Exchange bias and nanoparticle magnetic stability in Co-CoO composites

A series of Co-x(CoO)(1-x) thin films have been synthesized by sputtering Co in an oxygen atmosphere using different rf sputtering powers as a means to vary the concentration x. The highest exchange-bias field (H-E approximate to 2.7 kOe at T=90 K) was measured in a sample with an estimated metallic Co fraction of x=0.53. In this sample, and in those with lower Co concentrations, the temperature dependence of the magnetization shows a pronounced dip, both in zero-field- and field-cooled curves, at temperatures close to the Neel temperature of bulk CoO. This feature, which disappears with moderate annealing, is associated to the destabilization of small isolated Co nanoparticles when the exchange coupling with the CoO matrix vanishes above its Neel temperature. A simple structural model is proposed to account for the presented magnetization. X-ray diffraction and electrical resistivity results are also presented, which support the studies of magnetic properties.