Optimisation of the magnetic properties of mechanically milled R5.5Fe73.5-xCoxCr3B18 nanocomposites

Mechanical milling and subsequent annealing of R4.5R'Fe73.5-xCoxCr3B18 (R=Nd,Pr and R'=Tb,Dy) ingots has been found to produce hard magnetic nanocomposites of (R,R')2(Fe,Co)14B, (Fe,Cr)2B and α-(Fe,Co) phases. Here we report on the optimisation of the composition of such nanocomposites. Substituting different rare-earth metals has a significant effect on the magnetic properties. The replacement of Nd with Pr produces higher coercivity and remanence, and better loop squareness. However, the replacement of Tb with Dy produced inferior properties when the main rare-earth component was Nd. Improved properties were obtained with the combination of Pr and Dy or Tb. Substitution of Co for Fe was found to lower coercivity but increase the remanence. The best combination of properties measured was for Pr4.5Dy1Fe68.5Co5Cr3B1x, where Hc=0.41 MA/m, Jr=1 T, and (BH)max for the powder was 100 kJ/m3. These results will be discussed in terms of the grain size and the intrinsic properties of the hard and soft magnetic phases identified using X-ray diffraction. (orig.)