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

The effect of phase transformations on the magnetic properties of rare eart h lean (Nd0.95La0.05)(9.5)Fe82.5-xCr2B6+x (x=0 to 4.5) and (Nd0.95La0.05)(7 .5+y)Fe80.5-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, name ly alpha-Fe and R2Fe14B, were found in (Nd0.95La0.05)(9.5)Fe82.5-xCr2B6+x a lloy 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 o f the R2Fe14B phase, which led to an increase in the intrinsic coercive for ce from 7.1 kOe for x = 0 to 12.6 kOe for x = 4.2. A B-r = 9.6 kG H-i(c) = 9.5 kOe, 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 conte nt, or y, was found to suppress the formation of the metastable Fe3B and/or R2Fe23B3 phases and to yield an alpha-Fe/R2Fe14B mixture for y>1. This inc rease in total rare earth content not only increases the volume fraction of both the R2Fe14B and alpha-Fe phases but also decreases the average grain size of these phases as evidenced by transmission electron microscopy analy sis. This decrease in the average grain size may subsequently enhance the i ntrinsic coercivity and the remanence of the ribbons. A B-r of 9.5 kG, a H- i(c) of 13.2 kOe, and a (BH)(max) of 18 MGOe were achieved in (Nd0.95La0.05 )(11)Fe77Cr2B10. (C) 1998 American Institute of Physics. [S0021-8979(98)207 11-0].