The effect of tungsten addition on the magnetic properties and microstructure of SmFeN-alpha-Fe nanocomposites

The effect of tungsten addition, in the range of 0-17 at.%, on the magnetic properties and microstructure of SmFeN-alpha-Fe permanent magnet nanocompo sites is presented. The magnets were prepared by mechanical alloying of ele mentary powders of Sm, Fe and W, followed by annealing and nitriding. The addition of tungsten leads to initial increase and further decrease of the coercivity with a maximum value of 250 kA/m for 7 at.% W. The remanence , however, continuously decreases from 1.2 T for 0% W down to 0.7 T for 17 at.% W. The maximum value of (BH)(max) (120 J/m(3)), was obtained for 2 at. % W. The measurements of Curie temperature vs. W content showed a constant and decreasing values for alpha-Fe and the Sm2Fe17 phase, respectively. Thi s behaviour can be explained by the dissolution of some W atoms in the Sm2F e17 phase. X-ray diffraction analysis indicated, beside the alpha-Fe and Sm 2Fe17 phases, the existence of pure W in the microstructure. Addition of W leads to reduction of the a-Fe crystallite size (20 nm for 0% W, 18 nm for 7% W and 8 nm for 17% W). The high resolution electron microscopy investiga tions show randomly distributed small (below 20 nm) crystallites of tungste n. Thus for a small amount of W the remanence is still substantially enhanc ed. The increase of the coercivity, for W contents up to about 7 at.%, can possibly be attributed to increased magnetic anisotropy of the Sm-2(Fe,W)(1 7)N-3 phase. (C) 2000 Elsevier Science B.V. All rights reserved.