Order-disorder transition-induced twin domains and magnetic properties in ilmenite-hematite

The transition temperature (T_c) between disordered, $R3¯C$⁠, and ordered, $R3¯$⁠, ilmenite-hematite solid solutions in the ferrian ilmenite composition range between Ilm₆₀ and Ilm₈₀ has been redetermined by observing the presence or absence of transition-induced cation-ordered domains and the behavior of pre-existing domains annealed below the transition. The transition was reversed for Ilm₇₀ and is bracketed between 1000 and 1050 °C. The domains were shown by dark-field transmission electron microscopy to be twin related by a 180° rotation about an axis parallel to a and to vary in size depending on T_c and the temperature of quench.

A model of the twin-domain boundary indicates that such boundaries are disordered and partially Fe-enriched. Magnetic tem and observations on the same samples show that the room-temperature saturation magnetization is related to the surface area of the twin boundaries. Because the cation-ordered domains are ferrimagnetic with a strong magnetic moment and the disordered boundaries are probably antiferromagnetic with a weak magnetic moment, the quenched samples are essentially mixtures of two magnetic phases. The magnetization is therefore related to the proportion of each phase.

The twin boundaries act as the classic “x” phase, which allows ferrian ilmenite to acquire a self-reversed thermoremanent magnetization (trm). Our measurements indicate that when the surface area of the twin boundaries in Ilm₇₀ exceeds approximately 25 x 10⁶ m²/m³, the quenched samples acquire reverse trm during cooling in a 0.5-Oe field. When the boundary surface area is less than this critical threshold, the quenched samples acquire a normal trm.