Quench rate dependence of the initial magnetization in rapidly solidified neodymium-iron-boron ribbons

The quench rate dependence of the magnetization process is reported for rapidly solidified Nd-Fe-B ribbons. In optimally quenched ribbons, where the Nd2Fe14B grain size is less than the estimated single domain particle size, moment reversal during both magnetization and demagnetization is 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. Magnetization and demagnetization behaviors remain strongly correlated in underquenched ribbons, suggesting that maximum coercivity may be largely determined by the resistance to domain wall formation within grains smaller than the single domain particle limit.