In 1982, General Motors Corporation 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.
There are two principal neodymium magnet manufacturing routes:
Sintered Neo magnets are prepared by pulverizing an ingot precursor and liquid-phase sintering the magnetically aligned powder 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 from China and Japan.
Bonded Neo 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 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.
The magnetic field B is defined from the Lorentz Force Law, and specifically from the magnetic force on a moving charge:
The implications of this expression include:
When the magnetic force relationship is applied to a current-carrying wire, the right-hand rule may be used to determine the direction of force on the wire.
From the force relationship above it can be deduced that the units of magnetic field are Newton seconds /(Coulomb meter) or Newtons per Ampere meter.
This unit is named the Tesla. It is a large unit, and the smaller unit Gauss is used for small fields like the Earth's magnetic field. A Tesla is 10,000 Gauss. The Earth's magnetic field at the surface is on the order of half a Gauss.
A good permanent magnet should produce a high magnetic field with a low mass, and should be stable against the influences which would demagnetize it. The desirable properties of such magnets are typically stated in terms of the remanence and coercivity of the magnet materials.
When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed. The amount of magnetization it retains at zero driving field is called its remanence. It must be driven back to zero by a field in the opposite direction; the amount of reverse driving field required to demagnetize it is called its coercivity. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. The lack of retraceability of the magnetization curve is the property called hysteresis and it is related to the existence of magnetic domains in the material. Once the magnetic domains are reoriented, it takes some energy to turn them back again. This property of ferrromagnetic materials is useful as a magnetic "memory". Some compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are useful as "permanent magnets".
The table below contains some data about materials used as permanent magnets. Both the coercivity and remanence are quoted in Tesla, the basic unit for magnetic field B. The hysteresis loop above is plotted in the form of magnetization M as a function of driving magnetic field strength H. This practice is commonly followed because it shows the external driving influence (H) on the horizontal axis and the response of the material (M) on the vertical axis.
Besides coercivity and remanence, a quality factor for permanent magnets is the quantity (BB0/μ0)max. A high value for this quantity implies that the required magnetic flux can be obtained with a smaller volume of the material, making the device lighter and more compact.
The alloys from which permanent magnets are made are often very difficult to handle metallurgically. They are mechanically hard and brittle. They may be cast and then ground into shape, or even ground to a powder and formed. From powders, they may be mixed with resin binders and then compressed and heat treated. Maximum anisotropy of the material is desirable, so to that end the materials are often heat treated in the presence of a strong magnetic field.
The materials with high remanence and high coercivity from which permanent magnets are made are sometimes said to be "magnetically hard" to contrast them with the "magnetically soft" materials from which transformer cores and coils for electronics are made.
The ability of a permanent magnet to support an external magnetic field results from small magnetic domains "locked" in position by crystal anisotropy within the magnet material. Once established by initial magnetization, these positions are held until acted upon by forces exceeding those that lock the domains. The energy required to disturb the magnetic field produced by a magnet varies for each type of material. Permanent magnets can be produced with extremely high coercive forces (Hc) that will maintain domain alignment in the presence of high external magnetic fields. Stability can be described as the repeated magnetic performance of a material under specific conditions over the life of the magnet.
Factors affecting magnet stability include time, temperature, reluctance changes, adverse fields, radiation, shock, stress, and vibration.
The effect of time on modern permanent magnets is minimal. Studies have shown that permanent magnets will see changes immediately after magnetization. These changes, known as "magnetic creep", occur as less stable domains are affected by fluctuations in thermal or magnetic energy, even in a thermally stable environment. This variation is reduced as the number of unstable domains decreases. Rare Earth magnets are not as likely to experience this effect because of their extremely high coercivities. Long-term time versus flux studies have shown that a newly magnetized magnet will lose a minor percent of its flux as a function of age. Over 100,000 hours, these losses are in the range of essentially zero for Samarium Cobalt materials to less than 3% for Alnico 5 materials at low permeance coefficients.
Temperature effects fall into three categories:
These are losses that are recovered when the magnet returns to its original temperature. Reversible losses cannot be eliminated by magnet stabilization. Reversible losses are described by the Reversible Temperature Coefficients (Tc), shown in table 5.1. Tc is expressed as % per degree Centigrade. These figures vary for specific grades of each material but are representative of the class of material as a whole. It is because the temperature coefficients of Br and Hc are significantly different that the demagnetization curve develops a "knee" at elevated temperatures.