Tuning exchange bias in Fe/γ-Fe2O3 core-shell nanoparticles: Impacts of interface and surface spins

A comparative study has been performed of the exchange bias (EB) effect in Fe/gamma-Fe2O3 core-shell nanoparticles with the same thickness of the gamma-Fe2O3 shell (similar to 2 nm) and the diameter of the Fe core varying from 4 nm to 11 nm. Transmission electron microscopy (TEM) and high-resolution TEM confirmed the high quality of the core-shell nanostructures. A systematic analysis of magnetization versus magnetic field measurements under zero-field-cooled and field-cooled regimes using the Meiklejohn-Bean model and deconvoluting superparamagnetic and paramagnetic contribution to the total magnetic moment Langevin function shows that there exists a critical particle size (similar to 10 nm), above which the spins at the interface between Fe and gamma-Fe2O3 contribute primarily to the EB, but below which the surface spin effect is dominant. Our finding yields deeper insight into the collective contributions of interface and surface spins to the EB in core-shell nanoparticle systems, knowledge of which is the key to manipulating EB in magnetic nanostructures for spintronics applications. (C) 2014 AIP Publishing LLC.