What Is an Alnico Magnet?
An alnico magnet is a magnet made of aluminum, nickel, and cobalt.
Copper and other elements are sometimes added as additives, and they are used as strong permanent magnets. Due to supply concerns over raw materials, alnico magnets are not as common as ferrite magnets, and their disadvantage is that they are easily demagnetized.
Because of its high Curie temperature, it is resistant to high temperatures and cracking, and has excellent mechanical strength. Alnico magnets are manufactured by casting and sintering.
Uses of Alnico Magnets
Alnico magnets have a high Curie temperature (Curie point 850°C), which means that their magnetic properties do not change significantly with temperature, and they can be used at high temperatures. Due to their excellent mechanical properties, such as resistance to cracking, Alnico magnets are used in various meters, instruments, watt-hour meters, and crime prevention equipment.
- Magnets for Measuring Instruments
Ammeters, voltmeters, wattmeters, speedometers, etc. - Magnets for audio equipment
Level meters, peak meters, earphones, speakers, magnetic pickups for electric guitars, etc. - Magnets for rotating equipment
Coreless motors, stepping motors, servo motors, etc. - Communication, measurement and control
Reed switches, sensors, relays, etc. - Applied equipment
Solenoids, etc.
In other cases, Alnico magnets in the shape of a 5 cm bar are swallowed by cows to adsorb a bundle of wires and other iron pieces in the third stomach to prevent wound pericarditis.
Properties of Alnico Magnets
Alnico magnets come in three shapes:
- Bar type
Round bar type with a small outer diameter of 3 mmφ, indicated by outer diameter × length - Ring type
Shown by outer diameter x inner diameter x height - Square shape
Width x Height x Length
Alnico magnets are characterized by a magnetic force as strong as that of rare earth magnets, such as neodymium magnets. In addition, due to its high Curie temperature, Alnico magnets can be used at high temperatures with little change in magnetism due to temperature. It can also be manufactured in dimensions according to the intended use.
However, its disadvantages include fluctuating costs due to unstable prices of cobalt and nickel, the main raw materials, and its tendency to demagnetize. Since the coercive force is not that great, it may be demagnetized by external magnetic fields or mechanical shocks.
In addition, self-demagnetization often occurs in thin plate shapes with large antimagnetic fields. To compensate for the coercive force, the length in the direction of magnetization must be increased.
Other Information on Alnico Magnets
1. Invention of the Alnico Magnet
In 1931, Tokushichi Mishima invented MK steel, which is mainly composed of iron, nickel, and aluminum alloys. This MK steel was the beginning of a larger trend to utilize alloys for magnetic materials.
Later, Tokushichi Mishima sought to improve the magnetic properties of MK steel by adding cobalt and copper, and this is the foundation of the Alnico Magnet. 1934, Kotaro Honda, Kazumasa Masumoto, Yuki Shirakawa, and others added titanium to MK steel to develop NKS steel with high holding power.
In 1938, GE Corporation of the U.S. announced the Alnico Magnet, which was made by adding copper to NKS steel and became widely used. The composition of the Alnico magnet at this time was 8wt% aluminum, 14wt% nickel, 24wt% cobalt, 3wt% copper, and the rest iron.
2. Degradation of Alnico Magnets
The magnetic force of an alnico magnet decreases with age, temperature, and external magnetic field. Compared to other permanent magnets such as neodymium magnets and samarium-cobalt magnets, Alnico magnets have a lower coercive force, so they tend to lose their magnetic force as time passes since their manufacture.
If Alnico magnets are to be used for a long period, it is necessary to check periodically to see if the magnetic force has decreased. In addition, Alnico magnets have a high Curie temperature and are resistant to heat, but their magnetic force weakens at high temperatures. If the temperature is too high, the magnetism may not recover upon cooling, so it must be used within the proper temperature.
Alnico magnets are strongly affected by external magnetic fields due to their low coercive force. Therefore, in the presence of a strong magnetic field, they cannot retain their magnetic force, leading to a decrease in magnetic force. Another disadvantage is that partial damage, such as rust or deformation, makes it impossible to maintain the magnetism of the entire magnet.