Alnico Magnets
The exact supplier grade is the useful starting point, but it is not the whole specification. Geometry, magnetization direction, and the surrounding magnetic circuit affect how a magnet’s published properties translate into an assembly. Treat “Alnico” as a family name, then check the grade data against the design’s operating conditions.
What Alnico is
Alnico is an iron-based alloy family named for aluminum, nickel, and cobalt. Copper and titanium may also be present, and composition varies by grade. A family label therefore cannot tell you the performance of an individual magnet. Arnold Magnetic Technologies’ Alnico overview describes its alloy constituents and grade families.
Cast and sintered Alnico
Alnico magnets are made by casting or powder sintering. Casting can produce complex shapes. Sintering compacts and heats metal powder; a peer-reviewed manufacturing review reports that sintered Alnico tends to have somewhat lower magnetic properties but better mechanical characteristics and finer grain than cast material. These are process tendencies, not guarantees for every grade or part. Ask the supplier which process applies to the quoted component and compare its data. Kesler et al.’s review of permanent-magnet manufacturing discusses these trade-offs.
Process alone does not identify a magnet’s performance. Grade and orientation matter too. Arnold describes its grades 2, 3, and 4 as unoriented (isotropic), and grades 5, 6, 8, and 9 as anisotropic, with a preferred direction. Its statement that Alnico 8 has higher coercive force applies to Arnold’s grade family; grade numbers and naming should not be assumed to map identically across suppliers. Arnold’s grade descriptions are examples, not a universal codebook.
Read the magnetic properties as a set
Three common figures answer different questions:
- Remanence (Br) describes the magnetic flux density remaining after a material has been magnetized and the magnetizing field removed. It is not the field measured at an arbitrary point outside the finished part.
- Coercivity (Hc) is the reverse field required to bring magnetic flux density back to zero. Intrinsic coercivity (Hci) is the reverse field required to bring intrinsic polarization to zero. They describe resistance to demagnetization in different ways.
- Maximum energy product ((BH)max) is the maximum product of flux density and field strength on a material’s demagnetization curve. It is a material figure, not a direct prediction of pull force or assembled-device performance.
A supplier’s test method and conditions matter when comparing these values. Arnold’s technical note on magnetic properties and measurement explains the distinctions between material properties and measurements. An older MMPA specification also presents typical values by cast or sintered process and isotropic or anisotropic designation. Treat that table as an edition-specific reference, not a current certificate or proof that similarly numbered grades from different suppliers are interchangeable. MMPA 0100-00 (PDF)
Surface field and pull force depend on the part and measurement setup. The working point in an assembly depends on the magnet’s geometry and magnetic circuit, so a single “strength” number cannot stand in for the design point.
Check demagnetization and temperature in the design
Alnico’s relatively low coercivity can make it more susceptible to demagnetization in some magnetic circuits than higher-coercivity materials. Geometry and the final circuit matter. Arnold recommends magnetizing after assembly with pole pieces in the final circuit for relevant designs; the manufacturing review also identifies low coercivity as a key Alnico limitation. This is a design consideration, not a blanket verdict that every Alnico magnet will demagnetize easily.
Temperature figures need the same care. Alliance lists continuous working-temperature values from 450 to 550 °C across the grades shown on its Alnico data page. Arnold says heating-related irreversible loss depends on magnet dimensions and composition and notes a metallurgical change above 538 °C. These supplier statements have distinct scopes; neither establishes one safe operating limit for the entire Alnico family. A quoted maximum alone does not approve a particular shape, magnetic circuit, exposure duration, or acceptable loss. Request grade-specific curves and have the design checked for its actual conditions.
Mechanical details also belong in the specification. MMPA 0100-00 describes Alnico as hard and brittle and unsuitable for conventional machining. Confirm tolerances, finishing, and handling requirements with the supplier rather than assuming a finished magnet can be drilled or reshaped like a soft metal part.
Alnico specification checklist
Before comparing quotations or releasing a design, request and record:
- The supplier’s exact grade designation and whether it is cast or sintered.
- Dimensions, tolerances, and any finishing or handling requirements.
- Magnetization direction, including its intended direction relative to the part and assembly.
- Br, Hc, Hci, and (BH)max, with test conditions and the demagnetization curve for the quoted grade.
- Operating temperature, exposure duration, and acceptable irreversible loss, supported by grade-specific data and design review.
- The final geometry and magnetic circuit, including pole pieces and opposing fields that could shift the working point.
- The acceptance measurement, its location, and limits for the finished assembly.
This is a practical RFQ and design checklist, not a claim of compliance with a particular standard. IEC’s 60404-8-1:2023 listing describes minimum principal magnetic properties and dimensional tolerances for technically important hard permanent-magnet materials. The listing alone does not establish that a specific Alnico grade is covered or that a component conforms. Verify applicable material codes and requirements in the full standard and the supplier’s documentation.