Mössbauer study of the high-temperature phase of Co-substituted magnetites, ${\mathrm{Co}}_{\mathit{x}}$${\mathrm{Fe}}_{3\mathrm{-}\mathit{x}}$${\mathrm{O}}_4$. I. x≤0.04

Six Co-substituted magnetites ${\mathrm{Co}}_{\mathit{x}}$ ${\mathrm{Fe}}_{3\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_4$, with x≤0.04 have been studied by ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectroscopy at temperatures T between their respective Verwey transition and ≊900 K. The Curie temperature ${\mathit{T}}_{\mathit{C}}$ decreases from 868 K for x=0 to 859 K for x=0.03. All spectra were decomposed into tetrahedral (A-site) and octahedral (B-site) components. The obtained results are consistent with the fact that at low temperatures above the Verwey transition, the spins lie along a domain’s [100] axis, but flip to a [111] axis at a temperature strongly increasing with increasing x. The temperature variations of the hyperfine parameters (center shift δ, quadrupole shift ${\mathrm{\varepsilon }}_{\mathit{Q}}$, magnetic hyperfine field ${\mathit{H}}_{\mathrm{hf}}$ and dipolar field ${\mathit{H}}_{\mathrm{dip}}$) have been determined. The δ and ${\mathit{H}}_{\mathrm{hf}}$ values for the octahedral Fe are consistent with the well-known fast electron exchange between the involved ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ species.