CHARACTERIZATION OF HARD MAGNETIC 2-PHASE MECHANICALLY ALLOYED SM2FE17N3 ALPHA-FE NANOCOMPOSITES/

A range of exchange-coupled two-phase nanocomposites composed of hard magnetic Sm2Fe17N3 and soft magnetic alpha-Fe was prepared by mechanic al alloying with a view to optimizing the hysteresis loop shape. The m ain variables were the crystallization conditions, the nitriding treat ment, and the chemical additives. A model of the diffusion of nitrogen in the two-phase nanocomposite is proposed that explains how the pres ence of Fe permits the nitrogenation of samples at lower temperatures than in single phase Sm2Fe17 materials. Studies of samples both resin bonded and cold compacted measured in open and closed circuits reveale d that the con ect choice of demagnetizing factor used to correct dema gnetizing fields depends critically on the sample density. Transmissio n electron microscopy (TEM) studies of the materials prepared revealed grain sizes in the range 10-50 nm. The shape of the magnetic hysteres is loop and resulting magnetic propel lies reflects the grain size of both phases. image analysis of high resolution scanning electron micro scopy micrographs of etched samples showed that in general two to thre e soft grains cluster together and are surrounded by hard grains, but the grain sizes of both phases were found to be the same. The crystall ization of the hard phase from the mainly amorphous precursor is the p rimary factor determining grain size. Zr and Ta were the most successf ul additives In controlling the grain growth during crystallization, r educing the grain size from 20-30 to 10-20 nn. High resolution TEM ind icated the presence of a grain boundary phase between the crystallites of two tno phases. This phase was confirmed in Mossbauer studies of s amples where it seems to constitute 15 vol % of the samples and has a significant effect on the coupling between the two phases. Susceptibil ity measurements are an effective indicator of the degree of coupling between the hard and soft magnetic phases. (C) 1997 American Institute of Physics.