Effective anisotropy field variation of magnetite nanoparticles with size reduction J. M. Vargas, E. Lima Jr, R. D. Zysler, J. G.S. Duque, E. De Biasi and M. Knobel Eur. Phys. J. B, 64 2 (2008) 211-218 Published online: 25 July 2008 DOI: 10.1140/epjb/e2008-00294-6