The effect of boron and rare earth contents on the magnetic properties of La and Cr substituted $α-Fe/R2Fe14B$-type nanocomposites

The effect of phase transformations on the magnetic properties of rare earth lean $(Nd0.95La0.05)9.5Fe82.5−xCr2B6+x$ (⁠$x=0$ to 4.5) and $(Nd0.95La0.05)7.5+yFe80.5−yCr2B10$ (⁠$y=0$ to 4) melt spun ribbons has been investigated. The phase mixture, after optimum thermal processing, was found to be strongly dependent upon the rare earth and boron contents. Two magnetic phases, namely α-Fe and $R2Fe14B,$ were found in $(Nd0.95La0.05)9.5Fe82.5−xCr2B6+x$ alloy ribbons with $x$ ranging from 0 to 4.5. For a fixed rare earth content, increases in the boron concentration resulted in a higher volume fraction of the $R2Fe14B$ phase, which led to an increase in the intrinsic coercive force from 7.1 kOe for $x=0$ to 12.6 kOe for $x=4.2.$ A $Br=9.6 kG,$ $Hc=9.5 kOei$⁠, and $(BH)max=15.5 MGOe$ have been obtained in the alloy ribbons with $x=4.5.$ On the other hand, the increase in the total rare earth content, or $y,$ was found to suppress the formation of the metastable $Fe3B$ and/or $R2Fe23B3$ phases and to yield an α-$Fe/R2Fe14B$ mixture for $y>1.$ This increase in total rare earth content not only increases the volume fraction of both the $R2Fe14B$ and α-Fe phases but also decreases the average grain size of these phases as evidenced by transmission electron microscopy analysis. This decrease in the average grain size may subsequently enhance the intrinsic coercivity and the remanence of the ribbons. A $Br$ of 9.5 kG, a $Hci$ of 13.2 kOe, and a $(BH)max$ of 18 MGOe were achieved in $(Nd0.95La0.05)11Fe77Cr2B10.$