MOSSBAUER AND PAC STUDIES OF NANOCRYSTALLINE FE

High-purity Fe powder was mechanically milled under argon at ambient t emperature using an SPEX 8000 mill. The local atomic and magnetic stru cture was studied using Co-57/Fe Mossbauer and In-111/Cd perturbed ang ular correlations (PAC) spectroscopies. After 32 hours of milling, X-r ay diffraction revealed effective grain diameters of 18 nm and energy- dispersive X-ray analysis indicated a Cr impurity concentration of sim ilar to 5%, presumably introduced by mechanical attrition of steel bal l bearings used for milling. In addition to a spectral component very similar to bulk iron metal, the Mossbauer spectra exhibited hyperfine field shifts attributed to the Cr impurities. PAC spectra on Fe milled for 5 h, with no contamination, exhibited two components: (1) A sligh tly broadened magnetic interaction attributed to interior, defect-free sites of In/Cd probes with a mean hyperfine field sightly greater tha n in macroscopic grains. The defect-free site fraction grew appreciabl y during; milling, even though In is essentially insoluble in Fe. (2) An indistinct signal due to mixed magnetic and quadrupole interactions attributed to probes at surface or other defect sites.