Neodymium magnets use an alloy of neodymium, iron and boron with the composition Nd2Fe14B, from which the strongest permanent magnets are currently produced. It is one of the rare earth magnet materials and was developed in 1982. Rare earth magnets are a group of permanent magnets consisting mainly of ferrous metals (iron, cobalt, nickel) and rare earth metals (neodymium, samarium, praseodymium, dysprosium, terbium, gadolinium). Yttrium can also take on the role of a rare earth metal. They are characterized by the fact that they have a high magnetic remanence flux density and a high magnetic coercive field strength and thus a high magnetic energy density at the same time.
The most important intermetallic compounds are the low-cost neodymium-iron-boron (Nd2Fe14B) and samarium cobalt (SmCo5 and Sm2Co17). In addition, samarium-iron-nitrogen (Sm2Fe18N3) is also used commercially. The basic alloys are varied in order to achieve special properties and structures – for example, by substituting neodymium in Nd2Fe14B partly by praseodymium, dysprosium, terbium and iron partly by cobalt – or by disturbing the structure by foreign atoms such as aluminum, titanium, zirconium, copper or manganese. Finally, Nd2Fe14B requires an enrichment of rare earths between the crystals in order to achieve an acceptable coercivity field.
Mode of Action
The rare earths (lanthanides) are already ferromagnetic in themselves, but their Curie temperatures are below room temperature, so that if they are present in elemental form, their magnetism can only be observed at low temperatures. The atoms of the elements of the rare earths have high magnetic moments. This is a consequence of the incomplete filling of the 4f shell, which consists of up to seven unpaired electrons with aligned spins. Electrons in such orbitals are highly localized and therefore easily retain their magnetic moments and function as paramagnetic centers. Magnetic moments in other orbitals are often lost due to the strong overlap with the neighbors, e.g., electrons in covalent bonds form pairs with zero net spin.
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However, the magnetic polarization of rare earth magnets is essentially based on the spins of the 3D shells of the ferrous metals. The rare earth metals stabilize the magnetic orientation of the 3D shells because their crystalline structures exhibit a very high magnetic anisotropy. This means that a crystal made of the material is magnetized in a certain direction, but resists being magnetized in another direction.
The high magnetic moments at the atomic level combined with a stable orientation, i.e. a high magnetic anisotropy, result in a high magnetic coercive field strength at the macroscopic level.
Commercially available neodymium magnets are denoted by an N followed by a number. The number represents the magnetic strength of the permanent magnet. Usual values are in the range of N35 to N50. The following letter indicates an increased permissible operating temperature (gradations M–H–UH or similar are common), which corresponds to a higher coercivity at the same temperature.
Safety Precautions (for DIY Works)
Due to the high strength of the magnets, otherwise rather unexpected dangers arise. In the case of larger neodymium magnets, this includes, in particular, crushing due to improper handling and non-observance of safety distances to ferromagnetic materials such as iron or other magnets in the immediate vicinity. The adhesion forces of a neodymium disc with a diagonal of about 10cm and a thickness of about 1.5 cm can reach some 1000 N.
Smaller neodymium magnets are available in the relevant trade in the form of balls or cuboids and are used, among other things, for play or decoration purposes. If more than one small neodymium magnet is swallowed, there is a risk of death due to a possible intestinal perforation.
The strong magnetic field can damage or erase magnetic recordings even from a distance. During machining operations such as filing, sawing or drilling, neodymium magnet dusts and chips can ignite due to the heat generated during machining. In addition, the chips are not easy to separate from the base body (and possibly tools).
The material is prone to sharp-edged splintering, which is why appropriate protective clothing and goggles should be worn when processing neodymium magnets. Even if two magnets snap together unchecked, this breakage behavior can lead to dangerous situations.
