Magnetism
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 "" refers to iron, because permanent magnetism was first observed in lodestone, a form of natural iron ore called magnetite, FeO.
Magnet
In all units, it is convenient to employ two types of magnetic field, B and H, as well as the magnetization M, defined as the magnetic moment per unit volume.Materials that are not permanent magnets usually satisfy the relation M = "χH in SI, where "χ" is the (dimensionless) magnetic susceptibility. Most non-magnetic materials have a relatively small "χ" (on the order of a millionth), but soft magnets can have "χ" on the order of hundreds or thousands. For materials satisfying M = "χH, we can also write B = "μ"(1 + "χ")H = "μ""μ"H = "μH, where "μ" = 1 + "χ" is the (dimensionless) relative permeability and μ =μμ is the magnetic permeability. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called hysteresis loops, which give either B vs. H or M vs. H. In CGS, M = "χH, but "χ" = 4"πχ", and μ = μ.