Magnetic properties of some synthetic sub-micron magnetites

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 (χ), frequency-dependent χ(χFD), saturation remanence (SIRM), anhysteretic susceptibility (χARM), and coercivity of remanence ((BO)CR). The magnetites were produced in experiments designed to simulate possible pedogenic and biogenic pathways of magnetite formation. Their mean grain sizes range from 0.012 µm to 0.06 µm, 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, χ, χFD and χARM 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.