Permanent magnetic materials, also known as hard magnetic materials, refer to materials that are difficult to magnetize and difficult to demagnetize once magnetized. Their high coercivity is the most important category of magnetic materials. Next, let's start with permanent magnetic materials and gradually explore the world of magnetic materials (see in Fig. 1).

Fig. 1 Types of Rare Earth Permanent Magnetic materials

Rare earth permanent magnets are one of the most important magnetic products. Since their introduction in the 1960s, three generations of products have been mass-produced and applied. The fourth generation of rare earth iron nitrogen permanent magnet products is still in the research and development stage and may become the new generation of rare earth permanent magnet products in the future.

The first and second generation samarium cobalt permanent magnets are prepared by powder metallurgy processes. The main raw materials are samarium and cobalt. Since they are expensive and cobalt is a strategic resource, the mass production and large-scale application of samarium cobalt permanent magnets have been restricted and not widely used. They are mainly applied in the aviation, aerospace, and national defense industries.

The successful development of the third generation neodymium-iron-boron (NdFeB) rare earth permanent magnets is of great significance. It not only has astonishingly excellent properties and a record-breaking high maximum energy product (BH)max, but also replaces expensive strategic materials like cobalt and scarce resources such as samarium with inexpensive and abundant iron and neodymium. The characteristics of high quality and low cost, huge production potential, and broad application prospects of NdFeB permanent magnets have caused a great stir among magnet researchers and manufacturers around the world. Since Masato Sagawa in Japan announced the existence of NdFeB magnets in 1983, the records of various properties of these magnets have been repeatedly updated, and it has become the central topic of international academic conferences on multiple occasions.

By comparison of performance for permanent magnet materials in Table 1, it can be found that NdFeB is currently the rare earth permanent magnet material with the highest cost - performance ratio. Moreover, the magnetic properties of sintered NdFeB are superior to those of bonded NdFeB, while bonded NdFeB has better machining capabilities. 

Table 1. The Comparison of Performance for Permanent Magnet Materials