Types of Neodymium NdFeB Magnets
A neodymium magnet (also known as NdFeB, NIB or Neo magnet), the most widely used type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed independently in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet commercially available. They have replaced other types of magnets in many applications in modern products that require strong permanent magnets, such as motors in cordless tools, hard disk drives and magnetic fasteners.
Neodymium is a metal which is ferromagnetic (more specifically it shows antiferromagnetic properties), meaning that like iron it can be magnetized to become a magnet, but its Curie temperature (the temperature above which its ferromagnetism disappears) is 19 K (−254 °C), so in pure form its magnetism only appears at extremely low temperatures.However, compounds of neodymium with transition metals such as iron can have Curie temperatures well above room temperature, and these are used to make neodymium magnets.
The strength of neodymium magnets is due to several factors. The most important is that the tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA ~7 T – magnetic field strength H in units of A/m versus magnetic moment in A·m2). This means a crystal of the material preferentially magnetizes along a specific crystal axis, but is very difficult to magnetize in other directions. Like other magnets, the neodymium magnet alloy is composed of microcrystalline grains which are aligned in a powerful magnetic field during manufacture so their magnetic axes all point in the same direction. The resistance of the crystal lattice to turning its direction of magnetization gives the compound a very high coercivity, or resistance to being demagnetized.
The neodymium atom also can have a large magnetic dipole moment because it has 4 unpaired electrons in its electron structure as opposed to (on average) 3 in iron. In a magnet it is the unpaired electrons, aligned so they spin in the same direction, which generate the magnetic field. This gives the Nd2Fe14B compound a high saturation magnetization (Js ~1.6 T or 16 kG) and typically 1.3 teslas. Therefore, as the maximum energy density is proportional to Js2, this magnetic phase has the potential for storing large amounts of magnetic energy (BHmax ~ 512 kJ/m3 or 64 MG·Oe). This magnetic energy value is about 18 times greater than “ordinary” magnets by volume. This property is higher in NdFeB alloys than in samarium cobalt (SmCo) magnets, which were the first type of rare-earth magnet to be commercialized. In practice, the magnetic properties of neodymium magnets depend on the alloy composition, microstructure, and manufacturing technique employed.
The Nd2Fe14B crystal structure can be described as alternating layers of iron atoms and a neodymium-boron compound. The diamagnetic boron atoms do not contribute directly to the magnetism, but improve cohesion by strong covalent bonding. The relatively low rare earth content (12% by volume) and the relative abundance of neodymium and iron compared with samarium and cobalt makes neodymium magnets lower in price than samarium-cobalt magnets.
Other Neodymium NdFeB Magnets
Maximum Working Temperature
Although the Curie temperature for NdFeB material is about 310 ºC for 0% cobalt material to greater than 370 ºC for 5% cobalt, some irreversible loss of output may be expected at even moderate temperatures. Neo magnets also have a moderately high Reversible Temperature Coefficient of Induction which reduces total magnetic output as temperature rises. Selection of neo magnets instead of SmCo, is a function of the maximum temperature of the application, required magnetic output at typical working temperature and total cost of the system.
Neo magnets also have some limitations due to their corrosion behavior. In humid circumstance, a protective coating or plating is highly recommended. Coatings which have been applied successfully include; e-coating, powder coating, nickel plating, zinc plating, parylene and combinations of these coatings.