Introduction of AlNiCo Magnet
AlNiCo, short for aluminum-nickel-cobalt, represents a group of permanent magnet alloys primarily composed of iron, aluminum, nickel, and cobalt. Its inception dates back to 1931 when T. Mishima in Japan discovered its superior coercivity compared to existing steel magnets. Alnico alloys typically consist of 8-12% aluminum, 15-26% nickel, 5-24% cobalt, up to 6% copper, up to 1% titanium, and the remaining balance is iron.
Diverse magnetic properties are achieved in Alnico by modifying the composition and manufacturing process. Specialized casting techniques, such as those used in the production of Alnico 5-7, impart unique crystalline grain orientation.
Alnico held the distinction of being the strongest permanent magnet until the emergence of rare earth magnets in the 1970s. While it has been substituted in many applications, Alnico’s temperature stability and its ability to withstand high temperatures up to 500°C make it indispensable in specific use cases.
Design Considerations
To ensure the stability of Alnico magnets, it is crucial to design the magnet system with a high permeance coefficient, placing the working point above the knee of the J-H curve due to the low coercivity of Alnico. For instance, an Alnico 5 rod should have a length greater than five times its diameter.
Careless handling can easily demagnetize Alnico magnets, so it is advisable to magnetize them after assembly to prevent unintended demagnetization.
Alnico magnets exhibit remarkable temperature stability, making them ideal for temperature-sensitive applications. They experience minimal variation in output with temperature changes. The reversible temperature coefficient of induction is -0.02%/°C from ambient, while the reversible temperature coefficient of intrinsic coercivity is +0.01%/°C from ambient. This makes Alnico magnets an excellent choice for performance-critical applications, including medical and military sectors. Remarkably, Alnico magnets can be used at temperatures up to 530°C without irreversible losses in magnetizing.
