Comment on:: Exchange bias and vertical shift in CoFe2O4 nanoparticles [J. Magn. Magn. Mater. 313 (2007) 266]

In recent paper Mumtaz et al. [J. Magn. Magn. Mater. 313 ( 2007) 266] have reported magnetic characterization of CoFe2O4 nanoparticles prepared by co-precipitation. The authors have interpreted the horizontal shift of the hysteresis loops, traced after magnetic field-cooling, as a result of magnetic exchange between surface spins (spin-glass-like) and core (ferrimagnetic) spins. The observed vertical shift of these loops has been attributed to strongly pinned uncompensated spins at the core-shell interface. The present comment demonstrates that the work of Mumtaz et al. is deficient in proofs supporting their statements. A very simple and natural reason for the appearance of both shifts is the fact that the samples measured at low temperatures are unsaturated, i.e., the authors have actually measured minor hysteresis loops which are, unsurprisingly, shifted along both. field and magnetization axes. (c) 2007 Elsevier B.V. All rights reserved.