Curie temperatures (Tc) of the (Fe3O4)x(MgAl2O4)1–x solid solution have been determined from measurements of magnetic susceptibility (χ) vs. temperature. The trend in Tc vs. composition extrapolates to 0 K at x = 0.27. This behavior is rationalized in terms of the trend in cation distribution vs. composition suggested by Nell and Wood (1989), with Fe occurring predominantly on tetrahedral sites for x < 0.27.

High-temperature x-T curves are nonreversible because of the processes of cation ordering and exsolution, which occur in the temperature range 400–650 °C. The Curie temperature of single-phase material is shown to be sensitive to the state of nonconvergent cation order, with a difference in Tc of more than 70 °C being observed between a sample quenched from 1400 °C and the same sample after heating to 650 °C. This interaction between magnetic and chemical ordering leads to thermal hysteresis behavior such that Tc measured during heating experiments is approximately 10 °C higher than that measured during cooling. The hysteresis is due to a reversible difference in the state of cation order during heating and cooling caused by a kinetic lag in the cation-ordering behavior.

Samples with compositions in the range 0.55 < x < 0.1 undergo exsolution to a mixture of ferrimagnetic and paramagnetic phases after heating to 650 °C. Room-temperature hysteresis loops of the starting material and the high-temperature experiment products are compared. All starting materials are multidomain with coercivities Hc < 1.26 mT and Mrs/Ms < 0.051. Samples that exsolved during the experiments have coercivities up to 20 mT and Mrs/Ms up to 0.36. This change in hysteresis properties is caused by grain subdivision during exsolution and implies a transition in the magnetic domain state from multi- to single-domain.

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