Coercivity of Mischmetal-Cobalt Alloy Powders

Abstract

A single‐phase alloy of the type MMCo5 prepared by fusing cobalt with cerium‐rich mischmetal (MM: 54.5 w% Ce, 26 La, 13 Nd, 5 Pr) is the cheapest of several new candidate materials for fine‐particle permanent magnets. Results of experiments with powders produced by mechanical grinding and tempering are reported. The coercive force is limited by crystal imperfections introduced during milling. Maximum values of M Hc are between 2500 and 4600 Oe, depending upon the milling conditions and the magnetizing field. Vibration grinding produces higher Hc than ballmilling, but ballmilled powders align better in a field (loop squareness Mr/Mp ≈0.9 vs 0.7). Vacuum annealing at 300°–400°C improves the coercivity somewhat (3770 to 4600 Oe); higher temperatures reduce Hc drastically. Mortar grinding under liquid nitrogen yields considerably higher coercive forces than grinding at room temperature.