Recent Trends in Permanent Magnetism

Dr. Ralph Skomski
NCMN, University of Nebraska, Lincoln, NE 68588
Deeply influencing our lives, hundreds of permanent magnets are found in a typical household, ranging from toy magnets at fridges to hard-disk drives in computers and, abundantly, in cars. The performance of permanent magnets is described by the maximum energy product, which describes the material's ability to store magnetostatic energy in free space. In the 20th century, energy product has increased from about 1 kJ/m3 to more 400 kJ/m3. To a large extent, this progress reflects the discovery of rare-earth transition-metal intermetallics such as Nd2Fe14B and SmCo5. In these materials, the rare-earth atoms ensure anisotropy and coercivity, whereas the transition-metal atoms are responsible for magnetization and Curie temperature. The presently used rare-earth intermetallics are highly sophisticated structures, and room for further improvements is limited. In recent years, focus has therefore shifted from finding new intermetallic phases towards nanostructuring and, more generally, nanoscale processing. The nonlinear dependence of the energy product on the magnetization means that adding a soft phase with a high magnetization actually improves the hard-magnetic performance of the matrix phase. The corresponding theoretical energy products are of the order of 1000 kJ/m3 [1], and the predicted energy-product enhancement has been verified for an Fe-Pt system [2]. However, the relatively low magnetization of the hard-magnetic FePt phase limits the energy product of the system, and the generalization of the approach to other phases, most notably Nd2Fe14B, has remained a challenge. This is due to the demanding processing of complicated rare-earth transition-metal intermetallics and the need to maintain coercivity. In practice, coercivity development includes the control of imperfections such as metallurgical inhomogenities, grain boundaries, and surface irregularities.

[1] R. Skomski and J. M. D. Coey, Phys. Rev. B, 48, 15812 (1993).
[2] J. P. Liu, C. P. Luo, Y. Liu, and D. J. Sellmyer, Appl. Phys. Lett., 73, 3007 (1998).


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