Exchange bias training effect in NiFe2O4/NiO nanocomposites

Exchange bias field (HEB) accompanying vertical magnetization shift (?M) is observed in a granular system composed of ferrimagnetic (Ferri) NiFe2O4 nanoparticles embedded in an antiferromagnetic NiO matrix, after the sample is cooled from 350 to 10?K under a 40?kOe magnetic field. Consecutive hysteresis loops show that both HEB and ?M decrease with magnetic field cycling, which is referred to as the training effect. Furthermore, HEB shows a linear dependence on ?M throughout the training procedure, and HEB originates mainly from the cycle-dependent shift of the left coercivity (HC1) while the right coercivity (HC2) remains almost constant. This observed training effect is interpreted in the framework of the spin configurational relaxation model.