Exchange bias relaxation in CoO-biased films

Because the memory of the bias direction is carried by the antiferromagneti c order in exchange biased films, the stability of the antiferromagnetic or der is critical to the existence of the exchange bias field. Ferromagnetic resonance was used to measure the relaxation behavior of polycrystalline Co O films coupled to films of Ni80Fe20, probing the system on the time scale of the experiment, approximate to 10(3) s, and the time scale of the magnet ic precession, approximate to 10(-10) s. Unidirectional anisotropy (exchang e biasing) and isotropic resonance field shifts are observed at the lowest temperatures. Above the apparent exchange bias blocking temperature, isotro pic resonance field shifts persist. At still higher temperatures, diminishi ng resonance field shifts are accompanied by peaks in the FMR linewidth. Th e results highlight the effects of varying relaxation rates in the CoO rela tive to the two experimental time scales. (C) 2000 American Institute of Ph ysics. [S0021-8979(00)85308-6].