Planar Hall effects in exchange biased FeMn/NiFe bilayers were measured under magnetic fields rotated in the film plane. Detailed magnetic anisotropy analysis shows that there is no additional uniaxial anisotropy induced by the interfacial coupling in this system, while the instability of anti-ferromagnetic grains or the formation of many domains in ferromagnetic layer may play an important role responsible for the difference between exchange biasing fields obtained by reversible and irreversible measurements.