Exchange bias and vertical shift in CoFe2O4 nanoparticles

Magnetic properties of core-shell cobalt ferrite nanoparticles prepared by co-precipitation route in the range 15-48 nm have been studied. It is shown that the coercivity follows non-monotonic size dependence and exhibits a peak at around 26 nm. Field-cooled magnetization exhibited both horizontal (exchange bias) and vertical shifts. The exchange bias is understood as originating at the interface between a surface region (with structural and spin disorder) and a core ferrimagnetic region. The dependence of the exchange bias and vertical shift on the particle size and cooling field is found to have significant differences. These differences are explained in the light of recent results that suggest that there is a variation of the pinning strength amongst the interface spins and the vertical shift is affected by the more strongly pinned uncompensated spins. (c) 2007 Elsevier B.V. All rights reserved.