Investigation on the nature of the Verwey Transition in Cu-doped Fe₃O₄

Full text: Magnetite (Fe₃O₄), the oldest known magnet, is still a hotly debated material in scientific research, due to its complex magnetic, electronic and transport properties. One of the most interesting physical phenomena associated with Fe₃O₄ is the occurrence of a metal-insulator transition at ~120 K (T_V), the so-called Verwey transition, which was associated with charge ordering below T_V, accompanied by a structural transition from the cubic phase to the monoclinic phase. However, due to the twinning of crystal domains, the detailed crystallographic structure is not fully solved yet and different charge ordered and bond-dimerized ground states have been proposed. In order to overcome this problem, we have investigated Cu-doped Fe₃O₄ to approach the problem through the determination of the phase diagram of Fe_1-xCu_xFe₂O₄. Using neutron diffraction and high resolution X-ray synchrotron diffraction we have investigated both the crystallographic and magnetic structure of Cu-doped Fe3O4 in order to elucidate the effect of doping on the Verwey transition. Data obtained from both complementary diffraction techniques indicate that the Verwey transition temperature and the magnetic structure, in particular the magnetic moment, remain unchanged up to high doping levels of 85% Cu-substitution. This is a surprising result at first glance and required a systematic investigation. The analysis of our high resolution X-ray synchrotron diffraction data allowed us to extract detailed information on the precise doping mechanism, including the distribution of Cu-ions between tetrahedral and octahedral sites in the spinel structure. The diffraction data therefore provide valuable information on the detailed mechanism behind the Verwey transition. (author)