Strong anisotropy suppression at the CoO Ne\'{e}l temperature in perpendicularly exchange-biased CoO/CoPt multilayers

We have performed high-field torque magnetometry measurements on perpendicularly exchange-biased CoO/CoPt multilayers. From magnetometry measurements we observe perpendicular exchange bias that goes to zero at a blocking temperature of 200 K. Torque measurements were performed from 2 to 400 K in fields up to 9 T. There is significant hysteresis even at 9 T fields suggesting instabilities in the AF layer. This hysteresis persists up to the blocking temperature. Surprisingly, there is a large reduction in the uniaxial anisotropy with increasing temperature with a minimum at 250 K, the estimated Ne\'{e}l temperature of the CoO layers, followed by an increase in the anisotropy with increasing temperature. The anisotropy of Co/Pt multillayers without the CoO layers monotonically decreases with increasing temperature. This suggests that there is an additional planar anisotropy arising from the CoO that counters the anisotropy of the Co/Pt layers, and that the contribution from the CoO is maximized at the Ne\'{e}l temperature as observed by Grimsditch et al. [1]. These results show that the addition of antiferromagnetic layers may be used to tune the temperature dependence anisotropy response of magnetic systems. [1] M. Grimsditch et al., Phys. Rev. Lett. \textbf{90}, 257201 (2003).