedure. Changing the magnetization during field cooling shows that the exchange bias is an accumulative memory effect, dependent on the thermal and field history of the bilayer. The enhanced coercivity which accompanies the exchange bias, depends only on temperature and not on the field-cooling history. (C) 1999 American Institute of Physics. [S0021-8979(99)38208-6].