Neodymium magnet

Nickel plated neodymium magnet on a bracket from a hard drive.

Nickel-plated neodymium magnet cubes

High-resolution transmission electron microscopy image of Nd₂Fe₁₄B

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 in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet made. They have replaced other types of magnet in the many applications in modern products that require strong permanent magnets, such as motors in cordless tools, hard disk drives, and magnetic fasteners.

Description

The tetragonal Nd₂Fe₁₄B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA~7 teslas). This gives the compound the potential to have high coercivity (i.e., resistance to being demagnetized). The compound also has a high saturation magnetization (J_s ~1.6 T or 16 kG) and typically 1.3 tesla. Therefore, as the maximum energy density is proportional to J_s², this magnetic phase has the potential for storing large amounts of magnetic energy (BH_max ~ 512 kJ/m³ or 64 MG·Oe), considerably more than 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.

History and manufacturing techniques

In 1982, General Motors and Sumitomo Special Metals discovered the Nd₂Fe₁₄B compound. The effort was principally driven by the high material cost of the SmCo permanent magnets, which had been developed earlier. General Motors focused on the development of melt-spun nanocrystalline Nd₂Fe₁₄B magnets, while Sumitomo developed full density sintered Nd₂Fe₁₄B magnets. General Motors Corporation commercialized its inventions of isotropic Neo powder, bonded Neo magnets and the related production processes by founding Magnequench in 1986. Magnequench is now part of the Neo Materials Technology Inc. and supplies melt spun Nd₂Fe₁₄B powder to bonded magnet manufacturers. The Sumitomo facility has become part of the Hitachi corporation and currently manufactures and licenses other companies to produce sintered Nd₂Fe₁₄B magnets.

Sintered Nd₂Fe₁₄B tends to be vulnerable to corrosion. In particular, corrosion along grain boundaries may cause deterioration of a sintered magnet. This problem is addressed in many commercial products by providing a protective coating. Nickel plating or two layered copper nickel plating is used as a standard method, although plating with other metals or polymer and lacquer protective coatings are also in use.^[1]

Production

There are two principal neodymium magnet manufacturing routes:

• The classical powder metallurgy or sintered magnet process
• The rapid solidification or bonded 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 to tiny particles. This undergoes a process of liquid-phase sintering whereby the powder is magnetically aligned into dense blocks which are then heat-treated, cut to shape, surface treated and magnetized. Currently, between 45,000 and 50,000 tons of sintered neodymium magnets are produced each year, mainly in China and Japan.

Bonded Nd-magnets are prepared by melt spinning a thin ribbon of the Nd-Fe-B 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 magnets offer less flux than do sintered magnets but can be net-shape formed into intricately shaped parts and do not suffer significant eddy current losses. There are approximately 5,500 tons of Neo bonded magnets produced each year. In addition, it is possible to hot-press the melt spun nanocrystalline particles into fully dense isotropic magnets, and then upset-forge/back-extrude these into high-energy anisotropic magnets.

Properties

Magnetic properties

Some important properties used to compare permanent magnets are: remanence (M_r), which measures the strength of the magnetic field; coercivity (H_ci), the material's resistance to becoming demagnetized; energy product (BH_max), the density of magnetic energy; and Curie temperature (T_C), the temperature at which the material loses its magnetism. Neodymium magnets have higher remanence, much higher coercivity and energy product, but often lower Curie temperature than other types. Neodymium is alloyed with terbium and dysprosium in order to preserve its magnetic properties at high temperatures.^[2] The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.

Magnet	Mr (T)	Hci (kA/m)	BHmax (kJ/m3)	TC (°C)
Nd2Fe14B (sintered)	1.0–1.4	750–2000	200–440	310–400
Nd2Fe14B (bonded)	0.6–0.7	600–1200	60–100	310–400
SmCo5 (sintered)	0.8–1.1	600–2000	120–200	720
Sm(Co, Fe, Cu, Zr)7 (sintered)	0.9–1.15	450–1300	150–240	800
Alnico (sintered)	0.6–1.4	275	10–88	700–860
Sr-ferrite (sintered)	0.2–0.4	100–300	10–40	450

Physical and mechanical properties

Comparison of physical properties of sintered neodymium and Sm-Co magnets^[3]

Property	Neodymium	Sm-Co
Remanence (T)	1–1.3	0.82–1.16
Coercivity (MA/m)	0.875–1.99	0.493–1.59
Permeability	1.05	1.05
Temperature coefficient of remanence (%/K)	−0.12	−0.03
Temperature coefficient of coercivity (%/K)	−0.55..–0.65	−0.15..–0.30
Curie temperature (°C)	320	800
Density (g/cm3)	7.3–7.5	8.2–8.4
CTE, magnetizing direction (1/K)	5.2×10−6	5.2×10−6
CTE, normal to magnetizing direction (1/K)	−0.8×10−6	11×10−6
Flexural strength (N/mm2)	250	150
Compressive strength (N/mm2)	1100	800
Tensile strength (N/mm2)	75	35
Vickers hardness (HV)	550–650	500–550
Electrical resistivity (Ω·cm)	(110–170)×10−6	86×10−6

Hazards

The greater force exerted by rare earth magnets creates hazards that are not seen with other types of magnet. Neodymium magnets larger than a few centimeters are strong enough to cause injuries to body parts pinched between two magnets, or a magnet and a metal surface, even causing broken bones.^[4] If one accidentally becomes pinched between two magnets, or between a magnet and some magnetic material, such as steel, the natural reaction is to pull free. However, doing so makes the space between the magnets become thinner, and the pinch force increases dangerously. If possible, one should bear the pain and do whatever one can to keep the distance from closing up; only then should one pull free. Professionals who work with such magnets might include brass wedges as part of their tools.

Magnets allowed to get too near each other can strike each other with enough force to chip and shatter the brittle material, and the flying chips can cause injuries. There have even been cases where young children who have swallowed several magnets have had a fold of the digestive tract pinched between the magnets, causing injury or death.^[5] The stronger magnetic fields can be hazardous to mechanical and electronic devices, as they can erase magnetic media such as floppy disks and credit cards, and magnetize watches and other clockwork mechanisms and the shadow masks of CRT type monitors at a significant distance.

Applications

In technology

• 

  Ring magnets

• 

  Hard disk drive

Neodymium magnets have replaced alnico and ferrite magnets in many of the myriad applications in modern technology where strong permanent magnets are required, because their greater strength allows the use of smaller, lighter magnets for a given application. Some examples are

• head actuators for computer hard disks
• magnetic resonance imaging (MRI)
• magnetic guitar pickups
• loudspeakers and headphones
• magnetic bearings and couplings
• electric motors:
  • cordless tools
  • servo motors
  • lifting and compressor motors
  • synchronous motors
  • spindle and stepper motors
  • electrical power steering
  • drive motors for hybrid and electric vehicles. The electric motors of each Toyota Prius require 1 kilogram (2.2 pounds) of neodymium.^[2]
  • actuators

Other applications

In addition, the greater strength of neodymium magnets has inspired a new applications in areas where magnets were not used before, such as magnetic jewelry clasps, children's magnetic building sets (and other neodymium magnet toys) and as part of the closing mechanism of modern sport parachute equipment.^[6] The strength and magnetic field homogeneity on neodymium magnets has also opened new applications in the medical field with the introduction of open magnetic resonance imaging (MRI) scanners used to image the body in radiology departments as an alternative to superconducting magnets that use a coil of superconducting wire to produce the magnetic field. As with most solid-based magnets, the magnetic field gradient of neodymium magnets decreases towards the centers of their surfaces, thus there is a force that attracts metallic objects to the edges.

See also

• Lanthanide metals
• Neodymium magnet toys
• Samarium-cobalt magnet
• Transition metal substitutions like NdCoB

References

1. ^ Drak, M.; Dobrzanski, L.A. (2007). "Corrosion of Nd-Fe-B permanent magnets". Journal of Achievements in Materials and Manufacturing Engineering 20 (1–2). http://www.journalamme.org/papers_vol20/1369S.pdf. 
2. ^ ^{a b} As hybrid cars gobble rare metals, shortage looms, Reuters, August 31, 2009.
3. ^ Juha Pyrhönen, Tapani Jokinen, Valéria Hrabovcová (2009). Design of Rotating Electrical Machines. John Wiley and Sons. p. 232. ISBN 0470695161. http://books.google.com/?id=_y3LSh1XTJYC&pg=PT232. 
4. ^ Swain, Frank (March 6, 2009). "How to remove a finger with two super magnets". The Sciencepunk Blog. Seed Media Group LLC. http://scienceblogs.com/sciencepunk/2009/03/how_to_remove_a_finger_with_tw.php. Retrieved 2009-06-28. 
5. ^ "Magnet Safety Alert". U.S. Consumer Product Safety Commission. http://www.cpsc.gov/CPSCPUB/PUBS/magnet.pdf. Retrieved 7 August 2009. 
6. ^ United Parachute Technologies Options Guide: http://www.unitedparachutetechnologies.com/index.php?option=com_content&task=view&id=22

Further reading

• MMPA 0100-00, Standard Specifications for Permanent Magnet Materials
• K.H.J. Buschow (1998) Permanent-Magnet Materials and their Applications, Trans Tech Publications Ltd., Switzerland, ISBN 0-87849-796-x
• Campbell, Peter (1994). Permanent Magnet Materials and their Application. New York: Cambridge University Press. ISBN 0521249961. 
• Furlani, Edward P. (2001). Permanent Magnet and Electromechanical Devices: Materials, Analysis and Applications. London: Academic Press. ISBN 0122699513. 
• Brown, D (2002). "Developments in the processing and properties of NdFeB-type permanent magnets". Journal of Magnetism and Magnetic Materials 248 (3): 432–440. Bibcode 2002JMMM..248..432B. doi:10.1016/S0304-8853(02)00334-7. 
• The Dependence of Magnetic Properties and Hot Workability of Rare Earth-Iron-Boride Magnets Upon Composition.

External links

• Magnet Man Cool experiments with magnets
• Geeky Rare-Earth Magnets Repel Sharks, Genevieve Rajewski, 05.15.07 , wired.com
• Concern as China clamps down on rare earth exports, Cahal Milmo, 01.02.10, independent.co.uk