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 (resistance to loss of magnetism) and are used to make permanent magnets. Before the development of rare earth magnets in the 1970s, they were the strongest type of permanent magnet.
The composition of alnico alloys is typically 8–12% Al, 15–26% Ni, 5–24% Co, up to 6% Cu, up to 1% Ti, and the balance is Fe. The development of alnico began in 1931, when T. Mishima in Japan discovered that an alloy of iron, nickel, and aluminum had a coercivity of 400 oersted (Oe; 0.07957 kA/m), double that of the best magnet steels of the time.
Alnico alloys can be magnetised to produce strong magnetic fields, and have a high coercivity (resistance to demagnetization), and thus make strong permanent magnets. Of the more commonly available magnets, only rare-earth magnets such as neodymium and samarium-cobalt are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength of Earth's magnetic field. Some brands of alnico are isotropic and can be efficiently magnetized in any direction. Other types, such as alnico 5 and alnico 8, are anisotropic, with each having a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico's remanence (Br) may exceed 12,000 G (1.2 T), its coercivity (Hc) can be up to 1000 oersted (80 kA/m), its energy product ((BH)max) can be up to 5.5 MG·Oe (44 T·A/m). This means alnico can produce a strong magnetic flux in closed magnetic circuits, but has relatively small resistance against demagnetization. The field strength at the poles of any permanent magnet depends very much on the shape and is usually well below the remanence strength of the material.
Alnico alloys have some of the highest Curie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximum working temperature is normally limited to around 538 °C (1,000 °F). They are the only magnets that have useful magnetism even when heated red-hot. This property, as well as its brittleness and high melting point, is the result of the strong tendency toward order due to intermetallic bonding between aluminium and its other constituents. They are also one of the most stable magnets if they are handled properly. Alnico magnets are electrically conductive, unlike ceramic magnets.
|Magnetic Materials||Density (g/cm)||Maximum Energy Product BH (max) (MGOe)||Residual Induction Br (G)||Coercive Force Hc (Oe)||Intrinsic Coercive Force Hc (Oe)||Normal Maximum Operating Temp|
|Alnico 5 (cast)||7.3||5.5||12800||640||640||975 °F (524 °C)|
|Alnico 8 (cast)||7.3||5.3||8200||1650||1860||1,020 °F (549 °C)|
|Alnico 5 (sintered)||6.9||3.9||10900||620||630||975 °F (524 °C)|
|Alnico 8 (sintered)||7.0||4.0||7400||1500||1690||1,020 °F (549 °C)|
|Permanent magnetic alloy of sintered Alnico magnet|
|Instrinsic Coercivity||Normal Coercivity||Density||TC|