Nanocomposite Nd-Rich Nd-Fe-B Alloys: Approaching Ideal Stoner-Wohlfarth Type behavior

In theory, the coercive field of well aligned, noninteracting, single domain, magnetic particles, assuming K 1 ≫ K 2 , was shown [ 1 ] to be H c = 2 K 1 / M s – NM s , where K 1 and K 2 are the magnetocrystalline anisotropy constants, M s is the spontaneous magnetization and N is the demagnetization factor. For randomly oriented, noninteracting particles the Stoner–Wohlfarth model [ 2 ] predicts that the value of H c decreases to about half. However, experimentally obtained H c values in permanent magnets are four (two) times smaller for well oriented (randomly oriented) samples. This discrepancy was attributed to strong local stray fields, arising at sharp edges and corners, and the grain surface regions where the anisotropy constant K 1 is reduced [ 3 ]. The change in the interaction between magnetic grains also affects coercivity. If the interaction between the magnetic grains is reduced as in rapidly quenched (Pr, Nd)–Fe–B [ 4–6 ], thin films Nd 2 Fe 14 B/W [ 7 ], and sintered Pr 17 Fe 53 B 30 [ 8 ] (due to the excess of nonmagnetic phases) the coercivity was found to increase significantly. In order to understand this difference between the theoretically predicted and experimentally obtained results for H c , rapidly quenched, Nd-rich, Nd x Fe 13.1 B(2 < x < 150) ribbons were prepared. The motivation is to obtain randomly oriented, magnetically single domain, Nd 2 Fe 14 B particles embedded in a nonmagnetic Nd matrix. This can be achieved by appropriate annealing of the rapidly quenched Nd x Fe 13.1 B samples below 1000 K [ 9 ]. By changing the amount of the nonmagnetic Nd matrix the strength between Nd 2 Fe 14 B particles was tailored ranging from strongly magnetically interacting particles in pure Nd 2 Fe 14 B to nearly noninteracting Nd 2 Fe 14 B particles in Nd 147.6 Fe 14 B. The latter is expected to approach the high coercive field predicted by the Stoner-Wohlfarth model.