Magnet
Many materials have unpaired electron spins, and the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as magnetic). Because of the way their regular crystalline atomic structure causes their spins to interact, some metals are ferromagnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well as the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring ferromagnets were used in the first experiments with magnetism. Technology has since expanded the availability of magnetic materials to include various man-made products, all based, however, on naturally magnetic elements.
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.