Alnico is an acronym referring to a family of iron alloys which in addition to iron are composed primarily of aluminium (Al, nickel (Ni) and cobalt (Co, hence al-ni-co. They also include copper and sometimes titanium. Alnico alloys are ferromagnetic, with a high coercivity.
Alnico Magnets are produced by casting or sintering processes. [7] 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 optimum 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 [8] 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 widely used in industrial and consumer applications where strong permanent magnets are needed; examples are electric motors, electric guitar pickups, microphones, sensors, loudspeakers, traveling-wave tubes and cow magnets. |
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