A detailed study was made of the magnetic behaviour, particularly at the Verwey transition, for some stoichiometric magnetite samples as a function of their shape (film/bulk) and crystalline structure (single/polycrystal). The travelling solvent float-zone technique was first used to obtain crystalline bulk (110) oriented magnetite samples, while molecular beam epitaxy permitted the growth of a 68nm Fe3O4 (111) crystalline film on a α-Al2O3(0001) substrate. A 64nm Fe3O4 polycrystalline film, grown onto an Si/SiO2 substrate by using DC reactive magnetron sputtering method, completed the sample series. An anomalous magnetisation behaviour was seen in vicinity of the Verwey temperature and was shown to depend upon the applied magnetic field. A magneto-electronic model explained the origin of this behaviour. The maximum in the demagnetisation at the Verwey transition was very near to 0.1T. The amount and nature of structural defects were proved to determine the magnetisation process, and the approach to saturation was well-described by the M = MS(1-b/Bn) relationship; with n = 2 in bulk magnetite crystals and n = 0.5 in epitaxial magnetite films. These dependences existed at 10 to 300K; excluding the isotropic point (near 130K), and demonstrated that the antiphase boundaries were responsible for the much more progressive approach to saturation observed in thin films than in bulk samples. In epitaxial films, n was found to decrease down to 0.33 at the isotropic point; due to an enhanced super-exchange interaction through antiphase boundaries.

Study of the Electronic Paraprocess and Antiphase Boundaries as Sources of the Demagnetisation Phenomenon in Magnetite. F.Delille, B.Dieny, J.B.Moussy, M.J.Guittet, S.Gota, M.Gautier-Soyer, C.Marin: Journal of Magnetism and Magnetic Materials, 2005, 294[1], 27-39