The Verwey transition in magnetite as studied by means of definite impurity doping

Abstract

Abstract The effect of low-dose cation doping (0:005 < x < 0:08) of magnetite single crystals, Fe3–xMxO4 (M = Ni,Mg, Co, Al, Ti, Ga), has been studied by means of the magnetic after-effect (MAE) spectroscopy with respect to (i) the Verwey transition, (ii) the low-temperature (4 K < T < 125 K ≃ T V) charge transport mechanisms and (iii) the zero-crossing of the crystal anisotropy. The observed low-temperature shifting of the transition (T V) is in fair agreement with previous conductivity measurements. Variations of the MAE spectra clearly indicate the low-temperature tunnelling (4 K < T < 35 K) to be far more affected by smallest impurity doping than variable long-range hopping (50 K < T < 125 K) – this outstanding sensibility of the tunnelling processes against impurities or any other defects is also true when compared with the corresponding T V shifting. All samples undergo a doping-induced temperature splitting, ΔT VC, between the Verwey transition (spontaneous jump of the susceptibility at T V) and the zero-crossing of the crystal anisotropy (giving rise to a delayed susceptibility maximum) – in contrast to perfectly stoichiometric Fe3O4 single crystals where both effects are coincident. This range of temperature-splitting ΔT VC, found to be extremely large in the case of Co2+ doping, is characterized by destabilized magnetic domain structures due to locally disordered anisotropy distribution in the lattice.