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Types and Development of Permanent Magnet Materials

There are many types of permanent magnetic materials and they are widely used.The permanent magnetic materials used at present mainly go through three stages: metal permanent magnetic material, ferrite permanent magnet material and rare earth permanent magnet material.

The first stage: metal permanent magnetic materials, a large category o alloyed permanent magnetic materials with iron and iron group elements as important components,it developed and applied earlier, also called permanent magnet alloy.It mainly includes two kinds of permanent magnet alloys: Al-Ni-Co magnets and Fe-Cr-Co magnets.The development and production of this kind of material began in the early 20th century and was made by casting process. Therefore, it is also called casting permanent magnet material.Since 1931, alumino-nickel-cobalt series magnetic steels have been obtained by adding Al,Ni,Co into Fe after pouring and heat treatment. Since then, aluminum-nickel-cobalt magnets have dominated permanent magnetic materials until the 1960s.The magnetic energy product of this kind of material is low, but its Curie temperature is very high (up to 890℃), the temperature stability is very good, the magnetic inductance temperature coefficient is low, therefore, the use in some special devices can not be replaced, and still has the stable market demand up to now.

The second stage: ferrite permanent magnet, also known as permanent magnet ferrite, is a mixture of Fe2O3 and strontium (or barium) in a certain proportion, pre-fired, crushed, pulverized, pressed, sintered and grind.Ferrite permanent magnet materials were discovered in the 1930s. the coercivity of such permanent magnets is generally only 0.5 T,the remanent magnetism is about 0. 4 T, the magnetic energy product is low (25~36kJ/m3), its raw material is cheap, the craft is simple, the price is low, therefore obtained rapid development in the 1970s, its output occupied the first place.

The third stage: rare earth permanent magnetic material, which is a kind of high performance permanent magnetic material based on the intermetallic compound formed by rare earth element RE (Sm,Nd,Pr etc) and transition group metal element TM (Fe,Co etc.)Since the 1960's, rare earth permanent magnetic materials began to develop. The development of rare earth permanent magnetic materials has experienced three generations, the first generation is SmCo5 rare earth magnet, the second generation is Sm2Co17 rare earth magnet, and the third generation is NdFeB rare earth magnet.

The following figure shows the relationship between the magnetic energy product of permanent magnetic materials and the age.It can be seen from the figure that the permanent magnet material has been studied and developed for about 100 years, from about 1 MGOe of carbon steel at first to about 60MGOe of Nd-Fe-B today.The performance has been improved by leaps and bounds. At present, the maximum magnetic energy product has reached over 90% of the theoretical value of Nd-Fe-B.

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