HEMATITE-ILMENITE (FE2O3-FETIO3) SOLID-SOLUTIONS - DETERMINATIONS OF FE-TI ORDER FROM MAGNETIC-PROPERTIES

The saturation magnetization, M(s)(T), of samples with compositions sp anning the ilmenite-hematite solid solution series has been measured a t temperatures from 77 to 298 K. These data have been used to extrapol ate to the saturation magnetization at 0 K. The saturation magnetizati on at 0 K is directly related to the distribution of Fe3+, Fe2+, and T i4+ between the A and B sublattices of the ilmenite structure and can, therefore, be used to determine the order parameter, Q(Ms), for each composition and annealing temperature. Samples with compositions x(ilm ) greater-than-or-equal-to 0.5 (x(ilm) = mole fraction ilmenite) have been annealed below the order-disorder transition and above the solvus (Burton and Davidson, 1988) to eliminate twin domain boundaries (TDBs ) and prevent the development of compositional modulations. Both types of microstructures have been shown to affect the saturation magnetiza tion ( Nord and Lawson, 1992). For samples with x(ilm) = 0.6, Q(Ms) va ries from 0 for samples quenched from 1573 K to 0.66 for samples annea led at 923 K. For samples with x(ilm) = 0.7, Q(Ms) varies from 0.53 fo r samples quenched from 1573 K to 0.66 for samples annealed at 1073 K. For samples with x(ilm) = 0.85, no variation in Q(Ms) occurs as a fun ction of annealing temperature (Q(Ms) = 0.6). The decreasing variation in Q(Ms) with annealing temperature as compositions become more ilmen ite rich may indicate that quenching the high-temperature disordered s tate becomes more difficult. For ilmenite, there appears to be no sign ificant disorder. This implies that for samples with compositions betw een x(ilm) = 0.85 and x(ilm) = 1.0, the degree of order must abruptly increase. High coercivities exhibited by samples with compositions x(i lm) = 0.6 and x(ilm) = 0.7 are related to the development of high TDB surface areas and short-range ordering, which result from quenching th rough the order-disorder transition. With subsequent annealing below t he order-disorder transition, coercivities are drastically reduced. Th is decrease in coercivity is correlated with a significant decrease in TDB surface area. The higher coercivities associated with composition s of x(ilm) = 0.2 and x(ilm) = 0.4 suggest that these samples may also develop some short-range order upon quenching.