Initial Magnetization Behavior of Rapidly Quenched Neodymium-Iron-Boron Magnets

Abstract

ABSTRACTInitial magnetization and demagnetization data are reported for three forms of rapidly solidified Nd-Fe-B permanent magnet materials: melt-spun ribbons, hot pressed magnets, and die upset magnets. In all three materials the results are consistent with domain wall pinning at grain boundary phases as the coercivity mechanism. Optimally quenched ribbons are comprised of randomly oriented single domain Nd2Fe14B grains, and both initial magnetization and demagnetization are controlled by strong domain wall pinning at grain boundaries. Maximum coercivity is accompanied by a low initial permeability. Coercivity is reduced in overquenched ribbons by partial retention of a magnetically soft amorphous or very finely crystalline microstructure. Coercivity decreases in underquenched ribbons because wall pinning weakens as the grain size increases above optimum. Correlation of magnetization and demagnetization behaviors suggests that maximum coercivity in ribbons is largely determined by the resistance to domain wall formation in grains smaller than the single domain particle limit. Grain size is much less important in the aligned die upset magnets. Domain walls are initially free to move until they become strongly pinned at grain edges, and complete magnetization requires an applied field greater than the coercive field. Hot pressed magnets show a mixture of ribbon and die upset behavior.