Alnico magnets are traditionally classified using numbers assigned by the Magnetic Materials Producers Association (MMPA), for example, alnico 3 or alnico 5.
Alnico magnets are produced by casting or sintering processes. Anisotropic alnico magnets are oriented by heating above a critical temperature and cooling in the presence of a magnetic field. Both isotropic and anisotropic alnico require proper heat treatment to develop optimal magnetic properties-without it alnico's coercivity is about 10 Oe, comparable to technical iron, which is a soft magnetic material. After the heat treatment alnico becomes a composite material, named "precipitation material"-it consists of iron- and cobalt-rich precipitates in rich-NiAl matrix.
Alnico's anisotropy is oriented along the desired magnetic axis by applying an external magnetic field to it during the precipitate particle nucleation, which occurs when cooling from 900 °C (1,650 °F) to 800 °C (1,470 °F), near the Curie point. Without an external field there are local anisotropies of different orientations due to spontaneous magnetization. The precipitate structure is a "barrier" against magnetization changes, as it prefers few magnetization states requiring much energy to get the material into any intermediate state. Also, a weak magnetic field shifts the magnetization of the matrix phase only and is reversible.
Alnico magnets are ideal for high temperature applications up to 550ºC
. Cast AlNICo Magnet is very stable, has good corrosion resistance and a typical hardness of 50 Rockwell C. Once cast, the material is so hard that the only machining possible is grinding
. Cast AlNICo Magnet represents the most versatile magnet material available. The range of properties can be accurately designed for specific applications by changes to element analysis and heat treatment.