D. Chambaere

Nature of the morin transition in al-substituted hematite

Abstract The Mossbauer spectra of α(Fe1-cAlc)2O3 with c=0.048 indicate the presence of two distinct spin phases below 245 K, one of which gradually disappears with increasing temperature. A careful study of the hyperfine parameters as a function of temperature reveals complex spin reorientation phenomena in both phases.

Influence of small aluminum substitutions on the hematite lattice

Abstract The effect of small Al substitutions (up to 8 mole%) in hematite, αFe 2 O 3 , is determined from the 57 Fe isomer shifts measured by the Mossbauer effect. Using the Debye approximation, the characteristic Mossbauer temperatures and the intrinsic isomer shifts are evaluated. Both parameters show a distinct maximum at 4 mole% Al substitution.

The electric field gradient at the iron sites inβFeOOH

The temperature behaviour of the hyperfine parameters of iron in paramagnetic chlorine containingΒFeOOH is studied by57Fe Mössbauer spectroscopy. Applying external magnetic fields, the sign and asymmetry parameterη of the electric field gradient (EFG) at the compounds two iron sites is determined. It is shown that an external field enhances the distributive character of the hyperfine parameters. Finally, heat treatment experiments allow new data on the nature of both iron coordination types to be presented.

The βFeOOH to αFe2O3 phase transformation: Structural and magnetic phenomena

It is shown that under ambient atmospheric conditions heating causes the crystal structure of βFeOOH (synthetic akaganeite) to degenerate gradually into a quasi amorphous intermediate state, before the final phase transformation to αFe2O3 (hematite) takes place. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Mössbauer Spectroscopy, this amorphization process is monitored and the structural, morphological and magnetic features of the intermediate phase as a function of the isochronal heating temperature are discussed: the crystallites develop macropores on their surface, the adsorption capacity raises up to 10 percent of the initial mass, a third type of Fe3+ coordination, having an extremely large quadrupole splitting, is created and the Néel temperature, after an initial decrease, exhibits a sharp increase at higher heating temperatures. The magnetic behaviour of this intermediate phase at low temperatures and in high external fields suggests this antiferromagnet undergoes magnetic phase transitions (metamagnetism and spin flop) at unusually low critical fields.

A Mössbauer effect study of MgFe2O4

Abstract A slowly cooled sample of the ferrimagnetic spinel MgFe2O4 has been studied with 57Fe Mossbauer spectroscopy over a wide temperature range both with and without high magnetic fields. The observed temperature dependence of the A and B site hyperfine parameters is discussed. Conclusions about the spin structure, the magnetic exchange interactions and the supertransferred hyperfine fields are presented.

An 57Fe Mössbauer effect study of poorly crystalline γ-FeOOH

Four synthetic lepidocrocite samples with a significant difference in crystallinity have been studied by the 57Fe Mössbauer effect technique at temperatures ranging between 12 K and 360 K. In the magnetically ordered region, the spectra display broad hyperfine field distributions, the profile of which could be derived numerically. In a broad transition region of approximately 20 K, an additional doublet must be included in the fitting model. From Mössbauer thermoscanning measurements at zero-velocity and with and without the application of an external magnetic field, it is found that the doublet is due to Fe3+ species in a paramagnetic state. Neither the hyperfine field nor the Néel temperature distributions are markedly affected by the crystallinity. In the pure paramagnetic state, the broadened doublets are best described by a distribution of quadrupole splittings in the range 0.3–1.9 mm/s. By numerical manipulations of the derived probability profiles it was possible to distinguish between the iron species in the outer surface layers of the γ-FeOOH particles and those in the innermost layers, the relative amount of each being correlated with the measured surface areas. The centre shift is quite uniform and its temperature dependence, determined in detail for one of the samples, is perfectly described by the Debije approximation for the second-order Doppler shift. From the obtained Debije temperature, a Mössbauer fraction f of 0.79 at room temperature was calculated, which is in good agreement with the results obtained for a reference lepidocrocite/hematite mixture. Finally, line shape simulations aimed to explain the external field spectrum at 180 K provided strong indications that both the sign and the asymmetry parameter of the electric field gradient are non-uniform as well.

On the Neel temperature of βFeOOH: structural dependence and its implications

Abstract Using 57 Fe Mossbauer thermoscanning, the magnetic transition temperature T N of a number of synthetic βFeOOH samples (akaganeite) is studied. Mossbauer spectra in external fields up to 6 T reveal the sextet-doublet transition to be a true magnetic order-disorder transition and that no superparamagnetic relaxation occurs. It is shown that the value of T N in particular and the strength of the magnetic interactions in general are highly variable and depend upon the chemical circumstances during synthesis; this is due to spin reduction, induced by a variable amount of interstitial water molecules in the compound.

A Mössbauer investigation of the quadrupole splitting in β-FeOOH

Abstract High resolution 57Fe Mossbauer effect measurements at room temperature show that the spectra of pure, synthetic β-FeOOH in its paramagnetic state consist of a superposition of two discrete doublets, due to the existence of two different electric field gradients (EFG) which are proven to be related to the presence of halogen ions in this compound.

On the influence of the dual iron co-ordination on the hyperfine field in βFeOOH

Abstract 57 Fe Mossbauer measurements of βFeOOH (synthetic akaganeite) between 4.2 and 56 K are reported, in order to study the compounds low temperature antiferromagnetic state. Two of the three observed hyperfine fields are identified and characterized in terms of the two different iron co-ordinations, while the third contribution is shown to be a thermally induced effect.

Mössbauer effect study of the magnetic structure in δ-FeOOH

Two samples ofδ-FeOOH with different particle sizes have been studied in an external field of 4 T and as a function of temperature. They were found to have a ferrimagnetic structure due to an unequal occupancy of antiferromagnetically coupled octahedral ferric ions. The large surface contribution, which is characterized by a canted spin structure and by highly deformed Fe3+ co-ordinations, strongly influences the magnetic properties observed with Mössbauer spectroscopy.