Influence of microstructure on the interphase exchange coupling of Nd2Fe14B + 10 wt%α-Fe nanocomposites obtained at different milling energies

Abstract

The influence of the microstructure on the magnetic interphase exchange coupling of Nd2Fe14B + 10 wt% α-Fe nanocomposites obtained through mechanical milling was studied as a function of the milling energy (shock, friction and the ball impact energy) by varying only the size of the grinding media (steel balls with diameters of 5, 10, 15, and 20 mm). The mean crystallite sizes in both Nd2Fe14B and α-Fe were studied through X-ray diffraction, by the Scherrer method. The variation in milling energy had little impact on the mean crystallite sizes for α-Fe; approximately 10 nm after annealing at 700 °C for 1.5 min and 20 nm after annealing at 800 °C for 1.5 min. However, after annealing, the Nd2Fe14B crystallites present average sizes ranging from 50 nm (for the lowest impact energy used) to 20 nm (for high impact energy), with no significant variation of the grain sizes due to annealing temperature in the investigate range. Increasing the milling energy does not lower the mean crystallite size beyond 20 nm for the hard magnetic phase. The influence of the microstructure on the magnetic interphase exchange coupling between the hard and soft magnetic phases is reflected in the demagnetization curves and dM/dH vs. H plots for the annealed samples. The samples in which the Nd2Fe14B phase presents lower mean crystallite sizes show an improvement in both coercivity and energy product over the samples with larger grains. Further improvement in the magnetic interphase coupling can be observed as the average crystallite sizes decrease to 20 nm, for which the highest values for Hc = 0.65 T and (BH)max = 110 kJ/m3 were recorded.