Alnico Magnets - General Information

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 optimum magnetic properties — without it alnico's coercivity is about 10 Oe, comparable to 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 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.

However, in recent years, many applications have been replaced Alnico with rare earth magnets, whose stronger fields (Br) and larger energy products (BHmax) allow smaller size magnets to be used for a given application.

Today, Alnico are used mainly in high temperature applications which require low coercivities (the ability to demagnetize and re-magnetize easily), in mass produced instruments and in older designed products. On a cost per pound basis, this material is comparable to the cost of Neodymium magnets, although they are in most applications much less powerful than Neodymium magnets. Cast magnets can be made in fairly complex shapes 

Excellent temperature stability up to 1,000° F - 90% of room temperature magnetization is retained up to this temperature.
High residual induction - Alnico magnets can produce powerful fields in certain configurations.
Material does not corrode.
Cast Alnico magnets can be produced in relatively complex shapes.
Tooling for cast magnets is relatively low, since sand molds are generally used for the casting process.

Key Challenges

Alnico materials have low coercivities, meaning that they are easily demagnetized.
These magnets are relatively costly since they contain both nickel and cobalt.
Cast Alnicos often have casting pores and voids within them - which can be problematic from cosmetics point of view or because large voids may lower expected magnetic flux.
The material is extremely hard and brittle and difficult to machine.

Quick Facts

Density - 0.265 lbs per cubic inch
Saturation magnetizing field required - about 5kOe
Manufacturing methods - casting (most common), or sintering.
Shapes available - blocks, bars, discs, rings, horseshoes, etc.
Grades available - from about 0105 to 0519. (First 2 digits represent BHmax, and second two digits represent Intrinsic Coercivity, Hci.)
Sizes - off tool very large Alnico magnets can be cast (for example horseshoe magnets weighing 500 pounds), or small magnets sintered (for example sintered discs, 1/16" in diameter)

Surface Treatments

No special surface treatments are required for Alnico magnets since they are resistant to oxidation. They can, however, be easily plated using a variety of metals, such as nickel or chrome.

Cast Alnico materials commonly contain casting voids and hairline cracks within the material. Finish machining can expose these voids and hairline cracks.

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