Magnetic field induced irreversible changes of the exchange anisotropy in C
o/CoO bilayers were investigated. Cobalt films were grown by dc magnetron s
puttering and then partially oxidized. They were then field cooled in 7000
Oe to 4.2 K to induce the exchange bias. A variable magnitude magnetic fiel
d was applied in the film plane at various angles with respect to the excha
nge bias direction. The effects of this variable magnetic field on the exch
ange coupling were studied by the reversible anisotropic magnetoresistance
technique. Three qualitatively different behaviors of the exchange anisotro
py direction were observed. The particular behavior was determined by the m
agnitude of the applied magnetic field and the rotation angle. A simple phe
nomenological model of the exchange anisotropy was developed to explain the
experimental results, which allows for explicit quantitative separation of
the unidirectional and the rotatable parts of the exchange anisotropy. (C)
2000 American Institute of Physics. [S0021-8979(00)85708-4].