Influence of the grain boundary phase characteristics on the magnetic properties of Nd-Fe-B magnets

The effects of the grain boundary phase (GBP) thickness, ferromagnetism and orientation with the c-axis of the Nd2Fe14B phase on the magnetic properties of Nd-Fe-B magnets were investigated by micromagnetic simulation. Increasing the thickness of the non-ferromagnetic GBP parallel to the c-axis decreases the demagnetization field at the corners of magnets which act as the nucleation sites of initial reversed magnetic domain. However, the demagnetization field at the corners of magnets increases with the increased thickness of the non-ferromagnetic GBP perpendicular to the c-axis. This results in the high coercivity increasing rate with the same nonferromagnetic GBP thickness increasing parallel to the c-axis than that perpendicular to the c-axis. For the magnet models with ferromagnetic GBP the initial nucleation of reversed magnetic domain starts at the ferromagnetic GBP. And the nucleation field of the reversed magnetic domain in the ferromagnetic GBP perpendicular to the c-axis is far higher than that in the ferromagnetic GBP parallel to the c-axis. Moreover, it is much easier for the reversed magnetic domain in the ferromagnetic GBP parallel to the c-axis to propagate to the neighboring Nd2Fe14B grains than that in the ferromagnetic GBP perpendicular to the c-axis. This leads to the much more substantial deteriorating coercivity with the increased thickness of ferromagnetic GBP parallel to the c-axis than that perpendicular to the c-axis. This work clarifies the influence mechanism of the GBP characteristics on the magnetic properties of Nd-Fe-B magnets.