Abstract
Magnetic properties of Nd 4.5Fe 77 − x Cr x B 18.5 and results from high-resolution scanning transmission microscopy (HRSTEM) analysis of Nd 4.5Fe 77B 18.5 and Cr-doped Nd 5.5Fe 71Cr 5B 18.5 nanocomposite permanent magnets are reported. The magnets were prepared from amorphous alloys via a crystallization heat treatment. With increasing Cr content, the intrinsic coercivity ( H cJ) increased and the remanence ( B r) decreased considerably. The average grain size of crystallites was approximately 20 nm and found to be smaller in the Cr-doped sample than in the non-doped one. Two types of regions with the Fe/Nd atomic ratio of approximately 7 were detected by HRSTEM; some regions contain Cr and others do not. Presence of grains of the non-magnetic Nd 1.1Fe 4B 4 phase with negligible Cr contents were also revealed in both Nd 4.5Fe 77B 18.5 and Nd 5.5Fe 71Cr 5B 18.5 samples. The experimental results suggest that, in Cr-doped Nd-Fe-B nanocomposite magnets, Cr is concentrated in Fe and/or Fe x B phases and depleted in the Nd 1.1Fe 4B 4 and some of Nd 2Fe 14B phases after crystallization. The increase in H cJ probably originates from depression of intergrain exchange couplings through Cr concentrated in the soft magnetic phases.
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