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 (HE), coercivity enhancement (Delta 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 Delta sigma of similar to 0.03 erg/cm(2), comparable to those of NiO/Co and NiO/permalloy bilayers, is observed. Finally, it has been found that the coercivity enhancement Delta H-C = H-C (FC) - H-C (ZFC) scales with the interfacial exchange energy in the present system. (C) 2007 Elsevier Ltd. All rights reserved.