Magnetic properties of ilmenite70‐hematite30: Effect of transformation‐induced twin boundaries

Twin domains and boundaries arising from the order-disorder transition (Tcr ∼ 1025°C) in ilmenite70-hematite30 have little effect on intrinsic magnetic properties (TCurie, Jsaturation) but a large effect on extrinsic properties (isothermal remanent magnetization (IRM), thermoremanent magnetization (TRM), and alternating field (AF) and thermal demagnetization). At twin domain sizes between 800 and 1000 A, JIRM reaches a maximum, and the orientation of acquired TRM changes from reverse to normal, suggesting that the twin domains and magnetic domains are coincident below this size and the twin boundaries are also magnetic walls. Above 800–1000 A, free-standing magnetic walls can form, reducing the IRM. Wall pinning by the twin boundaries is also demonstrated by AF demagnetization of both IRM and TRM. Thermal demagnetization of TRM reconfirms the Fe-enriched nature of the twin boundaries (“x” phase), a necessary condition for self-reversal. Samples annealed within the miscibility gap (below 600°C) show increased Curie temperatures caused by compositional fluctuations (observed by transmission electron microscopy) forming within the two-phase field.