NANOSTRUCTURE AND MAGNETIC-PROPERTIES OF CHROMIUM-DOPED FE3B-ND2FE14BEXCHANGE-COUPLED PERMANENT-MAGNETS

Magnetic properties of Nd4.5Fe77-xCrxB18.5 and results from high-resol ution scanning transmission microscopy (HRSTEM) analysis of Nd4.5Fe77B 18.5 and Cr-doped Nd5.5Fe71Cr5B18.5 nanocomposite permanent magnets ar e reported. The magnets were prepared from amorphous alloys via a crys tallization heat treatment. With increasing Cr content, the intrinsic coercivity (H-cl) increased and the remanence (B-r) decreased consider ably. The average grain size of crystallites was approximately 20 nm a nd found to be smaller in the Cr-doped sample than in the Iron-doped o ne. 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 Nd1.1Fe4B4 phase with negligibl e Cr contents were also revealed in both Nd4.5Fe77B18.5 and Nd5.5Fe71C r5B18.5 samples. The experimental results suggest that, in Cr-doped Nd -Fe-B nanocomposite magnets, Cr is concentrated in Fe and/or FexB phas es and depleted in the Nd1.1Fe4B4 and some of Nd2Fe14B phases after cr ystallization. The increase in H-cl probably originates from depressio n of intergrain exchange couplings through Cr concentrated in the soft magnetic phases.