Magnetite reveals ambient field strength at low temperatures

Magnetite (Fe 3 O 4 ) is the most important and oldest known magnetic mineral on Earth (Figure l). We have come a long way from the magnetite loadstone compasses of ancient China; magnetite and titanomagnetite have been established as the principal carriers of geologically important remanent magnetizations in rocks, the study of which led to the plate tectonic paradigm. We now recognize that magnetite plays an important role in the biosphere. Some organisms contain intra‐cellular particles of Fe 3 O 4 that they use for spatial orientation and navigation. When preserved in rocks, these particles—called “magnetofossils”— can provide important insight into the origin and development of life here, and perhaps, on other planets [e.g., Thomas‐Keprta et al ., 2000]. Magnetite is now used in the medical field and in nanotechnology research. Nanoparticles of magnetite are used as a contrasting agent in magnetic resonance imaging and are being developed to deliver site‐specific drugs for the treatment of cancer [ Alexiou et al ., 2000]. Such applications add to a long list of industrial uses of magnetite that range from magnetic ink to magnetic recording media.