Temperature dependence of magnetization reversal and angular torque in Co CoO

The temperature dependence of the magnetization reversal process, reversible and irreversible exchange bias anisotropy, uniaxial anisotropy, and rotational hysteresis of polycrystalline exchange-biased Co CoO bilayers have been investigated. A dichotomy in the experimental results is found for thin and thick CoO layers with the boundary near 7 nm. The uniaxial anisotropy in thin (thick) CoO films is small (large), whereas the rotatable anisotropy is large (small). At low temperatures, the asymmetry in the magnetization reversal is due to instabilities at the antiferromagnet interface. Near the blocking temperature, a different asymmetry is due to a broad distribution of large grains. Near the latter temperature range, the smaller grains dissipate energy as evidenced by an increased rotational anisotropy and training of the hysteresis loop. All features can be understood qualitatively with a distribution of independent antiferromagnetic grains coupled to the ferromagnetic film.