The Magnetic Properties of Naturally Occurring Goethite

Abstract

It has been found that goethite acquires a thermoremanent magnetization (TRM) when cooled in the presence of a magnetic field from 120°C. It is believed that this TRM is due to the presence of antiferromagnetism in goethite with a Neel temperature of 120°C. A few spins have no mates because of small grain size or the presence of imperfections and impurities. This TRM is related to antiferromagnetism. Similarly, hematite acquires a TRM at its Neel temperature which is due to unbalanced spins. In addition hematite is known to have a ‘parasitic ferromagnetism’. Hematite derived from heating goethite above 350°C showed two different types of behaviour. Hematite composed of particles less than 1/2μ diameter showed no spontaneous magnetization, but did show a weak thermoremanence whereas hematite composed of larger particles developed a spontaneous magnetization. These observations indicate that parasitic ferromagnetism exists only in grains larger than 1/2μ.