Permanent magnetic materials, Alnico

M-type ferrites are mainly used as permanent magnet material. They have largely replaced the alnicos as preferred permanent magnet material, as a result of the lower material and processing costs. These ferrites were first introduced under the trade name Ferroxdure, the isotropic form in 1952 (22) and the anisotropic (crystal oriented) form in 1954 (23), and are widely available commercially under various trade names such as Oxid and Koerox. They cover about 55% of the world market of permanent magnet materials, corresponding to 1100 million U.S. doUars (1991), as weU as 55% of the U.S. market, at 300 million. 

The advantageous magnetic properties of Fe-Ni-Al alloys with a suitable composition were first discovered by Mishima in 1931, which initiated the development of the Alnico alloys for applications as permanent magnet materials (see, e.g. Jelling-... 

A useful permanent magnetic material must have an ordering temperature well above the operating temperature of many electrical and electronic devices which approaches sometimes 170°C (—450 K). Typical examples are Alnico, Ba-ferrite and PtCo. Simultaneously at the operating temperatures no deterioration should take place which irreversibly diminishes the magnetic properties. 

Chromium—Cobalt—Iron Alloys. In 1971, a family of ductile Cr—Co—Fe permanent-magnet alloys was developed (79). The Cr—Co—Fe alloys are analogous to the Alnicos in metallurgical stmcture and in permanent magnetic properties, but are cold formable at room temperature. Equivalent magnetic properties also can be attained with substantially less Co, thereby offering savings in materials cost. 

McCurrie, R. A. (1982). The structure and properties of alnico permanent magnet alloys. In Ferromagnetic Materials, Vol. 3. Ed. E. P. Wohlfarth. North-Holland, Amsterdam, pp. 127-88. 

Magnetic filler n. Any permanently magnetizable material in powder form that may be incorporated into plastics to produce molded or extruded-strip magnets. Major ones in use are Alnico, rare earths, and, most used in plastics, hard ferrite. 

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