Magnet
Rare earth (lanthanoid) elements have a partially occupied "f" electron shell (which can accommodate up to 14 electrons). The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore, these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare-earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets.
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.