Magnetism is a class of physical phenomena that are mediated by
magnetic fields.
Electric currents and the
magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. The most familiar effects occur in
ferromagnetic materials, which are strongly attracted by magnetic fields and can be
magnetized to become permanent
magnets, producing magnetic fields themselves. Only a few substances are ferromagnetic; the most common ones are
iron,
nickel and
cobalt and their alloys. The prefix
ferro- refers to
iron, because permanent magnetism was first observed in
lodestone, a form of natural iron ore called
magnetite, Fe3O4.