Polycrystalline NiFe/FeMn bilayers were fabricated by DC magnetron sputtering and studied by ferromagnetic resonance and vibrating sample magnetometer. The isotropic resonance field shift and the rotatable anisotropy are correlated to each other because they both undergo the maximal values around 2.5 nm and then decrease with increasing the antiferromagnetic layer thickness. These results can be explained in terms of thermally activated transition of antiferromagnetic spins. (C) 2009 Elsevier B.V. All rights reserved.