Alnico Magnets

Composition, cast and sintered

Alnico is an iron-based alloy with aluminium, nickel and cobalt as the principal additions, usually with copper and often titanium. The magnetic behaviour comes from a fine two-phase microstructure formed during controlled cooling, frequently in an applied magnetic field — which is why the higher-performing grades are directional, or grain-oriented, and must be magnetised along the axis they were processed on.

cast alnico

Poured into moulds. Achieves the higher magnetic properties and permits complex shapes including horseshoes and long slender rods. Surface finish is rough as-cast and dimensional tolerance is loose without machining.

sintered alnico

Pressed and sintered from powder. Roughly 10–15 percent lower magnetic properties than cast, but better dimensional control, better mechanical strength, and economic in small parts at volume.

isotropic grades

Alnico 2 and similar. No preferred direction; can be magnetised on any axis. Lower energy product.

anisotropic grades

Alnico 5 and above. Heat-treated in a field, so performance is high along one axis only. The orientation axis must be stated on the drawing.

machinability

Alnico is hard and brittle but can be ground, and unlike the rare earths it is tough enough to be handled without constant chipping. It cannot be turned or drilled conventionally.

Grade reference

Compare the H_cb column against NdFeB, where the equivalent figure runs to 10,000 Oe and above, and SmCo higher still. Alnico's coercivity is lower by an order of magnitude, and that single fact drives everything about how it has to be used.

Where Alnico still wins

Temperature

Alnico operates at 450–550 °C, with a Curie temperature above 800 °C. No rare earth comes close — Sm₂Co₁₇ stops at 350 °C and the best NdFeB at around 230 °C. For sensors and instrumentation in furnaces, exhaust paths, process equipment and similar service, Alnico is often the only permanent magnet option that survives.

Reversible temperature coefficient

Alnico 5's reversible coefficient of B_r is around −0.02 %/°C — better even than SmCo, and roughly six times more stable than NdFeB. In meters, galvanometers, reference devices and precision sensing this is the property being bought.

Ageing

Properly stabilised Alnico loses very little flux over decades. Instruments built in the 1950s and 1960s are still in calibration on their original magnets. Where a design has a thirty-year service life and no recalibration path, that record matters.

No rare earth content

Alnico contains no neodymium, dysprosium, terbium or samarium. It is insulated from rare-earth price volatility and export-control exposure, though it does contain cobalt and carries that exposure instead. For designs where rare-earth supply risk is the driving concern, Alnico belongs in the evaluation alongside the options in rare-earth-free and reduced-Dy options.

The low-coercivity problem

Alnico's coercivity is low enough that it can be partially demagnetised by conditions that no other permanent magnet material would notice. This is not a defect — it is inherent to the material and it has to be engineered around.

Design consequences

• Geometry drives stability. Alnico wants to be long in the magnetisation direction and small in cross-section — a high permeance coefficient. Short, wide Alnico is a self-demagnetising shape. This is the opposite of the NdFeB instinct, where a thin disc is perfectly stable.
• Opposing fields are dangerous. A nearby coil, a stray field or another magnet can partially demagnetise Alnico at levels that NdFeB would shrug off. Where opposing fields exist, Alnico 8 or 9 are the grades to use.
• Disassembly demagnetises. Taking an Alnico assembly apart opens the circuit. Any design that will be serviced needs either keepers fitted during service or a documented re-magnetisation step.
• It can be re-magnetised in place. This is the compensating advantage. Alnico's low coercivity means it saturates at a modest field, so re-magnetisation is straightforward and can often be done in the assembled device — unlike NdFeB, which needs 30 kOe and a dedicated fixture.
• Stabilise deliberately. Where absolute stability matters, Alnico is commonly knocked down a few percent by a controlled partial demagnetisation, leaving it on a flat, repeatable part of the curve. The magnet is slightly weaker and far more predictable.

Specifying Alnico

Alnico specifications need a few things that rare-earth specifications do not.

1. Cast or sintered. They are different processes with different properties, tolerances and minimum geometries. Do not leave it to the supplier.
2. Grade and orientation axis. For anisotropic grades, the axis is as important as the grade. Mark it on the drawing.
3. Magnetised or unmagnetised, and keepered or not. If magnetised, state that keepers are required and who supplies them.
4. Machined dimensions versus as-cast. Cast Alnico has a rough surface and loose tolerance. Identify which surfaces are ground and to what.
5. Stabilisation requirement, if the application needs it, with the intended knockdown stated.
6. Acceptance criterion. Flux measured in a defined fixture is more meaningful than surface gauss for Alnico, because the open-circuit reading depends heavily on geometry and on whether a keeper was recently fitted.

For retention, bonding and mounting of Alnico in an assembly — where its mechanical toughness permits methods that would be unsafe on sintered rare earths — see assembly and retention design.