Alnico Magnets - General Information

Alnico Round Horseshoe Magnet

Alnico (composed of alloys Aluminum, Nickel and Cobalt) magnets were invented in the 1920s. They are used today in high temperature applications, in applications which require low coercivities (the ability to demagnetize and re-magnetize easily), or in mass produced instruments and legacy applications where the material has been designed in. 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. Alnico magnets are manufactured through either a casting or sintering process. Cast magnets can be made in fairly complex shapes - such as the shape shown below - a 4-pole round horseshoe magnet.

Key Benefits

• 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.

CAUTION!

• Alnico magnets are very hard and brittle and can be broken unless carefully handled.
• Alnico magnets are easily demagnetized - even when exposed to repelling magnetic fields - such as when two similar Alnico magnets are placed in repulsion.
• Cast Alnico materials commonly contain casting voids and hairline cracks within the material. Finish machining can expose these voids and hairline cracks. The following is an extract from MMPA standards regarding such imperfections:

  "These are materials used primarily for their magnetic capabilities as permanent magnets without regard to mechanical properties. These materials, prepared by good metallurgical practice, by their very nature will unavoidably contain a degree of physical imperfection. It is not generally recommended that these materials be used for structural or decorative purposes unless the physical requirements of the magnet are previously suitably specified."