Alnico magnets have existed for almost a century and are used in many applications. Their discovery provided solutions to applications requiring temperature stability, higher magnetic flux density, lower coercive field, and higher corrosion resistance. Alnico magnets are composed primarily of aluminum (Al), nickel (Ni), cobalt (Co), and iron (Fe). The name Alnico is an acronym derived from the first two letters of the three main elements in the alloy: aluminum, nickel, and cobalt. Since the development of rare earth magnets in the 1970s, some applications using Alnico magnets have replaced them with less expensive, more powerful magnets; however, their high-temperature stability still makes them indispensable in specific applications.

Alnico magnets were first invented by a Japanese metallurgist, Tokushichi Mishima, in the early 1930s. He discovered that adding the proper amounts of inexpensive aluminum to nickel steel created a strong permanent magnet. These alloys, comprised of nickel, aluminum, and iron,  yielded a coercivity double that of the best steel magnets available. He determined they had an extremely high coercive force, a strong residual magnetism, and a resistance to being quenched compared to the available magnets. Mishima filed for the patent, “Magnet Steel Containing Nickel and Aluminium,” in 1932 and was granted the patent in 1936. These are the MK-Steel magnets, named after Mishima’s childhood home, “Mitsujima ka.” Mishima later discovered that substituting cobalt for part of the iron in the alloy improved its magnetic properties, ductility, tenacity, and ease of mechanical working. Other elements, including copper (Cu) and titanium (Ti), improved the coercivity of the alloy, allowing adaptation for further applications. 

Because of their low-temperature coefficient, Alnico magnets began to replace expensive electromagnets in devices like motors, sensors, and gauges. After their invention, they were used in military equipment in WWII, including radios, military tracking systems, radar, and electric motors. Other industries engineered Alnico magnets to provide solutions for various products, including generators, audio equipment like microphones and loudspeakers, and household uses like the magnetic seal on refrigerator doors. Their ability to be adapted and engineered to replace more expensive magnets at the time made them a highly utilized choice for various applications.

Alnico magnets have been engineered over time to meet the demands of varying applications based on the magnetic properties and performance characteristics required. They can be isotropic or anisotropic. Isotropic magnets are magnetized randomly, whereas anisotropic magnets are magnetized, so their magnetic properties are stronger in a specific direction. Anisotropic magnets are exposed to a strong magnetic field during manufacturing to align their magnetic domains in a particular direction. This typically results in a stronger magnet than isotropic magnets due to aligned magnetic domains. Isotropic magnets are also more prone to demagnetization than anisotropic magnets. Alnico magnets are mainly manufactured through two processes: casting or sintering.

A cast alnico magnet is created by melting the precise amounts of raw elements in an induction furnace to form a molten mixture. This is then poured into resin-bonded sand molds to create the desired magnet shape. By this method, several magnets can be poured at once. The alloy is then cooled and solidified, ready for fettling or careful surface grinding to remove excess material. Anisotropic magnets are created by exposure to an extreme heat treatment at this step in the manufacturing process and then exposed to a strong magnetic field while they cool and harden. Some magnets are complete after this phase, while others are machined to meet specific dimensional requirements.

Sintering is the other process used to manufacture Alnico magnets using a powdered metallurgical technique. Elements are ground into a fine powder and mixed to form the alloy. This powder is then subject to significant pressure and molded to the desired shape. Heat and pressure are applied to create a solid mass. The product is heated so the particles are bonded together without reaching the melting point. This process removes any porosity between particles, resulting in a cohesive solid. During the cooling process, an external magnetic field is applied to create an anisotropic magnet, or this process is eliminated if an isotropic magnet is the intended outcome. The result is a strong, durable magnet with enhanced density and magnetism. When complex geometries are required, sintering alnico is the suitable process.

Alnico magnets are available in several grades. Today’s composition of Alnico alloys is typically 8-12% Al, 15-26% Ni, 5-24% Co, up to 6% Cu, up to 1% Ti, and balanced with Fe. Once magnetized, these magnets have between 5-17 times the magnetic force of magnetite or Iodestone (a naturally occurring magnet that attracts Fe). Below is a chart from Thomas & Skinner, Inc. describing the different grades of Alnico magnets manufactured at our facility.

Alnico 1: Alnico 1 magnets are isotropic cast magnets with relatively low magnetic strength. They are primarily used in specialized applications that require less magnetic force, such as precision measuring instruments and low-sensitivity devices.

Alnico 2: Alnico 2 magnets are the second weakest alnico magnets. They are frequently used in electric guitar pickups to produce a clean and defined sound. These magnets are isotropic and manufactured by casting or sintering.

Alnico 3: Alnico 3 magnets contain no Co and are the weakest of the Alnico magnets. They are isotropic cast magnets and are also used in applications like electric guitar pickups.

Alnico 5: Alnico 5 magnets are versatile and offer a balance of magnetism and temperature stability. They are the most commonly used alnico magnet and are manufactured in several cast versions and one sintered version. They are anisotropic magnets and have exceptional magnetic output. Alnico 5 magnets are used in many applications, including electric guitar pickups, motors, and generators.

Alnico 6: Alnico 6 magnets are anisotropic and manufactured as one cast and one sintered version. They are often utilized in electric motors, generators, and various sensing devices where consistent magnetic output and temperature stability are critical.

Alnico 8: Alnico 8 is a strong, durable magnet resistant to demagnetization. It is stronger than its predecessors and is favored for applications requiring its enhanced attributes. Alnico 8 magnets have the highest coercive resistance among Alnico grades. These magnets are used in gauges and sensors where precision measurement in extreme temperatures is required, as well as microphones and loudspeakers, electric motors, relays and switches, and guitar pickups. Alnico 8 is manufactured in a few anisotropic cast and anisotropic sintered grades.

Alnico 9: Alnico 9 magnets have superior magnetic properties that make them suitable for demanding environments in applications for industries such as defense and aerospace. They are anisotropic cast magnets.

Each Alnico grade’s unique combination of elements and attributes optimizes performance across various applications. Thomas & Skinner, Inc. has manufactured high-quality,  high-performance metals for over a century and is respected for engineering solutions across multiple industries. The Alnico chart denotes the grades manufactured in Thomas & Skinner’s facility in Indianapolis, Indiana. As innovators, we have engineered custom Alnico grades that meet our customers’ unique demands, remain stable under extreme temperatures, and resist demagnetization. With our unparalleled quality and performance, Thomas & Skinner manufactures high-performance Alnico magnets for applications, including sensors and measuring instruments, precision defense technologies, magnetic separators, aerospace systems, audio equipment, and medical devices.

References

1. Tokushichi, Mishima. Magnet steel containing nickel and aluminium. U.S. Patent 2,027,994, issued January 14, 1936.
2. Jun Cui, Matthew Kramer, Lin Zhou, Fei Liu, Alexander Gabay, George Hadjipanayis, Balamurugan Balasubramanian, David Sellmyer, Current progress and future challenges in rare-earth-free permanent magnets, Acta Materialia, Volume 158, 2018, Pages 118-137, ISSN 1359-6454, https://doi.org/10.1016/j.actamat.2018.07.049.
3. (n.d.). Alnico Magnets: Types and Manufacturing Process. Industrial Quick Search. https://www.iqsdirectory.com/articles/magnet/alnico-magnet.html
4. (n.d.). Alico Magnets – High Performance. Thomas and Skinner, Inc. https://thomas-skinner.com/high-performance-magnets/alnico-magnets/