An X-ray magnetic circular dichroism (XMCD) study of Fe ordering in a synthetic MgAl₂O₄ – Fe₃O₄ (spinel – magnetite) solid solution series; implications for magnetic properties.

Fe L₂,₃-edge XAS and XMCD studies have been used to unravel structural trends in the MgAl₂O₄-Fe₃O₄ solid solution where thermodynamic modelling has presented a challenge due to the complex ordering arrangements of the end-members. Partitioning of Fe³⁺ and Fe²⁺ between tetrahedral (Td) and octahedral (Oh) sites has been established. In the most Fe-rich samples, despite rapid quenching from a disordered state, Fe²⁺_Td is not present, which matches the ordered, inverse spinel nature of end-member magnetite (Mgt) at room-T. However, in intermediate compositions Al and Mg substantially replace Fe and small amounts of Fe²⁺_Td are found, stabilized or trapped by decreasing occurrence of the continuous nearest neighbour Fe – Fe interactions which facilitate charge redistribution by electron transfer. Furthermore, in the composition range ~Mgt₀.₄₋₀.₉, XAS and XMCD bonding and site occupancy data suggest that nano- scale, magnetite-like Fe clusters are present. By contrast, at the spinel-rich end of the series, Mgt₀.₁₇and Mgt₀.₂₃ have a homogeneous long-range distribution of Fe, Mg and Al. These relationships are consistent with the intermediate and Fe-rich samples falling within a wide solvus in this system such that the Fe- clusters occur as proto-nuclei for phases which would exsolve following development of long-range crystalline order during slow cooling. \ud Unit cell edges calculated from the spectroscopy-derived site occupancies show excellent agreement with those measured by X-ray powder diffraction on the bulk samples. Calculated saturation magnetic moments (M_s) for the Fe-rich samples also show excellent agreement with measured values but for the most Mg-rich samples are displaced to slightly higher values; this displacement is due to the presence of abundant Mg and Al disrupting the anti-parallel alignment of electron spins for Fe atoms