Rare earth elements (lanthanide elements) are ferromagnetic metals, which means that they can be magnetized as iron as permanent magnets, but their Curie temperature (the temperature of its ferromagnetism disappears) is lower than room temperature, so their pure form magnetic properties only appear at low temperatures. However, they form compounds with transition metals such as iron, nickel, and cobalt, and the Curie temperature of some of these compounds is much higher than that of room temperature. The rare earth magnets
are made from these compounds.
The higher strength of rare-earth magnets is mainly due to two factors. First of all, their crystal structures have very high magnetic anisotropy. This means that the crystal of the material is preferentially magnetized along the specific crystal axis, but it is hard to magnetize in other directions. Just like other magnets, rare earth magnets
are composed of microcrystals, which are arranged in a powerful magnetic field during the manufacturing process, so their magnetic axis points to the same direction. The resistance of the lattice to its magnetization direction makes these compounds possess very high coercivity (anti magnetism).
Secondly, the atoms of rare earth elements can have high magnetic moments because their orbital electronic structures contain many unpaired electrons. In contrast to other elements, almost all electrons are in opposite spins in pairs, so their magnetic fields cancel out. This is because their magnetic field is offset. This is the result of full filling of the f shell. The f shell may contain as many as 7 unpaired electrons. In magnets, it is an unpaired electron that is aligned in the same direction to produce magnetic field. It makes the material have high remanence (saturation magnetization J s). Because the maximum energy density BH Max is proportional to J s 2, so these materials have the potential to store large amounts of magnetic energy. The magnetic energy product BH max of neodymium magnet is about 18 times larger than that of "ordinary" magnet. This makes the rare earth magnets smaller than other magnets with the same field strength.