Suppression of the perpendicular anisotropy at the CoO Néel temperature in exchange-biased CoO/[Co/Pt] multilayers

We performed high field torque magnetometry measurements on CoO/[Co/Pt] magnetic multilayers that exhibit perpendicular exchange bias. We find that the antiferromagnet CoO layers strongly modify the uniaxial anisotropy of the multilayer structures. The strongest effects due to the CoO layers occur in the vicinity of the Néel temperature, where we observe a suppression of the first-order anisotropy and a smaller enhancement of the second-order anisotropy. This results in a nonmonotonic variation of the anisotropy with temperature and for selected samples a transition from perpendicular to in-plane and back to perpendicular anisotropy with increasing temperature.