Changes in remanence, coercivity and domain state at low temperature in magnetite

Submicron magnetite crystals with mean sizes of 0.037, 0.10 and 0.22 μm undergo major changes in hysteresis properties and domain states in crossing the Verwey transition (T_v ≈ 120 K). The 0.037 μm crystals are single-domain (SD) both in the cubic phase at room temperature T₀ and in the monoclinic phase below T_v. The 0.10 and 0.22 μm crystals have a mixture of SD and two-domain (2D) states at room temperature T₀, but mainly SD structures below T_v, in agreement with micromagnetic calculations. Coercive force H_c increases on cooling through T_v, by a factor 3-5 in the submicron magnetites and 40 in a 1.3 mm single crystal, because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite. As a result, domain walls and SD moments are so effectively pinned below T_v that all remanence variations in warming or cooling are reversible. However, between ≈ 100 K and T₀, remanence behavior is variable. Saturation remanence (SIRM) produced in monoclinic magnetite at 5 K drops by 70-100% in warming across T_v, with minor recovery in cooling back through T_v (ultimate levels at 5 K of 23-37% for the submicron crystals and 3% for the 1.3 mm crystal). In contrast, SIRM produced in the cubic phase at 300 K decreases 5-35% (submicron) or >95% (1.3 mm) during cooling from 300 to 120 K due to continuous re-equilibration of domain walls, but there is little further change in cooling through T_v itself. However, the submicron magnetites lose a further 5-15% of their remanence when reheated through T_v. These irreversible changes in cycling across T_v, and the amounts of the changes, have potential value in determining submicron magnetite grain sizes. The irreversibility is mainly caused by 2D→SD transformations on cooling through T_v, which preserve or enhance remanence, while SD→2D transformations on warming through T_v cause remanence to demagnetize.