Hard Magnetic Properties of Nanocrystalline Fe-Rich Fe–Nd–B Alloys Prepared by Partial Crystallization of Amorphous Phase

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Abstract
When Fe-rich Fe–Nd–B amorphous alloys containing 88 to 90 at% Fe are annealed for 60–300 s at 923–1023 K, the annealed alloys have the nanostructure consisting of bcc-Fe, Fe14Nd2B and remaining amorphous phases and exhibit rather good hard magnetic properties. The best hard magnetic properties of remanence (Br), coercive field (iHc) and maximum energy product ((BH)max) are 1.14 T, 260 kA/m and 117 kJ/m3, respectively, for Fe90Nd7B3, 1.28 T, 252 kA/m and 146 kJ/m3, respectively, for Fe89Nd7B4 and 1.22 T, 240 kA/m and 130 kJ/m3 for Fe88Nd8B4. The mean particle sizes of these crystallites in the optimum annealing treatment are 20 to 40 nm and the thickness of the intergranular amorphous layer is 10 to 30 nm. The amorphous layer contains Nd concentrations much higher than the nominal concentration and the enrichment seems to be the reason for the residual existence of the amorphous phase at the high temperatures. The nanoscale Fe14Nd2B particles are surrounded by the bcc-Fe and amorphous phases. The three constituent phases have ferromagnetism and their Curie temperatures for the Fe89Nd7B4 alloy annealed for 300 s at 923 K are about 1040 K for bcc-Fe and 630 K for Fe14Nd2B. The further increase in annealing temperature and time causes the decrease in hard magnetic properties presumably because of the grain growth of bcc-Fe and Fe14Nd2B phases resulting from the disappearance of the residual amorphous phase. The coexistence of bcc-Fe, Fe14Nd2B and amorphous phases on a subnanoscale is important for the achievement of the rather good hard magnetic properties and hence the bcc-Fe and amorphous phases seem to act as an effective magnetic exchange-coupled medium. The simultaneous achievement of the high Br and (BH)max values in the residual existence of the amorphous phase for the Fe-rich alloys containing about 90 at% Fe is believed to be the first evidence and has significant engineering importance because of the expectations of high deformability and high cost performance.