155Gd Mössbauer investigation of the magnetic order and spin-reorientation in Gd3Ag4Sn4

In a recent study of the magnetic order in Gd₃Ag₄Sn₄ by neutron powder diffraction and ¹¹⁹Sn Mössbauer Spectroscopy we showed that both the Gd(2d) and Gd(4e) sublattices order antiferromagnetically at 28.8(2) K. We also demonstrated that the ‘magnetic event’ around 8 K is in fact a ‘plane to axis’ spin-reorientation of the Gd magnetic structure. Here, we extend our study with ¹⁵⁵Gd Mössbauer Spectroscopy. The initial magnetic ordering at 30(2) K is clear for both sites and substantial changes in the hyperfine fields are observed at 8 K when the magnetic structure reorients.     

Mössbauer studies of hyperfine fields in disordered Fe2CrAl

Heusler-like alloy Fe₂CrAl was prepared and studied. Structure determination was done by X-ray. The structure was found to conform to the B₂ type. Magnetic hyperfine fields in this sample were studied by the Mössbauer effect. The Mössbauer spectra were recorded over a range of temperature from 40 to 296 K. The Mössbauer spectra showed the co-existence of a paramagnetic part with a magnetic hyperfine portion at all recorded temperatures. Even with the distribution in the magnetic hyperfine field, the average hyperfine field follows the (T/T _c)^3/2 law. The paramagnetic part of the hyperfine field is explained in terms of the clustering of Cr atoms.     

A Mössbauer effect investigation of the magnetic behaviour of (Fe,Co)S1 + x solid solutions

The Mössbauer spectra of (Fe, Co)S_{1 + x} were recorded at room temperature and 4.2 K for samples of varying composition to study the magnetic behaviour of the solid solutions. The Mössbauer spectra are split magnetically at iron concentrations above 16% Fe. For samples with less than 16%Fe, the Mössbauer spectra show no evidence of magnetic splitting down to 4.2 K. The room temperature centre shift data appear to vary continuously with composition and the hyperfine magnetic field decreases with decreasing Fe²⁺ concentration. A Mössbauer spectrum of ⁵⁷Fe:CoS at 4.2 K in an external field of 25 kOe showed no evidence of magnetic splitting beyond that caused by the applied field, indicating a net zero internal field.A high spin to low spin transition in Fe²⁺ is ruled out as being responsible for the observed magnetic behaviour on the basis of the centre shift data. The Mössbauer data are interpreted to indicate a substantial increase in electron delocalization towards the ligands as the 〈M-S〉 distance decreases with decreasing Fe²⁺concentration. This causes a reduction in the magnitude of the internal magnetic field contributions as well as a decrease of shielding of the nucleus, giving rise to the observed Mössbauer parameters.The Mössbauer spectrum of ⁵⁷Fe:CoS at room temperature is compared with the spectrum of FeS above the 6.7 GPa phase transition at room temperature. The similarities of the centre shift and the 〈M-S〉 distance in the two phases indicate that covalency may also be responsible for the observed high pressure behaviour of FeS, and not the presence of Fe³⁺ as was originally suggested.     

Relation between magnetic hyperfine field distribution and cation order in natural hedenbergite

Mössbauer and magnetic measurements have been performed at various temperatures between 1.5K and 300K on a natural hedenbergite Ca_0.96 Mg_0.19 Fe_0.82Mn_0.02Si₂O₆. The magnetic measurements are consistent with a ferromagnetic alignment of the Fe²⁺ moments inside each zig-zag M1 chain and antiferromagnetic alignment of the moments of neighbouring chains. Above T_N (Mössbauer)=29±2K, a single quadrupole doublet attributed to Fe²⁺ in the M1 sites is observed. At 4.2K, the Mössbauer spectra can be accounted for by a discrete distribution of six hyperfine field components which can be related to all the possible Fe²⁺?Mg²⁺ configurations up to second cation neighbours surrounding a central⁵⁷Fe²⁺ probe.     

Mössbauer study of iron oxide modified montmorillonite

Montmorillonite particles were modified by iron oxides using the precipitation process with the aim to monitor the differences in the structural and magnetic properties of intercalated and adsorbed Fe³⁺. The Mössbauer spectra recorded at 5 K in zero and 6 T external fields, IR spectra and TG curves measured in zero and 32 mT fields identified the ferrihydrite pillars in an interlayer space of the montmorillonite structure and γ-Fe₂O₃ nanoparticles adsorbed on the mineral surface. The temperature dependent Mössbauer spectra (25–300 K) reflect the superparamagnetic behaviour of maghemite nanoparticles and ferrihydrite pillars with the blocking temperatures of about 80 and 25 K, respectively.     

The magnetic properties of the MgZn ferrite system by Mössbauer spectroscopy

Samples of the magnetism-zinc ferrite series Zn_xMg_1?xFe₂O₄ (x = 0.0 to 1.0) have been studied by the Mössbauer effect technique at 77 K. Mössbauer spectra for x = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octachedral ions (B-sites), while for x=0.7 it shows relaxation behaviour and for x?0.8 it exhibits a paramagnetic quadrupole doublet. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of the AB and BB supertransferred hyperfine interactions. Analysis of the average Mössbauer line width as a function of zinc concentration suggests that the relaxation spectrum observed at x=0.7 (77 K) is possibly due to domain wall oscillations.     

High-Pressure Mössbauer Spectroscopy of Perovskite Iron Oxides

Using a diamond anvil cell and a low-temperature cryostat with a superconducting solenoid, high-pressure ⁵⁷Fe Mössbauer spectroscopy with a radioactive source at 4.5 K under external magnetic fields up to 7 T has been developed. Pressure-temperature magnetic phase-diagrams for perovskite iron oxides, SrFeO₃, CaFeO₃ and Sr₂LaFe₃O₉ are presented up to about 70 GPa. High-pressure Mössbauer measurements under external magnetic fields proved that the electronic ground states of these oxides switch to uniform-charge and ferromagnetic (and most probably metallic) states under extremely high pressure.

     

Electronic and magnetic properties of the iron borate Fe2BO4

Polycrystalline Fe₂BO₄ was prepared by solid state reactions and its electronic and magnetic properties were investigated by Mössbauer spectroscopy and magnetization measurements. The Mössbauer spectra of Fe₂BO₄ below 270 K indicate the presence of Fe²⁺ and Fe³⁺ sites in the structure, in a ratio 1 : 1. Above this temperature electron delocalization sets in between the divalent and trivalent iron ions and Fe^2.5+ states are observed. The temperature dependence of the Mössbauer spectra and magnetization measurements clearly show the onset of magnetic order below 155 K.     

Low temperature 151Eu and 57Fe Mössbauer study of mechanically alloyed EuFeO3

Low temperature ⁵⁷Fe Mössbauer spectra of mechanically alloyed EuFeO₃ prepared by mechanical alloying depicts an interesting transformation in its hyperfine magnetic state, from a triple phase magnetic system at room temperature to a single phase ferromagnetic state at 20 K. The hyperfine magnetic field increased by 12% at 20 K from its room temperature. The isomer shift and quadrupole splitting values exhibit a peak around 200 K. Low temperature ¹⁵¹Eu Mössbauer measurements show that the line-width increased to its maximum value at 80 K which is 45% compared to its room temperature value not enough to suggest splitting.     

Evidence of magnetic broadening in Mössbauer spectra of superconducting FeTe 0.8 S 0.2

Magnetic properties of the FeTe_0.8S_0.2 superconductor were studied by Mössbauer spectroscopy. Low-velocity Mössbauer spectra that were recorded in the temperature range from 5.7 K up to 300 K show a paramagnetic doublet with a broadening at temperatures below 77 K. The broadening can be explained by the appearance of a distribution of hyperfine magnetic fields due to the magnetic ordering of a part of the sample. The magnetically ordered fraction starts to decrease at temperatures below 20 K indicating a possible competition with the onsetting superconductive state.     

237Np and 57Fe Mössbauer study of NpFeGa5

⁵⁷Fe and ²³⁷Np Mössbauer ōmeasurements have been performed for NpFeGa₅, which is one of the so-called neptunium 1-1-5 compounds. The ⁵⁷Fe Mössbauer spectra below T _N = 118 K show the magnetically ordered state. The magnitude of the hyperfine magnetic field at the ⁵⁷Fe nucleus is determined to be 1.98 ± 0.05 T at 10 K. From the ²³⁷Np Mössbauer spectrum at 10 K, the hyperfine magnetic field at the ²³⁷Np nucleus is 203 T and the hyperfine coupling constant is determined to be 237 T/μ_B using the Np atomic magnetic moment of 0.86 μ_B determined by the neutron diffraction study.     

Studies of ultrafine amorphous particles prepared by chemical reduction

Ultrafine amorphous FeNiB powder has been prepared by borohydride reduction by mixing the aqueous solutions. The influence of the mixing way and the drying procedure was observed. The paramagnetic doublet component of the Mössbauer spectra indicates the existense of the Fe³⁺, whose state was discussed. The Mössbauer measurements have been performed at room temperature (RT) and at 77K. The effect of the magnetic fields on the amorphous powders was somehow analogous to that on amorphous ribbons produced by a melt spinning method. The spontaneous magnetization of the flaky powder sample tended to locate on the sample plane.     

Goldanskii-Karyagin effect in FePS3

Mössbauer studies have been performed on FePS₃ in powder form between 125 K and 450 K. The Mössbauer spectrum consists of an asymmetric doublet, ratio～0.93 at room temperature. The asymmetry is attributed to the Goldanskii-Karyagin effect and the lattice anisotropy is found to be ～1.4, with the largest vibrations parallel to thec ^? axis of the monoclinic unit cell. The Debye temperature is ～200 K.     

An57Fe Mössbauer effect study of the highT c superconductor GdBa2Cu3O7−y

Split source⁵⁷Fe Mössbauer effect spectroscopy has been performed between 4 K and 295 K on the superconducting perovskite GdBa₂Cu₃O_7?y. No evidence is seen for magnetic splitting at low temperatures as reported in some split absorder⁵⁷Fe Mössbauer experiments on this material. There is evidence for phonon mode softening, as observed for¹¹⁹Sn Mössbauer spectra of some other highT _c superconductors.     

Depth selective conversion electron spectrometer for magnetic characterisation of thin films

A high transmission spherical electrostatic electron spectrometer for Mössbauer conversion electron spectroscopy was built. Depth selective Mössbauer studies can be carried out from T=4 K to 350 K in a 10^?8 Torr vacuum changing the electron energies and/or the electron escape angles.     

151Eu and57Fe Mössbauer effect studies of magnetic interactions in europium transition element oxides solution

The solid state solutions of europium transition element oxides Eu (Fe0.8M0.2)O₃ (M=Sc,Cr,Mn,Co) are synthesized. The X-ray diffraction of the compound shows that all the compounds possess the perovskite structures. Both the¹⁵¹Eu Mössbauer spectra and the⁵⁷Fe Mössbauer spectra are measured. The hyperfine magnetic field and non-axisymmetric electric field gradient are observed in the¹⁵¹Eu Mössbauer spectrum. The⁵⁷Fe Mössbauer spectrum shows that there are four components of hyperfine fields corresponding to four kinds of different neighbours of the Fe ion.     

Mössbauer study of spin structure transformation from an incommensurate to a commensurate state

We present crystallographic and magnetic properties of NiCr_1.98 ⁵⁷Fe_0.02O₄ by using X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), and Mössbauer spectroscopy. The lattice constants a₀ were determined to be 8.318 Å. The ferrimagnetic Neel temperature (T _N) for NiCr_1.98 ⁵⁷Fe_0.02O₄ is determined to be 90 K. The Mössbauer absorption spectra for all chromites at 4.2 K show two well developed sextets superposed with small difference of hyperfine fields (H _hf) caused by Cr^3?+? ions in two different magnetic sites. The values of the isomer shifts show that the charge states of Fe are Fe^3?+? for all temperature range. Ni-chromites Mössbauer spectra below T _N present aline broadening due to a Jahn–Teller distortion and show that spin structure behavior of Cr ions change from an incommensurate to a commensurate state.     

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.     

Study of the FeII ion role in the magnetic properties of the AFM ring Cr7Fe using Mössbauer spectroscopy

In recent years the synthesis of antiferromagnetic rings traced a path to the observation of interesting quantum coherence phenomena in heterometallic Single Molecule Magnets. Because of the presence of different magnetic centers, it is crucial to understand the distribution of the molecular spin on all the sites. ⁵⁷Fe Mössbauer spectroscopy is an efficient and powerful technique to contribute to this knowledge in antiferromagnetic wheels containing iron ions. We analyze the Cr₇Fe^II wheel: the Mössbauer line shape evolution of low temperature spectra (2.1 K) at different external fields is illustrated and the mean spin value of the iron ion is evaluated.     

Relaxation effects in Mössbauer spectra of bridged seven-coordinate Fe3+ pairs

The compound [Fe₂L(H₂O)₄] (ClO₄)₄.H₂O which contains pairs of Fe³⁺ ions within a binucleating macrocycle derived from Schiff base condensation of 2,6-diacetylpyridine and 1,3-diamino-2-hydroxy-propane has been studied by magnetic susceptibility measurements and⁵⁷Fe Mössbauer spectroscopy between 4.2 K and 300 K, and its crystal structure determined. The spectra show relaxation effects at all temperatures. Spectra taken at 4.2 K in applied fields of about 3 T showed thatV _zz is positive and η～0. The spectra were fitted using a stochastic model of a magnetic hyperfine field relaxing parallel to thez axis, giving relaxation times of 10^?9?10^?10 s.