Angular dependence of exchange coupling in ferromagnet/antiferromagnet bilayers

Angular dependence of the exchange coupling in a NiFe/CoO bilayer film has been measured. Different angular dependence and symmetry have been observed for the exchange field $H_E$ and the coercivity $H_C.$ The values of $H_E$ and $H_C$ are due to unidirectional and uniaxial components of the magnetic anisotropy energy, in which the odd and the even terms lead to $H_E$ and $H_C,$ respectively. The proposed free energy accounts for the experimental results.