The Magnetization Process In Monoclinic Pyrrhotite (Fe₇S₈) Particles Containing Few Domains

Abstract

Monoclinic pyrrhotite was prepared by the silica tube technique. All characteristic properties (X-ray, magnetic) point to the successful synthesis of Fe&. The material was separated into seven size fractions in the particle size range 1-30 pm, in which the particles are believed to contain only few domains. The magnetization process was studied at room temperature. The characteristics of the finest fraction (<0.8pm) are consistent with coherent rotations of monodomain moments and a triaxial symmetry of anisotropy within the basal plane. The in-plane anisotropy constant is about 3 x lo3 J m-3. In the sized fractions (1-30 pm) the variation of ratio of isothermal remanence to saturation magnetization (M,/M,) can be fitted by models of increasing complexity based on the statistically expected number of domains in particles of a particular size. The data can also be fitted by (Mrs/MS) = 0.52exp (-0.047z1'2) where is the mean particle size in pm. The form of this expression is suggestive of the relevance of Poisson statistics to the magnetization process, but the numerical parameter within the exponential is physically implausible. Unlike titanomagnetite (Fe2.4Ti0.604) particles of the same size range, the variation of coercive force (H,) cannot be modelled on the basis of independent surface and volume pinning of domain walls. There is evidence for only one wall pinning mechanism in the 1-30pm range in Fe,S,. A pleasing fit to the data is obtained by H, = 75.5 exp (-0.025t1'2) kA m-'. A similar dependence is found for H,,, the coercive force of isothermal remanence. The susceptibility in the 1-30 pm range appears to be essentially the 'intrinsic' susceptibility and, if so, again provides evidence for only one wall pinning mechanism.

Rock Magnetic Properties of Hydrothermally Formed Iron Sulfides from Middle Valley, Juan de Fuca Ridge

“…A hysteresis loop for Sample 139-856H-6R-1, 22-24 cm, containing mainly monoclinic pyrrhotite, is shown in Figure 4. The rectangle shape is typical for monoclinic pyrrhotites and has been observed previously (e.g., Menyeh and O'Reilly, 1991 (Table 2). Saturation remanence varies between about 9 and 4000 mA × m 2 /kg.…”

“…The grain size of pyrrhotite can be estimated from B c and B cr values. A comparison with the results of samples investigated by Clark (1984), Dekkers (1988), and Menyeh and O'Reilly (1991) points to magnetic grain sizes of pyrrhotites between 10 and 50 µm.…”

Magnetic properties of synthetic analogues of pyrrhotite ore in the grain size range 1-24 m

“…4, the samples could not be saturated completely in a field of 800 kA m −1 . Similar effects have also been observed by Dekkers (1988) and Menyeh & O’Reilly (1991). It is expected that the pyrrhotites are first saturated in the basal plane of easy magnetization.…”

“…Schwarz (1975). Magnetic domains of natural pyrrhotites in a rock matrix have been studied by Soffel (1977, 1981) and Halgedahl & Fuller (1981, 1983), while the coercive force and other hysteresis parameters that are dependent on grain size have been measured for ore samples by Clark (1984), Dekkers (1988) and Worm et al (1993), and for synthetics by Menyeh & O’Reilly (1991, 1997) at room temperature. Measurements at elevated temperatures have been carried out by Menyeh & O’Reilly (1995).…”

Magnetic Hysteresis Properties of Fine Particles of Monoclinic Pyrrhotite Fe7S8.

“…The resultant value of 18.3 ± 0.3 A m2kg-1 is consistent with that expected from the magnetic spin structure of Fe7S8, the magnetically hard c-axis of the crystal, and the fields employed here (Menyeh and O'Reilly, 1991). The hysteresis parameters coercive force (Hc) of 24 kA m-1, and ratio of remanence to saturation (Mrs/MS) of 0.386 correspond to the range of grain size (about 1-50 µm) in the crushed, but undifferentiated sample.…”