Magnetic Precipitation-Hardening

Abstract

Co5Sm intermetallic compounds have recently been developed for permanent magnets with relatively high coercive fields (He ). Basically He is high because of the very large magnetocrystalline anisotropy (K 1) of these hexagonal crystals. Magnetization reversal in a reverse magnetic field apparently is not caused by rotational processes within single magnetic domains because He would then be even larger than is observed (3 × 104 Oe). Rather, the reverse magnetization nucleates at defects in the material, mostly in the surface region of the sintered grains, and then spreads through the bulk by domainwall motion. The 180° domain walls separating the old from the new magnetization directions seem to be held up by ‘pinning centres’ during their motion as is shown by a finite susceptibility of the measured hysteresis loop. The nature of these defects is difficult to ascertain in Co5Sm because domain walls are extremely narrow (width δ ≍ 60 Å) in such a material of high K1 and the pinning defects will be of a correspondingly small size*. In precipItation alloys of Co–Cu–Fe with cerium studied by Nesbitt et al. the high coercivity has been shown to be due to domain-wall pinning.