Search

To understand the particular properties of thin α-Fe2O3 films, hematite nanoparticles with sizes between 20 and 140 nm have been examined by integral low-energy electron Mössbauer spectroscopy (ILEEMS) and transmission Mössbauer spectroscopy (TMS) at room temperature and 80 K. By comparing the results of both Mössbauer variants, surface effects of the Morin transition are studied. The results clearly indicate that the Morin transition takes place at higher temperature in the interior of the particles as compared to the outer layers. It is found that the lowering of the Morin transition is much stronger at the surface of smaller particles. [ABSTRACT FROM AUTHOR]

Thin α-Fe2O3 films on semi-quartz substrates have been prepared by radio frequency (RF) sputtering of a magnetite target in a pure argon gas flow with a bias power applied. The films were studied by X-ray diffraction and Integral Low-Energy Electron Mössbauer Spectroscopy (ILEEMS), the latter in the temperature range of 80 to 330 K. In ILEEMS electrons of ∼10 eV are detected and therefore it is an ideal instrument to study thin films with thicknesses below 500 nm. The first ILEEMS measurements on α-Fe2O3 films are reported and focus on the behaviour of the Morin transition, i.e., the reorientation of Fe3 + spins from the [111] crystallographic direction at low temperature, to the (111) basal plane at high temperature. Some films (∼400 nm) show the presence of both a weak- and antiferromagnetic hematite phase for all temperatures between 140 and 330 K. Others of about ∼100 nm thick only have a weak ferromagnetic phase for temperatures between 140 and 330 K of which the hyperfine field distribution shows two maxima, related to two distinct phases. Both phases stay in some films almost equal in intensity in the spectrum for the whole measured temperature range. In others a significant evolution in the spectral areas of both weak ferromagnetic phases as function of the temperature is observed. [ABSTRACT FROM AUTHOR]

Abstract: Amorphous Fe–B alloys can be prepared at room temperature by reduction with borohydride of iron-oxide particles in suspension. By varying the borohydride concentration, amorphous Fe–B alloys with boron contents between 2 and 13at.% have been produced by reduction of synthetic (nano-sized particles) and natural (micro-sized) hematite (α-Fe2O3) using sodium borohydride (NaBH4). The results presented in this paper were obtained from a systematic study of the effect of borohydride concentration on the resulting reaction products using a variety of experimental techniques, such as X-ray diffraction, wet chemical analyses, thermal analyses, scanning electron microscopy, transmission Mössbauer spectroscopy (TMS) and integral low-energy electron Mössbauer spectroscopy (ILEEMS). Three distinct NaBH4 concentrations have been applied. Beside unreacted hematite, amorphous Fe1−x B x alloys have been identified from the TMS spectra recorded at various temperatures between 15K and room temperature. The amount of Fe1−x B x increases strongly with increasing NaBH4 concentration, and for a given concentration with increasing specific surface area (SSA). Thermal analyses have suggested that for any given reduction condition, the boron content x in the formed amorphous alloy has a bimodal distribution. This is found to be consistent with the finding that the contribution of the Fe1−x B x phase to the total Mössbauer spectra consists of a superposition of a broad sextet and doublet. ILEEMS has further revealed that especially the surface layers of the hematite grains are affected by the reduction processes. [Copyright &y& Elsevier]

Abstract: Low crystalline ferrihydrite (2-XRD lines) was synthesized and thermally treated between 195 and 325°C in air. The obtained products were characterized by XRD, FT-IR and 57Fe Mössbauer spectroscopies. XRD and FT-IR did not show the formation of α-Fe2O3 at 220°C. The IR band at 330cm−1 for the sample obtained after heating at 245°C indicated that at this temperature some α-Fe2O3 has crystallized. XRD also showed the presence of α-Fe2O3 in the sample obtained at 245°C. XRD and FT-IR spectra of the sample obtained after heating at 325°C fully correspond to that of α-Fe2O3. The Mössbauer spectrum recorded at RT of the sample prepared at 220°C showed a broadened quadrupole doublet, whereas hyperfine-field distributions are observed at 78 and 12K, presumably due to non-uniform chemical environments for the Fe3+ and/or to the shape and size distributions of the α-Fe2O3 clusters. The RT Mössbauer spectrum of the sample heated at 245°C showed a superposition of a quadrupole doublet and a distributed sextet, the latter believed to be due to poorly crystalline α-Fe2O3. At 78K, two sextets were resolved. The sharper one, with a hyperfine field H hf =527kOe and quadrupole shift 2ε Q =−0.12mms−1, and fractional area of RA=0.22, is due to α-Fe2O3, which possibly shows the coexistence of antiferromagnetic and weakly ferromagnetic spin states. The much broader magnetic component is assigned to ferrihydrite. At 12K, hematite has H hf =535kOe and 2ε Q =−0.03mms−1. The ferrihydrite sextet is still considerably broadened. Mössbauer spectra at RT and 78K recorded for the sample obtained upon heating at 325°C correspond only to α-Fe2O3 fraction. For the sample obtained by heating at 245°C, a Mössbauer spectrum was recorded at 4.2K in an applied field of 6T. The ferrihydrite component clearly splits into a high- and low-field contribution. The heating of 2-line ferrihydrite caused a certain crystalline ordering and in the next stage the transformation to α-Fe2O3. [Copyright &y& Elsevier]

Mo¨ssbauer spectra for three Al-hematite powders were recorded at 4.2 K in applied field of 140 kOe. Particle sizes, as determined from X-ray diffraction line broadenings, are of order 100 nm. In zero field the powders are in a pure antiferromagnetic state. It is found that the applied-field is too low to produce a spin flop into the basal plane in all individual grains constituting the powders. Only portions undergo this transition, while remaining portions undergo a screw-rotation transition whereby the antiferromagnetic spin axis is canted away from the c-axis and rotates perpendicular to the external field. It is argued that the angle of the external field with respect to the c-axis determines which of the two distinct spin reorientation mechanisms takes place. A critical angle can be calculated from the numerical results of the spectral analyses. Its value depends on the field strength and on the Al substitution. [Copyright &y& Elsevier]