Formation of a high-coercive state in sintered Nd-Fe-B-Ga magnets by thermocycling

A comparative investigation of the effect of two types of low-temperature annealing, namely, (1) isothermal treatment and (2) thermocycling treatment (TCT), on the coercive force H c of sintered magnets of an Nd-Fe-B-Ga alloy has been performed. Powders of an Nd15.5Fe77.7B6.5Ga0.3 alloy were prepared from materials taken in three different conditions: cast alloy (CA), hydrided alloy (HA), and partly dehydrided alloy (PDA). The coercive force of the sintered magnets reaches different maximum values, depending on the time of grinding, which after isothermal annealing at 550°C are 680, 688, and 800 kA/m in the magnets prepared from the CA, HA, and PDA, respectively. It has been revealed that the subsequent application of a TCT consisting in the variation of the temperature in a range of 20–550°C additionally increases H c. In fine-grained magnets the increment in H c is 10–15%; in coarse-grained, 100–300%. Thus, after a TCT the extremal dependence of H c on the time of grinding of the powder virtually vanishes. To explain the observed effect of a TCT, an assumption is made on the effect of interphase stresses (arising at the boundaries between the grains and the intergrain phase enriched in Nd) on the mobility of atoms and the formation of defect-free grain boundaries, which favor an increase in H c upon annealing.