Magnetic properties of the magnetite-spinel solid solution: Curie temperatures, magnetic susceptibilities, and cation ordering

Curie temperatures (T-c) of the (Fe3O4)(x)(MgAl2O4)(1-x) solid solution have been determined from measurements of magnetic susceptibility (chi) vs. temperature. The trend in T-c 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 chi-T curves are nonreversible because of the processes of cation ordering and exsolution, which occur in the temperature range 400-650 degrees C. The Curie temperature of single-phase material is shown to be sensitive to the state of nonconvergent cation order, with a difference in T-c of more than 70 degrees C being observed between a sample quenched from 1400 degrees C and the same sample after heating to 650 degrees C. This interaction between magnetic and chemical ordering leads to thermal hysteresis behavior such that T-c measured during heating experiments is approximately 10 degrees 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 < 0.7 undergo exsolution to a mixture of ferrimagnetic and paramagnetic phases after heating to 650 degrees C. Room-temperature hysteresis loops of the starting material and the high-temperature experiment products are compared. All starting materials are multidomain with coercivities H-c < 1.26 mT and M(rs)/M(s) < 0.051. Samples that exsolved during the experiments have coercivities up to 20 mT and M(rs)/M(s) 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.