Low-temperature phase transitions of Fe3O4 with innocuous dopants

Abstract

Both Zn2+ and Cd2+ dopants on the A-sites in Fe3O4 lead to a loss in the furcation of the lambda anomaly at the Verwey transition but at markedly different dopant levels. Equilibrium adiabatic calorimetry from 5 to 350 K on new compositions of cadmium- and zinc-doped magneties, e.g. Cd0.002Fe2.998O4 and Zn0.01Fe2.99O4, permits an estimate of the mole ratios of Cd and Zn at which they have equivalent effects on the furcation of the lambda anomaly. These results are consistent with a mechanism in which the furcation of the Verwey transition in doped samples depends linearly on the deviation of the lattice constant from that of pure Fe3O4. Further studies on a deliberately oxidized sample demonstrate that the furcated lambda anomaly is characteristic of stoichiometric materials; apparently B-site vacancies have a much greater effect on the transition than A-site dopants. Heat capacities and thermodynamic properties of the three samples are summarized.