Recent Trends in Permanent Magnetism |
| Dr. Ralph Skomski |
| NCMN, University of Nebraska, Lincoln, NE 68588 |
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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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