Mössbauer effect study of the spin structure in natural hematites

Abstract

Abstract. Three natural hematites, ~-F%O3, from the region of Elba have been investigated by means of SVFe M6ssbauer spectroscopy at variable temperatures between 80 and 400K. The samples were selected on the basis of their different morphology as observed from powder X-ray diffraction and transmission electron microscopy. The mean crystallite diameters (MCD) along [104] are 1000, 280 and 40 nm respectively. Energy-dispersive analyses of X-rays revealed the presence of minor amounts of Si impurities in those two hematites having the largest MCD. All three hematites show the coexistence at low temperatures of antiferromagnetic-like (AF) and weakly-ferromagnetic-like (WF) spin states. The saturation values of the AF and WF magnetic hyperfine fields and quadrupole shifts have been determined, from which conclusions are drawn concerning the spin structure in relation to the crystallinity of the samples. The variations of the hyperfine parameters in the Morin-transition region indicate a gradual reorientation of both AF and WF spins towards the basal plane. As expected, the Morin transition itself is affected by the particle size. The two hematites exhibiting the largest particle dimensions still show an AF contribution for T > 270K. It is suggested, and argued that this unusual behaviour is due to the presence of Si 4+, and hence Fe 2+, in the lattice. The characteristic M6ssbauer temperatures and the intrinsic isomer shifts were evaluated from the temperature variation of the observed isomer shifts. Both parameters are not significantly affected by the morphology and are in excellent agreement with data obtained for synthetic hematites.

Characterisation of the magnetic iron phases in Clovis Class rocks in Gusev crater from the MER Spirit Mössbauer spectrometer

“…Hence, one may conclude that B hf,WF (WF state) equals 52.4 ± 0.5 T and B hf,AF (AF state) 53.3 ± 0.5 T, which compare well with the field values of the laboratory samples reported in Table 2. Similar results were obtained for the isomer shifts of hematite by Morris et al (2006a) in cases where the Ht component is more prominent and was decomposed into the two phases, but these authors mention larger differences (up to 2 T) between the hyperfine fields of the WF and AF phase than those commonly observed in the laboratory for natural terrestrial hematite (0.8 T, e.g., De Grave and Vandenberghe, 1990). Further, the quadrupole shifts mentioned by Morris et al (2006a) and by Dyar et al (2006) are average values for the involved temperature range (210-260 K), in which these values can vary significantly between À0.20 and À0.01 for the WF phase, and between 0.00 and 0.44 for the AF phase (De Grave and Vandenberghe, 1990).…”

“…Similar results were obtained for the isomer shifts of hematite by Morris et al (2006a) in cases where the Ht component is more prominent and was decomposed into the two phases, but these authors mention larger differences (up to 2 T) between the hyperfine fields of the WF and AF phase than those commonly observed in the laboratory for natural terrestrial hematite (0.8 T, e.g., De Grave and Vandenberghe, 1990). Further, the quadrupole shifts mentioned by Morris et al (2006a) and by Dyar et al (2006) are average values for the involved temperature range (210-260 K), in which these values can vary significantly between À0.20 and À0.01 for the WF phase, and between 0.00 and 0.44 for the AF phase (De Grave and Vandenberghe, 1990). In contrast, the quadrupole shift values mentioned in the present paper refer to a narrower temperature range and show much resemblance to those of natural hematites on Earth (À0.20 and 0.36 mm/s) (De Grave and Vandenberghe, 1990).…”

“…For instance, for a natural sample of terrestrial Ht the hyperfine fields were found to decrease systematically with increasing temperature from 52.3 to 52.0 T and from 53.0 to 52.5 T for WF and AF spin states, respectively (De Grave and Vandenberghe, 1990). For the quadrupole shifts, the corresponding ranges are between À0.14 and À0.19 mm/s and between 0.36 and 0.30 mm/s.…”

“…1. The adjusted hyperfine parameters are listed in Table 2 and compared with the results measured for laboratory samples, i.e., the aforementioned a-Fe/a-Fe 2 O 3 reference absorber (results for a-Fe and for the AF contribution by a-Fe 2 O 3 , window m5), for a natural terrestrial Ht (results for aFe 2 O 3 -WF at T = 215 K) (De Grave and Vandenberghe, 1990), and for stoichiometric Fe 3 O 4 at 215 K (De Grave et al, 1993). The area ratios for the outer to middle to inner absorption lines for the a-Fe sextet were found to be 3:3.3:1, reflecting the preferential orientation of the spins for the plane of the foil, which is not unusual.…”

“…Further, the quadrupole shifts mentioned by Morris et al (2006a) and by Dyar et al (2006) are average values for the involved temperature range (210-260 K), in which these values can vary significantly between À0.20 and À0.01 for the WF phase, and between 0.00 and 0.44 for the AF phase (De Grave and Vandenberghe, 1990). In contrast, the quadrupole shift values mentioned in the present paper refer to a narrower temperature range and show much resemblance to those of natural hematites on Earth (À0.20 and 0.36 mm/s) (De Grave and Vandenberghe, 1990). Finally, Klingelhö fer et al (2005) reported fixed values for the isomer shift of hematite at 0.416 mm/s, which are unacceptably high for a natural hematite at the mentioned temperatures (220-260 K) and when source and target are kept at the same temperature.…”