Controlling the exchange bias field in Co core/CoO shell nanoparticles

We have determined how the anomalous exchange bias effect in Co/CoO nanoparticles of 11 nm in diameter depends on the Co core and CoO shell dimensions. The oxidation of the Co nanoparticles used in this study is carefully controlled, yielding highly crystalline and oriented interfaces. The dimensions of the core and shell are determined from magnetization and small angle x-ray scattering measurements. The exchange bias field in Co/CoO core-shell nanoparticles depends nonmonotonically on the CoO shell thickness, reaching a maximum value of 7 kOe at 30 K when the core and shell dimensions are similar. We propose that lattice strain induces a net moment at the core-shell interface, and it is the variation of this moment with strain, which is responsible for the vanishing of HEB at both large and small Co shell thicknesses.