Silva, N. J. O. Amaral,V. S. Urtizberea,A. Bustamante,R. Millán,A. Palacio,F. Kampert,E. Zeitler,U. De Brion,S. Iglesias,A. A. Labarta,A. Shifted loops and coercivity from field-imprinted high-energy barriers in ferritin and ferrihydrite nanoparticles Phys. Rev. B, Condens. matter mater. phys. 10.1103/PhysRevB.84.104427 2011 2011