Elongated Iron-Cobalt: Ferrite, a New, Lightweight, Permanent Magnet Material

The preparation of fine particles of elongated iron-cobalt:ferrite is described. Coercive forces ranging from 2400 oe, for particles having 57% Fe, 31% Co, and 12% O, to over 7000 oe, for particles having 47% Fe, 26% Co, and 27% O, have been measured at −196°C. Compacts prepared from particles with the composition 57% Fe, 31% Co, and 12% O had the following magnetic properties when measured at 25°C: $\mathrm{Br}\text{\hspace{0.17em}=\hspace{0.17em}6200\hspace{0.17em}}\mathrm{gauss}$ , $\mathrm{Hc}\text{\hspace{0.17em}=\hspace{0.17em}1550\hspace{0.17em}}\mathrm{oe}$ , and ${\mathrm{(BH)}}_{\max }{\text{\hspace{0.17em}=\hspace{0.17em}4.0×l0}}^6\hspace{0.17em}\mathrm{gauss-oe}$ . These compacts have a physical density of 3.7 g/cc, and are stable magnetically at temperatures up to 100°C. Magnets exhibit a reversible decrease in remanence of less than 1% over the temperature range of −196°−+50°C. The observed magnetic properties can be accounted for in terms of a cobalt ferrite shell in exchange contact with its elongated iron-cobalt core.