Magnetic properties of some synthetic sub-micron magnetites

Abstract

The grain size dependence of various mineral (rock) magnetic parameters has been determined, using a series of essentially pure, fine-grained (single-domain, SD) and ultrafine-grained (superparamagnetic, SP) magnetites. The parameters measured include low-field susceptibility (x), frequency-dependent x(xFD), saturation remanence (SIRM), anhysteretic susceptibility (xARM), and coercivity of remanenceThe magnetites were produced in experiments designed to simulate possible pedogenic and biogenic pathways of magnetite formation. Their mean grain sizes range from 0.012 p m to 0.06 pm, and hence span the SP/SD boundary. Isothermal magnetic measurements were performed on two separate subsets of differing packing densities. The response of the magnetic parameters is modified by interaction effects, but they display continuous variation across the entire grain size range, confirming their value for rapid magnetic granulometry. Within the fine and ultrafine end of the magnetite grain size spectrum, x, xFD and xARM are notably responsive to grain size change. In terms of magnetic response (and also possibly of grain size, shape, and absence of cation substitution), these synthetic magnetites represent close analogues of those found in some soils and sediments.

How does Chinese loess become magnetized?

“…SP particles do not carry a stable remanence; therefore the P 1 component with a peak at 25 mT only reflects the concentration of magnetite/maghemite with SD size. Eyre (1996) and Spassov et al (2003a, b) reported higher coercivities of 30-35 mT for the low coercivity component in Chinese paleosols, but our value is consistent with the 21-29 mT range reported for synthetic SD magnetite (Maher, 1988). The high surface area to volume ratio of pedogenic particles makes them easy to oxidize into maghemite (van Velzen and Dekkers, 1999;Liu et al, 2003), which might explain the slightly higher coercivities documented by Eyre (1996) and Spassov et al (2003a, b).…”

Magnetic mineralogy of soils across the Russian Steppe: climatic dependence of pedogenic magnetite formation

“…High percentages ( s 6%) of frequency-dependent susceptibility (measured at 0.47 and 4.7 kHz) re£ect the presence of signi¢cant numbers of superparamagnetic (SP) ferrimagnetic grains, with grain diameters 6 V20 nm (e.g. Bean and Livingston, 1959;Dunlop, 1981;Maher, 1988;Dearing et al, 1996). Fig.…”

Formation of magnetite‐enriched zones in and offshore of a mesotidal estuarine lagoon: An environmental magnetic study of Tauranga Harbour and Bay of Plenty, New Zealand

“…Magnetic susceptibility is often used as an indicator for ferrimagnetic mineral concentration due to the particle size independence of this parameter [Heider et al, 1996] and because para-and antiferromagnetic minerals generally make a minor magnetic contribution to susceptibility [Thompson and Oldfield, 1986]. Magnetite percentage was calculated considering a value of 660 Â 10 À6 m 3 kg À1 for pure multidomain magnetite [Maher, 1988]. The ARM/ IRM ratio is a common magnetic grain size proxy, which is proportional to the relative content of single domain (SD) particles e.g., for equidimensional magnetite, the (30-100 nm grain size range) and pseudo-single domain (PSD) magnetite (i.e., the 100 nm-1 mm size range) [Muxworthy and Williams, 2006].…”