Mössbauer characterization of iron oxides and (oxy)hydroxides: the present state of the art

Mössbauer spectroscopy is a powerful direct technique for the identification and quantification of iron oxides and (oxy)hydroxides in soils and sediments. However, further characterization with respect to structural properties such as crystallinity, Al substitution, stoichiometry, water content, etc. is rather limited. With some examples of synthetic and natural goethite and hematite sample series it is illustrated that the hyperfine parameters depend on much more structural features than the Al content and crystallinity alone. Neither the Morin transition in hematite nor the Verwey transition in magnetite is directly applicable for analytical purposes in natural samples.     

Mössbauer study of the Morin transition

At hand of Mössbauer spectra on two commercial powder hematite samples, it was found that the finite slope shown by the Morin transition reveals the presence of an additional spectral component associated to a second-order phase transition. This extends over a temperature range much larger than the temperature width of the thermal hysteresis of the Morin transition, which is of first order. The new component is attributed to a change in the sign of the fourth-order single-ion anisotropy field due to crystal defects or cation substitution. The results of previous works are discussed from this point of view.     

A Mössbauer study of the morin transition on the surface and in the bulk of hematite single crystals

The Morin transition has been studied simultaneously by transmission and conversion electron Mössbauer spectroscopy on the surface and in the bulk of hematite single-crystal plates with orientation (111) and different origin. It was found that in both samples, the surface transition is shifted to higher temperatures with respect to the bulk.     

Surface effect on the Morin transition of α-Fe2O3 particles coated with surfactant

Uniform hematite particles with an average size of 400 Å were prepared for reducing the influence of the particle-size distribution on the measurement of the Morin temperature. A series of Mössbauer spectra at different temperatures have been recorded. The Morin transition occurs at 216 K for sample B coated with a surfactant-oleic acid, and it occurs at 192 K for uncoated sample A. The temperature intervals in the region of the spin flip are almost the same for both samples. These results show that aT _m shift could arise from surface effects such as spin pinning directly induced by a surfactant, and that the distortion produces restriction not only on the top few layers, but also on the whole volume of the particle.     

The effect of the particle morphology on the Mössbauer effect in αFe2O3

Several samples of hematite, one a natural specimen, are considered in order to study the specific effect of the morphology on the magnetic structure and especially on the Morin transition. It seemed that mainly the weak-ferromagnetic contribution is affected by the morphology, as well as the transition temperature and region.     

Electrical resistivity of a-Fe2O3 and comparison to TEP at the first order Morin transition

The electrical resistivity of hematite is measured in the region of the first order Morin transition and the above trasport property is compared to the thermoelectric power (TEP) in the same region. Some similarities can be found in the detailed behaviour, of the electrical resistivity which can be distinguished generally as a step-like change, at the temperature of the transition, and the behaviour of the thermoelectric power at the same temperature. The above behaviour of the two transport effects is compared to that observed in other materials at a first order transition.     

Low temperature reorientation of Fe spins in an irradiated hematite single crystal

Irradiation with alpha-particles of 29 and 50 MeV energies leads to essential changes in magnetic properties of hematite single crystals. When temperature decreases, the number of iron ions with high-temperature spin orientation decreases too but does not disappear completely, and at helium temperatures, in contrary, increases (the new transition). Appearance of the new transition is explained by superparamagnetic behaviour of the disordered zones produced by atom-atomic collision cascades. The splitting of the Müssbauer spectra in the temperature interval of the Morin transition as well as at the new transition undoubtedly shows that the both transitions are transitions of the first order.     

A specific heat study of natural haematite around the Morin transition and the effects of entrapped water

The Morin transition in samples of natural haemitite has been examined by specific heat and Mössbauer measurements. Specific heat measurements on as-received samples reveal an anomaly at ～ 273.2 K and irregular behaviour at higher temperatures. However, heat treatment of samples demonstrated that these effects were due to water trapped inside the natural specimens.No anomaly due to the Morin transition was observed in the specific heat although the expected changes in the Mössbauer hyperfine parameters were observed for the different magnetic phases below and above the Morin transition. The results are consistent with a coexistence of the two phases and a spread of temperatures ～ 40 K over which the transitions take place throughout the material of purity 99.6 wt%.     

Mössbauer studies of the Morin transition interval of uniform hematite particles

In the vicinity of the Morin temperature Mossbauer spectra of uniform, non-uniform hematite particles and⁵⁷Fe enriched on the surface of the uniform ones were measured. It was established that the wider the distribution of the particle size the larger the temperature interval of the Morin transition, but the surface effect did not affect the interval. The reason of existence of the Morin transition interval was explained based on theoretical analysis.     

Neutron diffraction evidence for the spin-reorientational transition in FeBO3 at high pressures

The spin-reorientational transition from the state with “easy-plane” anisotropy to the state with “easy-axis” anisotropy—a pressure analogue of the Morin temperature transition in hematite— was detected in iron borate at a pressure of P～17 kbar at room temperature by the neutron diffraction method.     

Study of neutron irradiation effect on Morin transition in hematite

It is established by Mössbauer spectroscopy and slow neutron diffraction that temperature range of Morin transition in hematite changes depending on neutron irradiation dose, and above some critical values the transition is not observed at all. A fraction of iron ions with high-temperature orientation of moments which cause the sample weak-ferromagnetic characteristics decreases with temperature fall but does not disappear completely, and near 4 K it even rises. It is established that radiation defects causing the changes in Morin transition disappear at annealing of hematite on air at temperatures of 400–500 K.     

Pressure induced spin-reorientation transition in FeBO3

Abstract

It was found out by means of neutron diffraction that “easy plain - easy axis” spin-reorientation transition takes place in FeBO₃ under quasihydrostatic pressure of approximately 17 kbar at room temperature. This is a pressure analog of the Morin transition in hematite.     

Morin transition suppression in Polycrystalline 57Hematite (α-Fe2O3) exposed to 56Fe(II)

We used the isotope selectivity of ⁵⁷Fe Mössbauer spectroscopy to investigate changes in the magnetic properties of polycrystalline hematite exposed to ferrous iron (Fe(II)). We found that sorption of ⁵⁶Fe(II), followed by interfacial electron exchange, alters the bulk magnetic properties of ⁵⁷hematite. After reaction with ⁵⁶Fe(II), we observed partial suppression of the Morin transition of ⁵⁷hematite to below 13 K. This is significantly lower than the Morin temperature (T _M) of ～230 K measured for isotopically enriched polycrystalline ⁵⁷hematite, as well as the T _M of 264?±?2 K reported for normal polycrystalline hematite.     

Interplay of surface conditions, particle size, stoichiometry, cell parameters, and magnetism in synthetic hematite-like materials

We have studied several synthetic hematite-like materials, produced via different reactions using various hydrothermal conditions and various temperatures of annealing in air, by bulk elemental analysis, weight loss measurements, scanning electron microscopy, powder X-ray diffraction, Mössbauer spectroscopy, and SQUID magnetometry. We conclude that hematite-like materials cannot be related to pure stoichiometric hematite via a single stoichiometric or physical parameter and that at least two degrees of freedom are required. This is most clearly seen when we introduce a plot of the cell parameter c versus the cell parameter a on which hematite-like materials do not fall on a single line but occupy an entire region that is bounded by hydrohematite-hematite and protohematite-hematite lines. A Morin transition boundary on this c-a plot separates a region where Morin transitions occur from a larger region where Morin transitions do not occur down to 4.2 K. Previous claims that particle size is the dominant factor controlling the Morin transition are understood in terms of correlations between stoichiometry and particle size that are produced at synthesis. Changing contents of incorporated molecular water and structural hydroxyls with associated cation vacancies have different characteristic effects on the crystal structure and move the sample coordinates in different directions on a c-a plot. It is also shown that an accessory sulphate content is adsorbed on the individual hematite crystallites and is not structurally incorporated. Mössbauer spectroscopy is used, as usual, to identify and characterize the spin structure. In addition, hyperfine field distributions from room temperature spectra, extracted by a new method, give a sensitive measure of sample conditions but not a unique one since several factors affect the extracted distributions in similar ways.     

VSM and Mössbauer study of nanostructured hematite

In the present work, we have synthesized nanostructured hematite samples using chemical precipitation method. The crystal structure and the grain size of the samples were studied using XRD. The zero field cooled and field cooled magnetization curves of the samples were recorded in the temperature range from 300 to 10 K. The variations of Morin transition temperature and blocking temperature with the grain size of the samples were investigated. The hysterics curves of the samples were recorded and the samples showed a superparamagnetic nature at room temperature whereas, at 10 K the samples showed open hysteresis curves. The sample with smaller grain size showed higher value of coercivity compared to samples with larger grain size. Mössbauer spectra of the samples were recorded and the grain size dependence on Mössbauer parameters was investigated.     

Superparamagnetism and the Morin transition in amorphous and crystalline iron oxide systems

This work studies the magnetic properties of microcrystalline iron oxides. These were produced from iron-nitrate under a variety of conditions using a simple hydrolysis and baking technique. In particular we wished to observe superparamagnetic behaviour of the particles produced. We also discuss the relaxation phenomena and Morin transitions observed in the hematite and ferrihydrite obtained and investigate how this is affected by noncrystallinity and particle volume.     

Mössbauer studies of the morin transition in bulk and microcrystalline α-Fe2O32

Mössbauer spectra of bulk and microcrystalline hematite have been analyzed at the Morin transition for both the weak-ferromagnetic (WF) and anti-ferromagnetic (AF) phases simultaneously, increasing precision in the extracted Mössbauer parameters. The Morin transition in bulk took place in less than 0.4 K, and in the microcrystals decreased linearily with expanding lattice in agreement with its increase under external pressure. Temperature hysteresis measurements in the microcrystals gave values of the fourth-order magnetic anisotropy energy which indicated possible surface spin-pinning effects. In bulk the magnetic field decreased by 7 ± 0.5 kOe in going from the WF to the AF phase on decreasing temperature, and the isomer shift decreased by $\text{0.014 ± 0.003}\text{mm}\text{sec.}$ With lattice dilation the AF quadrupolar interaction appeared to decrease, while the isomer shift change across the transition goes positive. In contrast the change in magnetic field is not simply related to this lattice dilation, indicating surface spin-pinning effects.     

Magnetic structure of hematite nanostructured in a porous glass

The magnetic and crystal structures of hematite (α-) in the form of nanoparticles with a mean diameter of 157(2) Å were synthesized within a porous glass and were studied by neutron and X-ray diffraction. The co-existence of two magnetic phases with different directions of magnetic moments, which in the bulk hematite are observed above and below the Morin transition was supposed.     

Magnetically Induced Anomalous Dichroism of Atomic Transitions of the Cesium D<Subscript>2</Subscript> Line

An alternative treatment of the well-known effect of a decrease in the Morin transition temperature in hematite with a decrease in the size of crystallites to the complete disappearance of the transition for nanoparticles smaller than 20 nm is proposed. In contrast to the standard speculative explanation of this effect in terms of the effect of surface and defectiveness of grains, we suggest that the decisive factor is an increase in the contribution of the shape anisotropy of particles with a decrease in their size, which is responsible for the spread of orientations of the axes of the resulting magnetic anisotropy with respect to the crystallographic axes. Our reasons are confirmed by a numerical analysis of Mössbauer spectra of hematite nanoparticles within the continuous model of magnetic dynamics of an ensemble of antiferromagnetic nanoparticles in the two-sublattice approximation generalized to the existence of weak ferromagnetism (Dzyaloshinskii interaction).     

Rotation of the plane of polarization and linear birefringence of sound in hematite below the morin point

The linear antiferromagnetic birefringence of sound in hematite (α-Fe₂O₃) residing in the collinear easy-axis phase (L ‖ C ₃) below the Morin point is experimentally studied. The plane of polarization of a linearly polarized transverse acoustic wave propagating along the trigonal axis C ₃ of a hematite crystal placed in a magnetic field H applied in the basal plane (H ⊥ C ₃, 3.5 ≤ H ≤ 15 kOe) is found to rotate after a temperature-driven orientational phase transition to the easy-axis state. The angle of rotation exhibits a 180° angular dependence on the direction of the magnetic field in the basal plane and varies from zero to ～π/2. Numerical estimates suggest that the conditions necessary for rotation of the plane of polarization through appreciable angles (～π/2) can be satisfied in the easy-axis phase at orientational phase transition temperatures close to the Morin point, which actually takes place in the fields employed. The results obtained are described sufficiently well by the theory of linear antiferromagnetic birefringence of sound (E.A. Turov) and confirm its main conclusions.