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 (TV≈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 TV. 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 TV, in agreement with micromagnetic calculations. Coercive force Hc increases on cooling through TV, 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 TV 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 TV, with minor recovery in cooling back through TV (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 TV itself. However, the submicron magnetites lose a further 5–15% of their remanence when reheated through TV. These irreversible changes in cycling across TV, 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 TV, which preserve or enhance remanence, while SD→2D transformations on warming through TV cause remanence to demagnetize.