Comment on "Grain-boundary structure and magnetic behavior in nanocrystalline ball-milled iron"

Del Bianco, Hernando, Bonetti, and Navarro [Phys, Rev. B 56, 8894 (1997)] p ublished Mossbauer spectroscopy and magnetization measurements on ball-mill ed iron. The observed changes were attributed to the increased grain-bounda ry fraction and this was supported by measurements on heat treated samples. The recovery of the physical properties of bulk iron after the heat treatm ents was attributed to grain growth and atomic rearrangement at the grain b oundaries. The effect of possible impurities (e.g., Cr originating from the milling tools) was not considered in any respect. We show that the results on the heat treated samples do not exclude the possibility of explaining s ome features of the Mossbauer hyperfine field distribution by impurities in stead of a reduced hyperfine field of the grain-boundary atoms. Heat treatm ents of small grain size Fe95Cr5 samples were performed under argon and in a vacuum of 10(-7) Pa. Significant reduction of the Cr content of the alloy phase can be observed at elevated temperatures, which is explained by Cr2O 3 formation in the surface region. [S0163-1819(99)04821-3].