Direct phase transformation from hematite to maghemite during high energy ball milling

Phase evolution and magnetic properties of a high-energy ball-milled hematite–alumina system

“…In other words, the blocking temperature for Mössbauer experiments is above 6 K for the milled phase ␣-(Fe 1Ϫ⌬W Al ⌬W ) 2 O 3 . Its hyperfine parameters are very close to those reported by Randrianantoandro et al 8 for ball-milled hematite. It is significant that the area ratio at 6 K shown by the two different magnetic components is nearly the same as that presented at RT ͑see 11 However, by reducing the particle size or introducing substitutional aluminum in bulk hematite, T M decreases or is even suppressed.…”

“…Additionally, the most probable hyperfine magnetic field B hf max ͓see the inset in Fig. 2͑e͔͒ decreased relative to the field of a graded and well crystallized hematite because iron substitution by a nonmagnetic atom ͑i.e., by aluminum͒ causes a reduction in the magnetic hyperfine field, as previously stated by de Grave et al 28 -30 One can argue about the possible formation of maghemite, as previously reported for high-energy ballmilling induced hematite phase transformations, 8,31 and whose B hf is ϳ50 T at RT. If Mössbauer data are ambiguous on this point, the XRD data are not, since the ␥-Fe 2 O 3 phase pattern could not be identified in the diffractogram in Fig.…”

“…This is a known product of the high-energy ball-milling process. [7][8][9][10] Thus, the single line present among the individual subspectra is attributed to SPM iron nanoprecipitates. Indeed, several authors have reported a singlet with similar values for the ␦ in iron implanted alumina samples [17][18][19][20][21] and as-sputtered Fe:Al 2 O 3 thin films, 22 and attributed it to nanostructured iron.…”

“…In fact, these situations have usually been reached through the high-energy ball-milling process. 7,8 Room-temperature Mössbauer spectra for some selected samples are shown in Fig. 2, with the respective hyperfine magnetic field (B hf ) and QS distributions ͑histograms͒ as inserts in this figure.…”

“…[5][6][7][8][9][10] Exploring and understanding the magnetic and electric modifications associated with the structural evolution in the ballmilled hematite, has been the driving force of many efforts conducted in recent years. [7][8][9][10] The crystal structure of hematite is isomorphous to alumina (␣-Al 2 O 3 ϭcorundum), with a close-packed oxygen lattice and Fe 3ϩ cations in octahedral sites. It is a complex magnetic material, being antiferromagnetic ͑AF͒ at low temperatures and undergoing a transition to a weak ferromagnetic state ͑WF͒ above the so-called Morin temperature (T M Х260 K), because of a spin canting.…”