Semihard Iron-Based Permanent-Magnet Materials

Permanent magnets generally require a favorable, but difficult-to-achieve combination of high magnetization, Curie point, and magnetic anisotropy. Thus there have been few, if any, viable permanent magnets developed since the 1982 discovery of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ [M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura, and S. Hirosawa, J. Appl. Phys. 57, 4094 (1985)]. Here we point out, both by direct first-principles calculations on the iron carbides and silicides ${\mathrm{Fe}}_{5}{\mathrm{C}}_{2}$, ${\mathrm{Fe}}_{5}\mathrm{Si}\mathrm{C},$ and ${\mathrm{Fe}}_{7}{\mathrm{C}}_{3}$ as well as a discussion of recent experimental findings, that there are numerous rare-earth-free iron-rich potential permanent-magnet materials with sufficient intrinsic magnetic properties to reasonably achieve room-temperature energy products of 20--25 MG Oe. This is substantially better than the performance of the best available rare-earth-free magnets based on ferrite, as well as shape-anisotropy-employing alnico. These magnets could plausibly fill, at low cost, the present performance ``gap'' [J. M. D. Coey, Scr. Mater. 67, 524 (2012)] between the best rare-earth-free magnets and rare-earth magnets such as ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ and $\mathrm{Sm}\text{\ensuremath{-}}\mathrm{Co}$.