Particle size and temperature dependence of exchange bias in NiO nanoparticles

Zero-field cooled (ZFC) and field cooled (FC) magnetizations are measured for NiO nanoparticles of size 5–55 nm. Variations with particle size ( d ) and temperature of the coercivity ( H C ), exchange bias field ( H E ), coercivity enhancement ( Δ H C ) are discussed. The results are interpreted in terms of exchange bias (EB) models. H E and the ferromagnetic magnetization ( M FM ) arising from uncompensated surface spins of otherwise antiferromagnetic (AF) NiO show inverse trends when presented as functions of particle size. An interfacial exchange energy Δ σ of ∼0.03 erg/cm2, comparable to those of NiO/Co and NiO/permalloy bilayers, is observed. Finally, it has been found that the coercivity enhancement Δ H C = H C ( FC ) − H C ( ZFC ) scales with the interfacial exchange energy in the present system.