Exchange bias effect in NiO/NiFe2O4 nanocomposites

A series of (100-x)NiO/(x)NiFe2O4 nanocomposites (x 0, 2.5, 5, 8.3, 12.5, 25) synthesized by a chemical coprecipitation method have been investigated. The exchange bias field H-E of the nanocomposites reaches a maximum at x = 2.5, and then decreases with increasing x. The decrease of H-E is attributed to the formation of isolated ferrimagnetic NiFe2O4 clusters, which is confirmed by observation with the use of high resolution transmission electron microscopy. The temperature dependence of H-E and the coercivity H-C for pure NiO is different from those with other samples, which is due to the exchange coupling between the uncompensated antiferromagnetic core and disordered surface shell of NiO nanoparticles. (C) 2011 American Institute of Physics. [doi:10.1063/1.3544505]