Nanostructure and magnetic properties of chromium-doped Fe3B-Nd2Fe14B exchange-coupled permanent magnets

Magnetic properties of Nd4.5Fe77 − xCrxB18.5 and results from high-resolution scanning transmission microscopy (HRSTEM) analysis of Nd4.5Fe77B18.5 and Cr-doped Nd5.5Fe71Cr5B18.5 nanocomposite permanent magnets are reported. The magnets were prepared from amorphous alloys via a crystallization heat treatment. With increasing Cr content, the intrinsic coercivity (HcJ) increased and the remanence (Br) 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 Nd1.1Fe4B4 phase with negligible Cr contents were also revealed in both Nd4.5Fe77B18.5 and Nd5.5Fe71Cr5B18.5 samples. The experimental results suggest that, in Cr-doped Nd-Fe-B nanocomposite magnets, Cr is concentrated in Fe and/or FexB phases and depleted in the Nd1.1Fe4B4 and some of Nd2Fe14B phases after crystallization. The increase in HcJ probably originates from depression of intergrain exchange couplings through Cr concentrated in the soft magnetic phases.