Mössbauer study on Zn1 − x Fe x O semiconductors prepared by high energy ball milling

We present the preparation of massive Zn_1???x Fe _x O ternary oxides using the mechanical mill. The Fe atom is a particular dopant since it presents two different oxidation states which allow us to vary the starting materials: Fe₂O₃, $\upalpha $ -Fe or FeO. Parameters such as initial concentrations, atmosphere and milling times were varied. X-ray diffraction and ⁵⁷Fe Mössbauer spectrometry (MS) were applied in order to analyze the structure evolution and iron incorporation in the wurtzite crystalline structure with milling time. At final stages, Fe atoms seem to be incorporated in the ZnO structure for those samples milled under Ar atmosphere. In all cases, two paramagnetic components, attributed to Fe atoms in both valence states, were observed by MS.     

Magnetic and Mössbauer studies of pure and Ti-doped YFeO 3 nanocrystalline particles prepared by mechanical milling and subsequent sintering

Single-phased nanocrystalline particles of pure and 10 % Ti ⁴⁺-doped perovskite-related YFeO ₃were prepared via mechanosynthesis at 450^°C. This temperature is ～150–350 ^°C lower than those at which the materials, in bulk form, are normally prepared. Rietveld refinements of the X-ray diffraction patterns reveal that the dopant Ti ⁴⁺ ions prefer interstitial octahedral sites in the orthorhombic crystal lattice rather than those originally occupied by the expelled Fe ³⁺ ions. Magnetic measurements show canted antiferromagnetism in both types of nanoparticles. Doping with Ti ⁴⁺ lowers the Néel temperature of the YFeO ₃ nanoparticles from ～ 586 K to ～ 521 K. The Ti ⁴⁺-doped YFeO ₃ nanoparticles exhibit enhanced magnetization and coercivity but less magnetic hyperfine fields relative to the un-doped nanoparticles. The ⁵⁷Fe Mössbauer spectra show ～ 15 % of the YFeO ₃ nanoparticles and ～22 of Ti ⁴⁺-doped YFeO ₃ ones to be superparamagnetic with blocking temperatures < 78 K. The broadened magnetic components in the ⁵⁷Fe Mössbauer spectra suggest size-dependent hyperfine magnetic fields at the ⁵⁷Fe nuclear sites and were associated with collective magnetic excitations. The ⁵⁷Fe Mössbauer spectra show the local environments of the Fe ³⁺ ions in the superparamagnetic nanoparticles to be more sensitive to the presence of the Ti ⁴⁺ ions relative to those in the larger magnetic nanoparticles.     

Microwave absorption and Mössbauer studies of Fe3O4 nanoparticles

Materials consisting of nanometer-sized magnetic particles are currently the subject of intensive research activities. Especially, much attention has been paid to their promising features for microwave magnetic properties. Well dispersed Fe₃O₄ nanoparticles of 30 nm have been synthesized by oxidization method with NaNO₂, and the microwave magnetic properties of the composites have been studied. The real and imaginary part of relative permittivity remained low and nearly constant in the region of 0.1–18 GHz, respectively. As a result, the resin composites having a thickness of 2.0–3.2 mm, and containing 20 vol% Fe₃O₄ in the form of nanoparticles with an average diameter of 30 nm, exhibited excellent electromagnetic wave absorption properties in the frequency range of 4.5–12.0 GHz.     

57Fe Mössbauer spectra study of coated α-Fe2O3 nanoparticles

Nano-sized -Fe₂O₃ particles coated with polar organic molecules have been studied using the Mössbauer spectroscopy method. The -Fe₂O₃ nanoparticles were prepared by the microemulsion method. The average particle size of the Fe₂O₃ particles is about 24 Å. Because the particle size is so small that the Mössbauer spectra of the -Fe₂O₃ samples only consist of a quadrupole-split central line. It was proved that the Isomer Shifts (DIS) and the Quadrupole Splitting (DQS) changed as the refluxing time prolongs and the refluxing temperature increases during the preparation of the Fe₂O₃ nanoparticles, which implied an enhancement of the surface electrofield gradient formed by the surface coated polar molecules during the refluxing process.     

Study of maghemite nanoparticles as prepared and coated with DMSA using Mössbauer spectroscopy with a high velocity resolution

Study of maghemite nanoparticles, native and coated with DMSA as magnetic fluid for biomedical applications, was carried out using Mössbauer spectroscopy with a high velocity resolution at 295 and 90 K. The obtained results demonstrated differences in Mössbauer hyperfine parameters for uncoated and DMSA-coated nanoparticles which were related to the interactions of DMSA molecules with Fe^3+? ions on maghemite nanoparticle’s surface.     

A Mössbauer and ESR study of LiNbO3-Fe2O3 for low Fe2O3 concentrations

Samples of the system LiNbO₃-Fe₂O₃ prepared by water quenching and by the double-roller quenching method in the range up to 24 mol% Fe₂O₃ were investigated by Mössbauer and ESR spectroscopy. In the water quenched samples up to 11 mol% Fe₂O₃ only the Fe³⁺ and the Fe²⁺ valence states could be detected. The Fe²⁺ concentration decreased with increasing Fe₂O₃ content. Above 11 mol% Fe₂O₃ magnetically split Mössbauer spectra indicated the presence of Fe₂O₃ clusters. The isomer shift values of Fe³⁺ as a function of Fe₂O₃ concentration showed jumps at 6 and 11 mol% Fe₂O₃, whereas no significant changes could be detected in the quadrupole splitting values. The ESR data already exhibited the existence of isolated Fe³⁺ ions and of clusters with Fe-Fe distances less than 8 Å for the lowest Fe₂O₃ concentration. The cluster signal intensity increased with increasing Fe₂O₃ content. The roller quenched samples showed increased Fe²⁺ concentration as compared to the water quenched samples, which suggests that slow quenching results in iron oxidation and cluster formation. For low Fe₂O₃ concentrations a valence state change Fe³⁺Fe²⁺ can easily be obtained by heat treatments in various atmospheres, whereas for higher Fe₂O₃ contents (9.8 mol%) precipitations of-Fe (in reducing atmosphere) and Fe₂O₃ (in air) could be observed in addition to the valence state changes of a remaining part of dissolved Fe ions. On the basis of the obtained results a model was suggested for the unusual behaviour of the lattice parameters observed in LiNbO₃-Fe₂O₃.     

Mössbauer study of magnetic order in RBa2Fe3O8

Complete replacement of copper by iron in RBa₂Cu₃O₇ leads to RBa₂Fe₃O₈ (R=Y, rare earth). Mössbauer spectroscopy measurements of⁵⁷Fe and¹⁵¹Eu in RBa₂Fe₃O₈ (R=Y, Eu, Ho, Er) at temperatures 4.2–800 K have been performed. Some of the spectra reveal two inequivalent iron sites, probably corresponding to iron in the Fe(2) site (fivefold oxygen coordination) and in the Fe(1) site (octahedral oxygen coordination). In all compounds the iron moments order antiferromagnetically at the same Néel temperatureT _N720 K. The¹⁵¹Eu Mössbauer spectra of EuBa₂Fe₃O₈ show that the Eu ion is trivalent and exposed to a small exchange field from the iron sublattices.     

Study of NiFe2O4 nanoparticles using Mössbauer spectroscopy with a high velocity resolution

The nanocrystalline NiFe₂O₄ particles prepared by solution combustion synthesis technique using different fuels such as ethylene-diamine-tetra-acetic acid (NA sample) and urea (NB sample) were studied using magnetic measurement and ⁵⁷Fe Mössbauer spectroscopy with a high velocity resolution. The temperature dependence of magnetization is different for the two samples. Mössbauer spectra demonstrate the necessity to use more than two magnetic sextets, usually used to fit the NiFe₂O₄ nanoparticles spectra. Evaluation of the different local microenvironments for Fe in both tetrahedral (A) and octahedral (B) sites, caused by different Ni^2?+? occupation of octahedral sites, demonstrates at least five different local microenvironments for both A and B sites. Therefore, the Mössbauer spectra were fitted by using ten magnetic sextets which are related to the spread ⁵⁷Fe location in octahedral and tetrahedral sites.     

A Mössbauer effect study of MgFe2O4

A slowly cooled sample of the ferrimagnetic spinel MgFe₂O₄ has been studied with ⁵⁷Fe Mössbauer 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.     

57Fe Mössbauer study of Lu2Fe3Si5 iron silicide superconductor

With the advent of Fe–As based superconductivity it has become important to study how superconductivity manifests itself in details of ⁵⁷Fe Mössbauer spectroscopy of conventional, Fe-bearing superconductors. To this end, the iron-based superconductor Lu₂Fe₃Si₅ has been studied by ⁵⁷Fe Mössbauer spectroscopy over the temperature range from 4.4 K to room temperature with particular attention to the region close to the superconducting transition temperature (T_c=6.1 K). Consistent with the two crystallographic sites for Fe in this structure, the observed spectra appear to have a pattern consisting of two doublets over the whole temperature range. The value of Debye temperature was estimated from temperature dependence of the isomer shift and the total spectral area and compared with the specific heat capacity data. Neither abnormal behavior of the hyperfine parameters at or near T_c, nor phonon softening were observed.     

X-ray diffraction and Mössbauer studies of X20Cr13 steel subjected to ball milling

The powders of X20Cr13 steel were subjected to ball milling process in a planetary ball mill. X-ray diffraction and Mössbauer spectroscopy studies revealed the refinement of the structure of this steel down to a nanocrystalline range practically without any phase transformations. Both techniques allowed to detect the alloyed ferrite as well as residual content of iron containing M₂₃C₆-type carbide, which was dissolved into the ferrite during milling. Hyperfine magnetic fields in ball milled steel samples did not differ significantly from those for the bulk steel disc.     

Mössbauer study of Fe-Re alloys prepared by mechanical alloying

The room temperature Mössbauer spectra of ⁵⁷Fe were measured for nanocrystalline iron-based solid solutions Fe _1?x Re _x , prepared by mechanical alloying with x in the range 0.01 ≤ x ≤ 0.04. The obtained data were analysed in terms of the binding energy E _b between two rhenium atoms in the Fe-Re system. The extrapolated value of E _b for x = 0 was used for computation of enthalpy of solution of rhenium in iron. The result was compared with that resulting from the cellular atomic model of alloys by Miedema as well as with value, derived from proper data for Fe-Re solid solutions obtained by melting in an arc furnace. From the comparison it follows that our findings are in agreement with the Miedema’s model predictions and previous Mössbauer studies.     

A Mössbauer spectroscopic study of 3d magnetism of R2Fe14B

Guided by the occupancies and iron magnetic moments μ³, ⁵⁷Fe Mössbauer parameters of Y₂Fe₁₄B at 250K, and in turn for other temperatures, of the sublattices of iron were deduced. Plots of μ(T) in reduced coordinates, through the established correlation between hyperfine field Hn and μ, show that the corresponding state of different iron sites is different and all experimental points fall below Brillouin function. The relation between exchange integral deviation parameter Δ and standard deviation of Fe-Fe interatomic distances S is linear, indicating electrostatic nature of exchange interactions between spins in neighboring atoms. It is inclined to the view that fluctuations of exchange integral is responsible for low T_c of R₂Fe₁₄B.     

A magnetic and Mössbauer study of melt-spun Nd60Fe30Al10

Nd₆₀Fe₃₀Al₁₀ alloys were rapidly quenched by the melt-spinning technique with different wheel surface speeds ranging from 5 to 30 m/s. The microstructure and the magnetic properties were strongly dependent on the quenching rate. A high quenching rate led to an amorphous structure with a low coercivity at room temperature, while a mixture of amorphous and crystalline phases was found after melt-spinning at 5 m/s, which exhibited hard magnetic properties at room temperature. For both the ribbons melt-spun at 5 and 30 m/s respectively, coercivity increased with decreasing temperature and reached a maximum at around 50 K. Maximum magnetization at 10 T increased dramatically at low temperature. Our magnetic study has shown that the presence of crystalline Nd was responsible for the increase of magnetization and the decrease of coercivity, as Nd became magnetically ordered at low temperatures. The Mössbauer study has shown that the magnetic microstructures of melt-spun ribbons were not uniform, as the spectra needed to be fitted by magnetic and non-magnetic components.     

Mössbauer study of FINEMET with different permeability

Stress field and magnetic field annealed FINEMET ribbons were investigated by ⁵⁷Fe Mössbauer spectroscopy, magnetic and XRD methods. The change in relative areas of the 2nd and 5th lines in the Mössbauer spectra indicated significant variation in magnetic anisotropy due to the different annealing. High velocity resolution Mössbauer spectroscopy was also used to control the model applied for the evaluation of Mössbauer spectra. A correlation was found between the permeability and the magnetic anisotropy of the annealed FINEMET samples. This can be applied to predict production parameters of FINEMET ribbons with more favorable soft magnetic properties for technological applications.     

Mössbauer study of Fe‐doped CuGeO3 system Cu0.9Ge0.9Fe0.2O3

The Fe‐doped system Cu_0.9Ge_0.9Fe_0.2O₃ has been investigated by means of X‐ray diffractometry, Mössbauer spectroscopy and superconducting quantum interference device. The structure of this system is orthorhombic and the lattice constants are a=4.784 Å, b=8.472 Å and c=2.904 Å, respectively. Magnetic measurements confirm that the spin‐Peierls transition appears in our sample at about 12 K, which is near to the spin‐Peierls transition temperature (T _sp) 14 K of pure CuGeO₃ system. The Mössbauer spectrum shows the superposition of two Zeeman sextets and a broad central line due to Fe³⁺ ions from room temperature to 4.2 K. The Mössbauer parameters show a discontinuity near T _sp. The jump of the magnetic hyperfine field at temperatures lower than T _sp means increasing of the superexchange interaction among the magnetic ions. The jump of the quadrupole splitting and the isomer shift values could be interpreted as due to decrement in symmetry of lattice sites and spontaneous thermal contraction.     

A 57Fe Mössbauer spectroscopy study of iron nanoparticles obtained in situ in conversion ferrite electrodes

Transmission Mössbauer spectroscopy and CEMS are powerful tools to study the changes in which iron-containing active materials of conversion electrodes are involved during lithium cell charge and discharge. The usual spectrum of pristine CoFe₂O₄ spinel with two sextets ascribable to Fe^3?+? ions in both tetrahedral and octahedral environments, changes dramatically after cell discharge to 0 V vs. Li, and can be interpreted as the result of iron reduction to the metallic state in the form of superparamagnetic metal nanoparticles dispersed in a Li₂O matrix. After cell charge to 3 V, the MS of the pristine sample is not recovered. Instead, two new doublets are visible with IS ascribable to Fe^3?+? ions. ⁵⁷Fe CEMS evidences the different environment of iron atoms in the surface of the nanodispersed material found in the used electrodes.     

Mössbauer study of magnetic Fe/FeSi multilayers

Many useful properties of magnetic multilayers depend on the coupling between the ferromagnetic layers. The coupling often oscillates with the thickness of non-magnetic spacer layers: it is ferro- or antiferromagnetic or even non-collinear near a critical thickness. We investigated the magnetron-sputtered Fe/FeSi multilayers with spacer thickness around 1.7 nm by means of Conversion Electron Mössbauer Spectroscopy with oblique incidence of the γ beam in order to gain information on the orientation of the local magnetic moments in the multilayer plane. The results show that the local moments make an angle of 45°–50° with the direction of the remanent magnetization. This is consistent with strong biquadratic coupling which in turn is expected at this spacer thickness from our magnetic measurements. An analysis of the distribution ofB _hf corresponding to different numbers of n.n. Si atoms in the bcc Fe structure points to weak diffusion of Si through the Fe/FeSi interface characterized by a diffusion length of about twice the substrate roughness.     

Mössbauer spectroscopy with high velocity resolution in the study of ordinary chondrites

Mössbauer spectroscopy with high velocity resolution was used to study of 11 ordinary chondrites from L and H chemical groups. Mössbauer spectra were recorded in 4,096 channels and then presented in 1,024 channels. An increase of velocity resolution allowed us to carry out more detailed analysis of ordinary chondrites and to decrease experimental error for evaluation of hyperfine parameters in comparison with previous chondrite Mössbauer spectra measured in 512 channels or less. Variations of hyperfine parameters were observed for corresponding iron bearing phases in chondrites. Two crystallographically non-equivalent octahedral sites M1 and M2 in olivine and pyroxene were revealed in Mössbauer spectra of bulk chondrite samples.