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

Abstract

Submicron magnetite crystals with mean sizes of 0.037, 0.10 and 0.22 Wm undergo major changes in hysteresis properties and domain states in crossing the Verwey transition (T V W120 K). The 0.037 Wm crystals are single-domain (SD) both in the cubic phase at room temperature T 0 and in the monoclinic phase below T V . The 0.10 and 0.22 Wm crystals have a mixture of SD and two-domain (2D) states at room temperature T 0 , 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 W100 K and T 0 , 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 s 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 51 5% 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 2DCSD transformations on cooling through T V , which preserve or enhance remanence, while SDC2D transformations on warming through T V cause remanence to demagnetize. ß

Magnetic Properties of Bacterial Nanoparticles

“…Coercive force is practically temperature independent above the Verwey transition (105 K) and increases by factor of 4 below this transition. The increase in H c in the vicinity of T v has been observed previously [3] for submicron magnetite nanoparticles with medium particle sizes ≤ 0.22 µm. This increase reflects the large increase in magnetocrystalline anisotropy in the cubicmonoclinic phase transition.…”

“…The results from measurements of remanence and coercive force measured in the monoclinic phase (below T v ) for fine grained magnetites with medium particle sizes of 0.037, 0.10 and 0.22 µm have showed that coercive force H c and remanence magnetization increase on cooling through T v in submicron magnetite particles [3]. These increases reflect the large increase in magnetocrystalline anisotropy and magnetostriction in the cubic → monoclinic transformation.…”

Rock magnetic characterization of ferrimagnetic iron sulfides in gas hydrate-bearing marine sediments at Site C0008, Nankai Trough, Pacific Ocean, off-coast Japan

“…4a; Muxworthy and McClelland 2000;Özdemir et al 2002). During warming of the RT-SIRM, the remanence is not fully recovered when passing the temperature of the Verwey transition.…”

Burial Diagenesis of Magnetic Minerals: New Insights from the Grès d’Annot Transect (SE France)

“…These measurements allow the recognition of the magnetic minerals based on their characteristic magnetic behavior below room temperature. The most common observation in the studied samples is the drop of the remanence acquired at room temperature (RT-SIRM) at 110-120 K, which corresponds to the Verwey transition of magnetite [47,48]. This feature is observed throughout the entire studied transect from SW to NE.…”