A neodymium magnet (also known as NdFeB, NIB or Neo magnet) is a permanent magnet made from an alloy of neodymium, iron, and boron that forms the Nd2Fe14B tetragonal crystalline structure. They are the most widely used type of rare-earth magnet.

Developed independently in 1984 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet available commercially. They have replaced other types of magnets in many applications in modern products that require strong permanent magnets, such as electric motors in cordless tools, hard disk drives and magnetic fasteners.

NdFeB magnets can be classified as sintered or bonded, depending on the manufacturing process used.Sintered NdFeB Magnets, What are Sintered NdFeB Magnets?Bonded NdFeB Magnets, What are Bonded NdFeB Magnets?

History

General Motors (GM) and Sumitomo Special Metals independently discovered the Nd2Fe14B compound almost simultaneously in 1984. The research was initially driven by the high raw materials cost of samarium-cobalt permanent magnets (SmCo), which had been developed earlier. GM focused on the development of melt-spun nanocrystalline Nd2Fe14B magnets, while Sumitomo developed full-density sintered Nd2Fe14B magnets.Chu, Steven. Critical Materials Strategy United States Department of Energy, December 2011. Accessed: 23 December 2011.

GM commercialized its inventions of isotropic Neo powder, bonded neo magnets, and the related production processes by founding Magnequench in 1986 (Magnequench has since become part of Neo Materials Technology, Inc., which later merged into Molycorp). The company supplied melt-spun Nd2Fe14B powder to bonded magnet manufacturers. The Sumitomo facility became part of Hitachi, and has manufactured but also licensed other companies to produce sintered Nd2Fe14B magnets. Hitachi has held more than 600 patents covering neodymium magnets.

Chinese manufacturers have become a dominant force in neodymium magnet production, based on their control of much of the world's rare-earth mines.

The United States Department of Energy has identified a need to find substitutes for rare-earth metals in permanent magnet technology and has funded such research. The Advanced Research Projects Agency-Energy has sponsored a Rare Earth Alternatives in Critical Technologies (REACT) program, to develop alternative materials. In 2011, ARPA-E awarded US$31.6 million to fund rare-earth substitute projects. Because of its role in permanent magnets used for wind turbines, it has been argued that neodymium will be one of the main objects of geopolitical competition in a world running on renewable energy. This perspective has been criticized for failing to recognize that most wind turbines do not use permanent magnets and for underestimating the power of economic incentives for expanded production.

Properties

Magnetic properties

In its pure form, neodymium has magnetic properties—specifically, it is antiferromagnetic, but only at low temperatures, below . However, some compounds of neodymium with transition metals such as iron are ferromagnetic, with Curie temperatures well above room temperature. These are used to make neodymium magnets.

The strength of neodymium magnets is the result of several factors. The most important is that the tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA ≈ 7T – 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 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 that their spin is in the same direction, which generate the magnetic field. This gives the Nd2Fe14B compound a high saturation magnetization (Js ≈ 1.6T or 16kG) and a remanent magnetization of 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 ≈ 512kJ/m3 or 64MG·Oe).

This magnetic energy value is about 18 times greater than "ordinary" ferrite magnets by volume and 12 times by mass. This magnetic energy 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, 26.7% by mass) and the relative abundance of neodymium and iron compared with samarium and cobalt makes neodymium magnets lower in price than the other major rare-earth magnet family, samarium–cobalt magnets.

Although they have higher remanence and much higher coercivity and energy product, neodymium magnets have lower Curie temperature than many other types of magnets. That Nd2Fe14B maintains magnetic order up to beyond room temperature has been attributed to the Fe present in the material stabilising magnetic order on the Nd sub-lattice. Special neodymium magnet alloys that include terbium and dysprosium have been developed that have higher Curie temperature, allowing them to tolerate higher temperatures than those alloys containing only Nd.

+ Magnetic properties of various permanent magnets Magnet Br (T) Hci (kA/m) BHmax (kJ/m3) TC (°C) (°F) Nd2Fe14B, sintered 1.0–1.4 750–2000 200–440 310–400 590–752 Nd2Fe14B, bonded 0.6–0.7 600–1200 60–100 310–400 590–752 SmCo5, sintered 0.8–1.1 600–2000 120–200 720 1328 Sm(Co, Fe, Cu, Zr)7, sintered 0.9–1.15 450–1300 150–240 800 1472 Alnico, sintered 0.6–1.4 275 10–88 700–860 1292–1580 Sr-ferrite, sintered 0.2–0.78 100–300 10–40 450 842

Physical and mechanical properties

+ Comparison of physical properties of sintered neodymium and Sm-Co magnetsTypical Physical and Chemical Properties of Some Magnetic Materials, Permanent Magnets Comparison and Selection. Property Neodymium Sm-Co Remanence (T) 1–1.5 0.8–1.16 Coercivity (MA/m) 0.875–2.79 0.493–2.79 Recoil permeability 1.05 1.05–1.1 Temperature coefficient of remanence (%/K) −(0.12–0.09) −(0.05–0.03) Temperature coefficient of coercivity (%/K) −(0.65–0.40) −(0.30–0.15) Curie temperature (°C) 310–370 700–850 Density (g/cm3) 7.3–7.7 8.2–8.5 Thermal expansion coefficient, parallel to magnetization (1/K) (3–4)×10−6 (5–9)×10−6 Thermal expansion coefficient, perpendicular to magnetization (1/K) (1–3)×10−6 (10–13)×10−6 Flexural strength (N/mm2) 200–400 150–180 Compressive strength (N/mm2) 1000–1100 800–1000 Tensile strength (N/mm2) 80–90 35–40 Vickers hardness (HV) 500–650 400–650 Electrical resistivity (Ω·cm) (110–170)×10−6 (50–90)×10−6

Corrosion

Sintered Nd2Fe14B tends to be vulnerable to corrosion, especially along grain boundaries of a sintered magnet. This type of corrosion can cause serious deterioration, including crumbling of a magnet into a powder of small magnetic particles, or spalling of a surface layer.

This vulnerability is addressed in many commercial products by adding a protective coating to prevent exposure to the atmosphere. Nickel, nickel-copper-nickel and zinc platings are the standard methods, although plating with other metals, or polymer and lacquer protective coatings, are also in use.

Temperature sensitivity

Neodymium has a negative coefficient, meaning the coercivity along with the magnetic energy density (BHmax) decreases as temperature increases. Neodymium-iron-boron magnets have high coercivity at room temperature, but as the temperature rises above , the coercivity decreases drastically until the Curie temperature (around ). This fall in coercivity limits the efficiency of the magnet under high-temperature conditions, such as in wind turbines and hybrid vehicle motors. Dysprosium (Dy) or terbium (Tb) is added to curb the fall in performance from temperature changes. This addition makes the magnets more costly to produce. The temperature dependence of the material's magnetic properties can be modelled within electronic structure calculations via application of the disordered local moment (DLM) picture of magnetism at finite temperature.

Grades

Neodymium magnets are graded according to their maximum energy product, which relates to the magnetic flux output per unit volume. Higher values indicate stronger magnets. For sintered NdFeB magnets, there is a widely recognized international classification. Their values range from N28 up to N55 with a theoretical maximum at N64. The first letter N before the values is short for neodymium, meaning sintered NdFeB magnets. Letters following the values indicate intrinsic coercivity and maximum operating temperatures (positively correlated with the Curie temperature), which range from default (up to ) to TH ().How to Understand the Grade of Sintered NdFeB Magnet?, Grades of Sintered NdFeB Magnets

Grades of sintered NdFeB magnets:"Grades of Neodymium magnets" (PDF) . Everbeen Magnet. Retrieved December 6, 2015.

N27 – N55 N30M – N50M N30H – N50H N30SH – N48SH N28UH – N42UH N28EH – N40EH N28TH – N35TH N33VH/AH

Production

There are two principal neodymium magnet manufacturing methods:

Classical powder metallurgy or sintered magnet process Sintered Nd-magnets are prepared by the raw materials being melted in a furnace, cast into a mold and cooled to form ingots. The ingots are pulverized and milled; the powder is then sintered into dense blocks. The blocks are then heat-treated, cut to shape, surface treated and magnetized. Rapid solidification or bonded magnet process Bonded Nd-magnets are prepared by melt spinning a thin ribbon of the NdFeB alloy. The ribbon contains randomly oriented Nd2Fe14B nano-scale grains. This ribbon is then pulverized into particles, mixed with a polymer, and either compression- or injection-molded into bonded magnets.

Bonded neo Nd-Fe-B powder is bound in a matrix of a thermoplastic polymer to form the magnets. The magnetic alloy material is formed by splat quenching onto a water-cooled drum. This metal ribbon is crushed to a powder and then heat-treated to improve its coercivity. The powder is mixed with a polymer to form a mouldable putty, similar to a glass-filled polymer. This is pelletised for storage and can later be shaped by injection moulding. An external magnetic field is applied during the moulding process, orienting the field of the completed magnet.Bonded neo powder

In 2015, Nitto Denko of Japan announced their development of a new method of sintering neodymium magnet material. The method exploits an "organic/inorganic hybrid technology" to form a clay-like mixture that can be fashioned into various shapes for sintering. It is said to be possible to control a non-uniform orientation of the magnetic field in the sintered material to locally concentrate the field, for instance to improve the performance of electric motors. Mass production is planned for 2017.