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.     

Mössbauer and magnetic characterization of TbRhSn

The intermetallic compound TbRhSn was investigated in detail by X-ray, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy. This compound undergoes a transition from a paramagnetic to an antiferromagnetic state at T _N = 20.8(2) K. The ¹¹⁹Sn Mössbauer spectrum recorded at 4.2 K can be well fitted as a composition of three subspectra with the same intensities, magnitudes of H _hf, ΔE _Q, and δ _is, in agreement with the model of triangular-like antiferromagnetic arrangements of equal magnetic Tb moments lying in the basal ab-plane deduced from neutron diffraction studies (Szytu?a et al., J Alloys Compd 244:94–98, 1996).     

Determination of the nuclear parameters of155Gd excited states using the Mössbauer effect

From Mössbauer spectra of GdAlO₃ and GdVO₄ above and below the Néel temperature and fitted using a transmission integral, we have determined the following parameters of the 86.5 keV and 105 keV levels:g(86)/g(0)=+1.217±0.005,Q(86)/Q(0)= +0.10±0.02,g(105)/g(0)=–0.55±0.02,Q(105)/Q(0)=+0.74±0.02, r ²₁₀₅/ r ²₈₆=+1.30±0.05. The linewidth observed for the 105 keV transition is less than the calculated natural linewidth.     

Interpretation of the 119Sn Mössbauer parameters

The variations of the ¹¹⁹Sn Mössbauer isomer shift δ are interpreted for tin compounds from a semi-empirical tight-binding calculation of the electronic density at the nucleus ρ(0). A molecular model is proposed in order to relate the variations of ρ(0) for the Sn(IV) chalcogenides to the changes in the Sn environment. The variations of the experimental values of the quadrupole splitting δ are linearly correlated to the values of the electric field gradients (EFG) calculated by the full-potential linearized-augmented-plane-wave (FP-LAPW) method. The value of the ¹¹⁹Sn nuclear quadrupole moment is found to be |Q| = 10.5 ± 0.2 fm². Finally, the relation between the EFG and the Sn environment is discussed for SnO.     

Mössbauer effect study of the magnetic structure in δ-FeOOH

Two samples of-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.     

Mössbauer spectroscopy and magnetic studies of orientated textured Fe3O4 ferrofluid

Nano scale magnetite based ferrofluid is synthesized by chemical co pre cipitation technique and stabilized with oleic acid. Magnetization and viscosity measurements were used to optimize for texturing purpose. The freeze-textured ferrofluid in two configurations, namely, (1) field texture system (FTS) and (2) zero field texture system (ZTS) are investigated by magnetization measurements at 298 K and Mössbauer spectroscopy measurements at 77 and 298 K. These results are analysed on the basis of the contributions from collective superparamagnetic reversal and the strength of the inter particle interactions.     

Mössbauer spectroscopy of Np3As4. Interpretation of the spectra and of the magnetic behaviour in a crystal field model

Np₃As₄ (Th₃P₄-type structure) is ferromagnetic below T_c=81 K. In this work, we present results of Mössbauer experiments performed between 4.2 and 150 K. The isomer shift with respect to NpAl₂ is 5.9 mm/s, suggesting a Np⁴⁺ valence state. The hyperfine field at 4.2 K is 377 T, corresponding to an ordered moment μ_ord=1.75μ_B. The quadrupolar interaction is very small both in the ferromagnetic and in the paramagnetic state (e²qQ<3 mm/s).The low value of μ_ord (free ion value 3.2μ_B) as well as the absence of a significant quadrupolar component are discussed in terms of crystal field effects.In particular, the contributions to the electric field gradient of both the lattice and the 5f electrons have been considered, taking into account the distortion of the 5f charge cloud in presence of a molecular field below T_c.We show that the reduction of μ_ord is linked to a rotation of the moment towards the [111] direction as in the case of the isostructural compound U₃P₄.The crystal field ground state which accounts for this effect leads to a vanishingly small quadrupolar interaction both in the ordered and in the paramagnetic states.As a consequence, the lattice contribution to the electric field gradient, itself, is likely to be very small. Two mechanisms are proposed for this lattice term reduction: either a compensation by the conduction electrons or a reduction of B⁰₂ due to the competing effects of the ligands surrounding Np⁴⁺.Lastly, the model gives a good agreement with the temperature variation of the hyperfine field.     

155Gd Mössbauer investigation on (GdxNd1−x)1+εFe4B4

Tetragonal (Gd_xNd_1–x)₁₊ Fe₄B₄ alloys have been investigated for 0.2x1 by Mössbauer spectroscopy, using the 86.5 keV¹⁵⁵Gd resonance. The Gd quadrupolar interaction e²qQ=12.67(5) mm/s for x=1, nearly independent of x, is the largest observed to date in metallic compounds of Gd. A crystal field term A ₂ ⁰ =–2450±50 K/a ₀ ² is inferred. This quadrupolar interaction shows some dispersion increasing when x decreases, reflecting the quasi incommensurate nature of the (Gd,Nd) and Fe+B sublattices in the (Gd_xNd_1–x)Fe₄B₄ structure (=0.109 for x=0 and =0.139 for x=1). The hyperfine field is perpendicular to the c axis for x0, but no unique direction is obtained for x=0.     

Mössbauer investigation of temperature transformations in bacterial ferrihydrite

Ferrihydrite nanoparticles formed as a result of the microorganism activity have been studied using Mössbauer spectroscopy, X-ray powder diffraction analysis, and X-ray fluorescence analysis. Three positions of trivalent iron with nonoverlapping ranges of quadrupole splittings have been revealed in bacterial ferrihydrite: QS{Fe³⁺(1)} = 0.49–0.83 mm/s, QS{Fe³⁺(2)} = 0.84–1.10 mm/s, and QS{Fe³⁺(3)} = 1.25–1.73 mm/s. It has been experimentally demonstrated that the Fe³⁺(3) positions are the centers of nucleation of the hematite phase in the course of heat treatment.     

Using Mössbauer spectroscopy as key technique in the investigation of nanosized magnetic particles for drug delivery

This paper describes how cobalt ferrite nanoparticles, suspended as ionic or biocompatible magnetic fluids, can be used as a platform to built complex nanosized magnetic materials, more specifically magnetic drug delivery systems. In particular, the paper is addressed to the discussion of the use of the Mössbauer spectroscopy as an extremely useful technique in supporting the investigation of key aspects related to the properties of the hosted magnetic nanosized particle. Example of the use of the Mössbauer spectroscopy in accessing information regarding the nanoparticle modification due to the empirical process which provides long term chemical stability is included in the paper.     

Mössbauer spectroscopy in the investigation of new mineral–related materials

New materials based on the composition of the mineral schafarzikite, FeSb $_{2}\textit {O}_{4}$ , have been synthesised. $^{57}$ Fe- and $^{121}$ Sb- Mössbauer spectroscopy shows that iron is present as Fe $^{2+}$ and that antimony is present as Sb $^{3+}$ . The presence of Pb $^{2+}$ on the antimony sites in materials of composition FeSb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ induces partial oxidation of Fe $^{2+}_{}$ to Fe $^{3+}$ . The quasi-one-dimensional magnetic structure of schafarzikite is retained in FeSb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ and gives rise to weakly coupled non-magnetic Fe $^{2+}$ ions coexisting with Fe $^{3+}$ ions in a magnetically ordered state. A similar model can be applied to account for the spectra recorded from the compound Co $_{0.5}$ Fe $_{0.5}$ Sb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ .     

Mössbauer investigations of magnetic system stratification in europium and thulium ferromanganites

Strontium-substituted ferromanganites of europium and thulium were investigated. Samples of Eu_0.65Sr_0.35Mn_{1 ? x} Fe _x O₃ and Tm_0.65Sr_0.35Mn_{1 ? x} Fe _x O₃ were prepared using ceramic processing. Based on the data from X-ray analysis, the samples with europium had a perovskite lattice, whereas the thulium samples were crystallized in an ilmenite lattice. Stratification of the magnetic subsystem was observed in the Mössbauer spectra of certain samples at T ≈ 300 K.     

Mössbauer spectroscopic investigation of tin-doped thiospinels

The tin-doped sulphur-containing compound of composition Fe_1.05Cr_1.90Sn_0.05S₄, in which tin is located on the octahedral sites as Sn^4?+? in the spinel-related structure, has been examined by ¹¹⁹Sn Mössbauer spectroscopy. The data complement results obtained by ⁵⁷Fe Mössbauer spectroscopy and show that tin increases the magnetic ordering temperature of pure FeCr₂S₄.     

Mössbauer spectroscopy studies of the magnetic properties of ferrite nanoparticles

Studies of the ferrite nanoparticles prepared by the chemical decomposition of iron chlorides with a various ratio ξ = Fe³⁺/Fe²⁺ are herein presented. The microstructure and the magnetic properties have been studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). The TEM studies show that the nanoparticles have almost a spherical shape with the diameter of (12 ± 2) nm for all samples. The measured XRD pattern was mainly composed of lines which were indexed with a cubic spinel structure. The analysis of the Mössbauer data shows that the microstructure of the nanoparticles consists of the core formed by nonstoichiometric magnetite and maghemite shell. A small amount of hematite, probably on the surface of the nanoparticles with ξ = 1.75, 2.0, was detected. At temperatures T ≤ 150 K the spin canting of surface maghemite with ξ = 2.25 was observed while for the samples with ξ = 1.75, 2.0 such effect was suppressed by the presence of hematite on the surface of the nanoparticles. Infield Mössbauer spectra with ξ = 1.75, 2.0 show that magnetic moments of the magnetite/maghemite core are parallel while magnetic moments of the surface hematite are perpendicular to the direction of the external magnetic field.     

Mössbauer spectroscopy of the chloroplast-targeted DnaJ-like proteins CDJ3 and CDJ4

The heat shock protein 70 (HSP70) in the chloroplast of Chlamydomonas reinhardtii, termed HSP70B, interacts with chloroplast-targeted DnaJ-like proteins (CDJs). In this work we focus on two CDJ co-chaperones (CDJ3 and CDJ4) of HSP70B which contain a redox-active Fe-S cluster (Dorn et al. Biochem. J. 427, 205 [2010]). We have performed Mössbauer spectroscopy on ⁵⁷Fe enriched CDJ3 an) CDJ4. Our results indicate that both proteins have unusual [4Fe4S] ²⁺ clusters showing structural inhomogeneity of the two [Fe ^2.5+-Fe ^2.5+] pairs. The spectra have been analyzed by means of two components with δ-values characteristic for Fe ^2.5+ centers, but the differences in ΔE _Q indicate variations in their tetrahedral coordination spheres.     

The investigation of magnetic state variations in α-Fe and its alloys by means of continuous temperature scanning of the Mössbauer effect

A new method for continuous temperature scanning of the Mössbauer effect was developed permitting the measurement in a broad temperature range with satisfactory precision. The temperature dependence of Mössbauer radiation transmission for -Fe, amorphous Fe-P and for alloys was measured from 20–800°C. A region of anomalous transmission was observed for crystalline -Fe and its alloys and a new method of Debye-temperature determination was proposed.