Hard Magnetic Materials: A Perspective

• Published in: IEEE transactions on magnetics Vol. 47; no. 12; pp. 4671 - 4681
• Main Author: Coey, J. M. D.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.12.2011; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Magnetic anisotropy; Magnetic films; Magnetic materials; Magnetism; Materials science; Neodymium alloys; Other topics in materials science; permanant magnets; Permanent magnets; Physics; Rare earths; permanant magnets; Magnetic anisotropy; Hard materials; Permanent magnets; Magnetic materials; Neodymium alloys; Magnetic properties; magnetic films
• ISSN: 0018-9464; 1941-0069
• DOI: 10.1109/TMAG.2011.2166975

The principles of operation of permanent magnets are summarized, and their development is reviewed. The key figure of merit, the energy product, improved exponentially over much of the 20th century, doubling roughly every 12 years. Yet it has not improved significantly in the last 20 years. Constraints on further development are explained, together with the limits of 1/4 μ 0 /M s 2 on energy product and 2K 1 /μ 0 M s on coercivity, where K 1 is the uniaxial anisotropy constant and Ms is the spontaneous magnetization. The challenge of making rare-earth free magnets with a large energy product is discussed, as well as nanocomposite megajoule magnets and the development of new magnetically hard thin-films with perpendicular anisotropy which are potentially interesting for spin electronics or magnetic recording.