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Properties

Alnico alloys can be magnetised to produce strong magnetic fields and have a high coercivity (resistance to demagnetization), thus making strong permanent magnets. Of the more commonly available magnets, only rare-earth magnets such as neodymium and samarium-cobalt are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength of Earth's magnetic field. Some alnico brands are isotropic and can be efficiently magnetized in any direction. Other types, such as alnico 5 and alnico 8, are anisotropic, each having a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico's remanence (Br) may exceed 12,000 G (1.2 T), its coercivity (Hc) can be up to 1000 oersteds (80 kA/m), its maximum energy product ((BH)max) can be up to 5.5 MG·Oe (44 T·kA/m). Therefore, alnico can produce a strong magnetic flux in closed magnetic circuits, but has relatively small resistance against demagnetization. The field strength at the poles of any permanent magnet depends very much on the shape and is usually well below the remanence strength of the material.

Alnico alloys have some of the highest Curie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximal working temperature is typically limited to around 538 °C (1,000 °F).⁴ They are the only magnets that have useful magnetism even when heated red-hot.⁵ This property, as well as its brittleness and high melting point, results from the strong tendency toward order due to intermetallic bonding between aluminum and other constituents. They are also one of the most stable magnets if handled properly. Alnico magnets are electrically conductive, unlike ceramic magnets. Alnico 3 has a melting temperature of 1200 - 1450 °C.⁶

As of 2018, Alnico magnets cost about 44 USD/kg (US$20/lb) or US$4.30/BHmax.⁷

Classification

Alnico magnets are traditionally classified using numbers assigned by the Magnetic Materials Producers Association (MMPA), for example, alnico 3 or alnico 5. These classifications indicate chemical composition and magnetic properties. (The classification numbers themselves do not directly relate to the magnet's properties; for instance, a higher number does not necessarily indicate a stronger magnet.)⁸

These classification numbers, while still in use, have been deprecated in favor of a new system by the MMPA, which designates Alnico magnets based on maximum energy product in megagauss-oersteds and intrinsic coercive force as kilo oersted, as well as an IEC classification system.⁹

Manufacturing process

Alnico magnets are produced by casting or sintering processes.¹⁰ Cast alnico is produced by conventional methods using resin bonded sand molds, which can be intricate and detailed, thereby allowing for complex shapes to be produced.¹¹ The produced alnico magnet typically has a rough surface.¹² This process has higher initial tooling costs for mold creation.¹³ Sintered alnico magnets are formed using powdered metal manufacturing methods. While sintering can also produce a range of shapes, it may not be as suitable for extremely intricate or detailed designs compared to casting.¹⁴¹⁵

Most alnico produced is anisotropic, meaning that the magnetic direction of the grains is randomly oriented when initially made. 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 optimal magnetic properties. Without it, alnico's coercivity is about 10 Oe, comparable to technical iron, a soft magnetic material. After the heat treatment alnico becomes a composite material, named "precipitation material"—it consists of iron- and cobalt-rich¹⁶ precipitates in a 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. There are local anisotropies of different orientations without an external field 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.

Uses

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, magnetron tubes, and cow magnets. In many applications they are being superseded by rare-earth magnets, whose stronger fields (Br) and larger energy products (B·Hmax) allow smaller-size magnets to be used for a given application.

The high-temperature resistance of alnico magnets leads to many uses that cannot be filled by less resistant magnets, such as in magnetic stirring hotplates.

Further reading

• MMPA 0100-00, Standard Specifications for Permanent Magnet Materials

References

1. Hellweg, Paul (1986). The Insomniac's Dictionary. Facts On File Publications. p. 115. ISBN 978-0-8160-1364-7. 978-0-8160-1364-7 ↩

2. Brady, George Stuart; Clauser, Henry R.; Vaccari, John A. (2002). Materials Handbook: An Encyclopedia for Managers. McGraw-Hill Professional. p. 577. ISBN 978-0-07-136076-0. 978-0-07-136076-0 ↩

3. Cullity, B. D.; Graham, C. D. (2008). Introduction to Magnetic Materials. Wiley-IEEE. p. 485. ISBN 978-0-471-47741-9. 978-0-471-47741-9 ↩

4. "Alnico Magnets & Custom Assemblies". Arnold Magnetic Technologies. Archived from the original on September 13, 2024. Retrieved September 13, 2024. https://www.arnoldmagnetics.com/products/alnico-magnets/ ↩

5. Hubert, Alex; Rudolf Schäfer (1998). Magnetic domains: the analysis of magnetic microstructures. Springer. p. 557. ISBN 978-3-540-64108-7. 978-3-540-64108-7 ↩

6. "ALNICO 3 Safety Data Sheet" (PDF). September 2, 2014. Archived (PDF) from the original on September 13, 2024. https://www.arnoldmagnetics.com/wp-content/uploads/2017/10/ALNICO3_SDS_US_090214_FINAL.pdf ↩

7. "Frequently Asked Questions". Total Magnetic Solutions. Magnet Sales & Manufacturing Company, Inc. Archived from the original on March 12, 2019. Retrieved March 12, 2019. http://www.magnetsales.com/design/faqs_frames/faqs_3.htm ↩

8. "Standard Specifications for Permanent Magnet Materials (MMPA Standard No. 0100-00)" (PDF). Magnetic Materials Producers Association. Retrieved 9 September 2015. http://www.allianceorg.com/pdfs/MMPA_0100-00.pdf ↩

9. "Standard Specifications for Permanent Magnet Materials (MMPA Standard No. 0100-00)" (PDF). Magnetic Materials Producers Association. Retrieved 9 September 2015. http://www.allianceorg.com/pdfs/MMPA_0100-00.pdf ↩

10. Campbell, Peter (1996). Permanent magnet materials and their application. UK: Cambridge University Press. pp. 35–38. Bibcode:1996pmma.book.....C. ISBN 978-0-521-56688-9. 978-0-521-56688-9 ↩

11. Cui, Jun; Ormerod, John (2022). "Manufacturing Processes for Permanent Magnets: Part I—Sintering and Casting". JOM. 74 (4): 1279–1295. Bibcode:2022JOM....74.1279C. doi:10.1007/s11837-022-05156-9. https://doi.org/10.1007%2Fs11837-022-05156-9 ↩

12. "AlNiCo Magnets". Stanford Magnets. Retrieved September 13, 2024. https://www.stanfordmagnets.com/alnico-magnets.html ↩

13. Rottmann, P.F.; Polonsky, A.T. (2021). "TriBeam tomography and microstructure evolution in additively manufactured Alnico magnets". Mater. 49: 23–34. doi:10.1016/j.mattod.2021.05.003. /wiki/Doi_(identifier) ↩

14. Cui, Jun; Ormerod, John (2022). "Manufacturing Processes for Permanent Magnets: Part I—Sintering and Casting". JOM. 74 (4): 1279–1295. Bibcode:2022JOM....74.1279C. doi:10.1007/s11837-022-05156-9. https://doi.org/10.1007%2Fs11837-022-05156-9 ↩

15. Dussa, Saikumar; Joshi, S. S. (2024). "Additively Manufactured Alnico Permanent Magnet Materials—A Review". Magnetism. 4 (2): 125–156. doi:10.3390/magnetism4020010. https://doi.org/10.3390%2Fmagnetism4020010 ↩

16. Chu, W.G; Fei, W.D; Li, X.H; Yang, D.Z; Wang, J.L (2000). "Evolution of Fe-Co rich particles in Alnico 8 alloy thermomagnetically treated at 800 °C". Materials Science and Technology. 16 (9): 1023–1028. Bibcode:2000MatST..16.1023C. doi:10.1179/026708300101508810. S2CID 137015369. /wiki/Bibcode_(identifier) ↩