Exchange bias in Co/Co3O4 bilayers

Abstract

Exchange biasing effect in Co/Co3O4(t) bilayers was carefully investigated by a vibrating sample magnetometer. It was surprising that a significant exchange bias effect appeared when the temperature varied from 90 to 220 K. In this temperature range bulk Co3O4 should be in a paramagnetic state because its Néel temperature is about 40 K. The exchange bias field (HE) of Co/Co3O4 fluctuated in the range of 110–130 Oe with various thickness of Co3O4 at 90 K and HE was zero when the temperature was above 220 K. X-ray photoelectron spectroscopy results indicated that a thin CoO layer formed near the interface, which may be due to the interdiffusion between Co and Co3O4 layers. Therefore, the exchange bias effect in Co/Co3O4 bilayers is thought to be dominated by the exchange coupling between the Co and CoO layers. Using the finite-size effect theory, we evaluated the average thickness of interfacial CoO layer to be 39 Å. The fluctuation of HE in Co/Co3O4 is probably due to thickness fluctuations of the CoO. Assuming that HE is inversely proportional to the thickness of CoO, we can conclude that the approximate thickness range of CoO is from 36 to 43 Å.