What Is A Permanent Magnet And What Are The Categories

A permanent magnet, also known as a hard magnet, is a material that can retain its magnetic properties over a long period of time once it has been magnetized. This ability to maintain a constant magnetic field区分s it from soft magnets, which lose their magnetism when the external magnetic field is removed. Permanent magnets can be found naturally, such as magnetite (a type of iron oxide), or they can be manufactured artificially. These magnets are widely used in various applications including electronics, electrical engineering, machinery, transportation, healthcare, and daily life products.

Permanent magnets are characterized by their wide hysteresis loop, high coercivity, and high remanence. They operate in the second quadrant of the demagnetization curve after being saturated and charged magnetically. To ensure maximum stored magnetic energy and stable magnetism, permanent magnets should have as high coercivity (Hc), remanence (Br), and maximum energy product ((BH)m) as possible.

Permanent magnets can be classified into two major categories based on their material composition: alloy permanent magnetic materials and ferrite permanent magnetic materials.

Alloy Permanent Magnetic Materials include:

1. Rare Earth Permanent Neodymium Magnets (e.g., Nd2Fe14B, or Neodymium Magnets): These are among the strongest magnets commercially available. They have extremely high magnetic properties, with a maximum energy product (BHmax) over 10 times higher than that of ferrite. NdFeB magnets can be further divided into bonded NdFeB, sintered NdFeB, and injection-molded NdFeB based on production processes. Bonded NdFeB magnets are made by mixing NdFeB powder with binders like resin, plastic, or low-melting-point metals, and then formed through compression, extrusion, or injection molding. Sintered NdFeB magnets are produced through air jet milling and sintering, resulting in high coercivity and excellent mechanical properties. Injection-molded NdFeB magnets offer high precision and can be easily made into complex shapes.

2. Samarium Cobalt (SmCo): SmCo magnets are divided into SmCo5 and Sm2Co17 based on their composition. As a second-generation rare earth permanent magnet, SmCo not only has a high maximum energy product (14-28 MGOe) and reliable coercivity but also exhibits good temperature characteristics. Compared to NdFeB, SmCo is more suitable for high-temperature applications (above 200°C).

3. Aluminum Nickel Cobalt (AlNiCo): AlNiCo is an alloy composed of aluminum, nickel, cobalt, iron, and other trace metallic elements. It can be produced through sintering or casting processes, allowing for the production of various sizes and shapes. Cast AlNiCo permanent magnets have the lowest reversible temperature coefficient and can operate at temperatures up to over 600°C.

Ferrite Permanent Magnetic Materials (Ferrite): Ferrite magnets are primarily made from raw materials like BaFe12O19 and SrFe12O19 through ceramic processing. They are hard and brittle, with good temperature resistance and low cost, making them the most widely used permanent magnets.

In summary, permanent magnets play a crucial role in numerous applications due to their ability to retain magnetism over time. Their diverse material compositions and production processes offer a range of properties suitable for various applications.