Iron , cobalt, nickel, and various alloys of these materials are called ferromagnetic. Ferromagnetic materials can be permanently magnetized through exposure to an external magnetic field . They are strongly drawn towards a magnetic field. Their magnetic susceptibility, which is a material specific constant that relates applied field and magnetic response linearly, is orders of magnitude stronger than the susceptibility of paramagnetic or diamagnetic materials.
Paramagnetic materials are drawn towards magnets, while diamagnetic materials are repelled. Neither material can become permanently magnetized—or carry a remanent magnetization—and this is independent of temperature for all practical purposes. Their magnetic susceptibility is weakly positive and negative, respectively. The strength of a material's magnetic susceptibility is solely dependent on crystal structure. Paramagnetism usually dominates over diamagnetism. Most rock forming minerals are diamagnetic (e.g., quartz , limestone ) or paramagnetic (e.g., micas, amphiboles).
Ferromagnetic behavior is different from diamagnetism and paramagnetism in several respects. First, it is strongly dependent on temperature. A ferromagnet looses its ability to carry a remanent magnetization and simply become paramagnetic if heated above its specific Curie temperature. A second fundamental property of ferromagnets is hysteresis. Hysteresis means that the application of an external field changes a ferromagnet irreversibly. The magnetic state of a ferromagnet depends not only on the strength of an applied field, but also on the history of the magnet. Any applied field can produce four different magnetic answers in a ferromagnet, once it has been magnetized initially. Additional to their susceptibility, ferromagnets are characterized by their coercivity, which is proportional to the field strength necessary to remagnetize it, and by their saturation remanence.
The most important variants of ferromagnetism are ferromagnetism and antiferromagnetism. Magnetite is the most abundant representative of the first family. It is a product of abiotic geochemical processes. Pure magnetite can be grown inter- and extra-cellularly by bacteria. These iron-oxide minerals have different crystal lattices resulting in dramatically differing magnetic properties. An important antiferromagnetic mineral is goethite, which is a product of weathering processes.
The properties of ferromagnets are not only determined by their crystalline structure, but also depend strongly on the grain-size of a particle. Ferromagnets develop magnetic domains above a critical volume. They are not capable of having a remanent magnetization below this volume, in which case they are called superparamagnetic. Magnetization carried by particles just above the critical threshold are extremely stable, because these single domain particles can only be magnetized parallel to their long, easy, axis. A further increase of the particle volume leads to the development of an increasing number of domains that destabilizes the remanent magnetization.
Both palaeomagnetic and rock magnetic research use the ferromagnetic properties of rocks. Palaeomagnetism uses the fact that minute amounts of ferromagnets acquire a magnetization parallel to the magnetic field of the earth at the time of the rocks' formation. This naturally occurring magnetization of rocks can be used in plate tectonics and magnetostratigraphy to reconstruct the former distribution of tectonic plates and continents, and to date sedimentary sequences.
Rock magnetic research uses the fact that it is relatively easy to measure the ferromagnetic properties of rocks. Additionally, rock magnetism is a fast and non-destructive method. Because composition and grain size distribution of any assemblage of iron-oxide minerals is a highly sensitive indicator of past environmental change, rock magnetism has become ever more important in environmental and palaeoclimatic research. Today, environmental magnetism is routinely incorporated in research projects designed to understand the environmental history of a site, material or region.
See also Paleomagnetics