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Magnetic Properties

It is often assumed that magnetite is “magnetic,” i.e., it acts as a magnet and will attract ferrous metals or neighboring magnetite particles. However, this is not in fact the case. Magnetite is ferrimagnetic, meaning that it is attracted to a magnet but is not itself magnetic (Dionne 2009). Filler-grade magnetite is a free-flowing powder which makes shipping and handling straightforward. Magnetite also has a low remanence so that once removed from a magnet, it does not readily remain magnetized. Lodestone is a rare form of naturally occurring magnetite which is a weak magnet and was the basis for the first compasses. The Curie temperature of magnetite is ^585 °C or ^1,085 °F. Above that temperature, it is no longer attracted to a magnet. Once cooled down below the Curie temperature, it is once again susceptible to a magnetic field.

Magnetite is the most magnetic (i.e., strength of attraction to a magnet) of all the naturally occurring minerals (Harrison et al. 2002) with a saturation magnetization of 480 G at room temperature (Pullaiah et al. 1975). For nanoparticles, that value can be higher with a maximum value reported to be 10 nm diameter particles (Thapa et al. 2004). It can be blended into polymers and coatings to impart magnetic susceptibility (Kong et al. 2010a). An example is magnetic paint available from several manufacturers. Magnetite also finds application in automotive mats where it provides vibration- and sound-damping properties due to its high density combined with the ability to place the mats precisely using robots equipped with electromagnets. One of the best-known uses of magnetite is in ferrofluids which are colloidal dispersions that can be manipulated with a magnetic field. The first reported such fluids were made by NASA by milling natural magnetite in the presence of oleic acid as a dispersant (Papell 1965). Ferrofluids and magnetorheological fluids are now made using synthetic nano-particulate magnetite.

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