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 And Cation Site Ordering

Fe L-2,L-3-edge XAS and XMCD studies have been used to unravel structural trends in the MgAl2O4-Fe3O4 solid solution where thermodynamic modeling 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, Fe-Td(2+) is not present, which matches the ordered, inverse spinel nature of end-member magnetite (Mgt) at room temperature. However, in intermediate compositions Al and Mg substantially replace Fe and small amounts of Fe-Td(2+) are found, stabilized, or trapped by decreasing occurrence of the continuous nearest neighbor Fe-Fe interactions that facilitate charge redistribution by electron transfer. Furthermore, in the composition range similar to Mgt(0.4-0.9), XAS and XMCD bonding and site occupancy data suggest that nanoscale, magnetite-like Fe clusters are present. By contrast, at the spinel-rich end of the series, Mgt(0.17) and Mgt(0.23) 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 that would exsolve following development of long-range crystalline order during slow cooling.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.