Grain boundary phases in (Ga, Cu)-doped Nd-Fe-B sintered magnet

The nano-scale non-ferromagnetic thin grain boundary (GB) phase plays an important role in enhancing coercivity of Nd-Fe-B magnets. However, it is a great challenge to simultaneously explore the structure, composition and magnetism of thin GB phases due to their ultrafine scale. Here, aside from common Nd₆Fe₁₃Ga, NdO and Nd₅Ga₃ phases, a kind of thin GB phase with excellent wettability has been found in the (Ga, Cu)-doped Nd-Fe-B sintered magnet, which is analyzed to be a new Nd-rich phase with the composition of Nd_49.6Pr_18.5Fe_6.8Co_6.7Cu_15.1Ga_3.3 (at.%). Using tilt-series electron diffraction, it is determined to be tetragonal with lattice parameters a = 13.2 Å and c = 5.4 Å. Lorentz microscopy further reveals that the new Nd-rich phase is non-ferromagnetic and its existence at thin GBs as thin as about 10 nm is beneficial for magnetically isolating adjacent grains and improving coercivity. Besides, a new Fe-rich phase with the composition of Nd_32.1Pr_8.5Fe_48.4Co_3.3Cu_1.9Ga_3.5Al_2.3 (at.%) has also been found in the magnet, which is needle-shaped and located at triple junctions. Tilt-series electron diffraction has determined it to be hexagonal with lattice parameters a = 17.6 Å and c = 29.2 Å.