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

Abstract

͓Phys. Rev. B 56, 8894 ͑1997͔͒ published Mössbauer spectroscopy and magnetization measurements on ball-milled iron. The observed changes were attributed to the increased grain-boundary fraction and this was supported by measurements on heat treated samples. The recovery of the physical properties of bulk iron after the heat treatments was attributed to grain growth and atomic rearrangement at the grain boundaries. The effect of possible impurities ͑e.g., Cr originating from the milling tools͒ was not considered in any respect. We show that the results on the heat treated samples do not exclude the possibility of explaining some features of the Mössbauer hyperfine field distribution by impurities instead of a reduced hyperfine field of the grain-boundary atoms. Heat treatments of small grain size Fe 95 Cr 5 samples were performed under argon and in a vacuum of 10 Ϫ7 Pa. Significant reduction of the Cr content of the alloy phase can be observed at elevated temperatures, which is explained by Cr 2 O 3 formation in the surface region.

Mössbauer study of the interface of iron nanocrystallites

“…Following these studies, n-Fe particles were prepared by other methods, including chemical 10 or cluster beam deposition 11 and mechanical milling. [12][13][14][15][16] These studies could not reveal an unambiguous common component in the Mössbauer spectra of different n-Fe samples which could be safely identified as the one belonging to grain-boundary atoms. On the other hand, the role of different impurities are emphasized in most of these works.…”

“…On the other hand, the role of different impurities are emphasized in most of these works. 10,11,[13][14][15] A large variety of the extensively studied soft magnetic nanocrystalline composite materials 17 also contain n-Fe grains besides nanosize amorphous granules. These materials are usually formed by partial crystallization of amorphous ribbons.…”

Mechanically induced phase transformations of the sigma phase of nanograined and of coarse-grained near-equiatomic FeCr alloys

“…A depletion of the Cr content inside grains might be related to the formation of Cr 2 O 3 at grain boundaries for long milling times. A preferential oxidation of Cr occurred indeed in ball-milled Fe 0.95 Cr 0.05 but during annealing in argon [39].…”

“…The average room-temperature hyperfine magnetic field at 57 Fe nuclei of nanocrystalline ␣-Fe is close to that of coarsegrained ␣-Fe, being reduced by at most 5%, once the unavoidable effects of contamination are properly taken into account [38][39][40][41]. Similarly, the mean hyperfine magnetic fields of coarse-grained bcc Fe 0.51 Cr 0.49 are essentially identical ( B = 16 T) when the alloy is filed (d = 250 nm) or when it is ball-milled (d = 25 nm) despite domain sizes d which differ by an order of magnitude [42].…”

Low temperature heat capacity and thermal stability of nanocrystalline nickel

“…2). Nanocrystalline samples can be generally described as a two-component system [12] consisting of the nanocrystallines and the grain or interphase boundary components. The heat-capacity enhancements in the nc-materials are usually associated with an increase in the con®gura-tional and vibrational entropy of the grain boundaries, which constitute a large volume fraction of the material.…”