Hyperfine magnetic fields in Fe−Co alloys and their tempera ture dependences

We obtained⁵⁷Fe hyperfine field parameters from Fe₁−x-Co _x alloys (0≤x≤0.6) from 77 K to 900 K. We first discuss the origin of the low temperature hyperfine fields in terms of the 3d and 4s electrons at⁵⁷Fe atoms. The⁵⁷Fe hyperfine magnetic field (hmf) of Fe-Co alloys depends more weakly on temperature than the hmf of pure Fe. This temperature dependence occurs because the alignment of the magnetic moments at both the Fe atoms and at the Co atoms depend on temperature in the same way as the bulk magnetization of Fe-Co alloys.     

Effect of atomic environment on the Fe hyperfine structure and the thermal stability of magnetic order in L10 ordered Fe-Pt alloys

The ⁵⁷Fe Mössbauer effect measurements were made for the L1₀ ordered Fe-Pt alloys with 39-62 at% Pt and the effect of local atomic environment on the hyperfine structure was investigated. Furthermore, the thermal stability of magnetic order was investigated for the alloys with high Pt concentration. From the analyses of the observed Mössbauer spectra, we found that dipole-field-like anisotropic transferred hyperfine fields are mainly responsible for the large difference in hyperfine field between Fe-site and Pt-site in the Fe-rich alloys. In the Pt-rich region far from stoichiometry, the existence of many Fe-sites occupied by excess Pt atoms causes a distribution of exchange fields. Therefore, the iron atoms in different local environments may have their several hyperfine fields with different temperature dependence. The anomalous temperature dependence of the averaged hyperfine field and line broadening observed for the 61, 62 at% Pt alloys can be understood from the co-existence of various sub-spectra with different temperature dependence. As a result, the thermal stability of magnetic order is largely reduced as the Pt concentration exceeds 60 at%.     

Short range ordering in undercooled Fe3Al

Alloys of Fe-27 at.% Al were prepared far from thermodynamic equilibrium by ultra-rapid quenching from the melt. The hyperfine magnetic field (hmf) distribution revealed a wide variety of local Al environments around⁵⁷Fe atoms. Upon low temperature annealing, the probabilities of these different Al environments changed at different rates. The “kinetic path” of short range ordering in Fe₃Al is consistent with that found in a previous study of long range ordering, where B2 order grows more rapidly than D0₃ order at lower temperatures.     

Mössbauer effect study of the magnetic state of iron atoms in Fe-Ni 36 wt.% Invar alloys

The⁵⁷Fe Mössbauer spectra of the Fe-Ni 36 wt.% Invar alloy at different temperatures (300–530 K) have been measured. The experimental results indicate that the hyperfine field distributions are characterized by dual peaks. The temperature dependence of the hyperfine field indicates that some iron atoms may transfer from the ferromagnetic state to the antiferromagnetic one with increasing temperature, and that the variation of the ratio of numberN ₀ of the iron atoms in the antiferromagnetic state to the numberN _h in the ferromagnetic state with temperature will obey the thermodynamic relationship.     

Temperature dependence of57Fe hyperfine field in Gd: YIG

The NMR spectra of⁵⁷Fe in Gd: YTG polyeryslallaline samples were measured between liquid-helium and room temperatures. In spectra of⁵⁷Fe on tetrahedral sites, beside the main line, weaker and broader peak at higher frequencies appears. It can be decomposed in to the three satellites corresponding to Fe³⁺ ion with one Gd³⁺ ion in the nearest (two satellites) and next nearest dodecahedral site. The temperature dependence of the stellite resonance frequency indicates, that the part of the transferred hyperfine field on⁵⁷Fe with the Gd³⁺ in the nearest c-site depends on the magnetic moment of Gd³⁺ ion. The resonance frequency of the satellite corresponding to the⁵⁷Fe with Gd³⁺ in the next nearest c-site follows the temperature dependence of the dipolar field, the change of the transferred field is small.     

Mössbauer studies of57Fe atoms isolated in CH4 and CO2 matrices

The Mössbauer absorption spectra of matrix isolated⁵⁷Fe atoms have been measured in the inert matrices CH₄ and CO₂ with matrix temperatures between ～3.3 and ～46 K. The isomer shift of the observed resonances is (?0.79±0.02) mm/s and (?0.76±0.05) mm/s with respect to iron metal at 300 K for⁵⁷Fe in CH₄ and for CO₂ respectively. This is within the experimental errors the same isomer shift as that of rare-gas matrix isolated⁵⁷Fe atoms. All spectra show quadrupole doublets due to the noncubic point symmetry of the lattice site occupied by the⁵⁷Fe atoms. The effective Debye temperatures as obtained from the temperature dependence of the resonance absorption areas are (46±4) K for the CH₄ matrix and (121±6) K for the CO₂ matrix annealed at ～20 K.     

Mössbauer effect study of Y(57FeMn)2

YMn₂ compound doped with⁵⁷Fe was investigated using⁵⁷Fe Mössbauer resonance in the temperature range 4.2–400 K. The magnetically split spectra were analyzed assuming two magnetically nonequivalent Fe sites with relative population dependent on iron concentration. In the transition temperature region a coexistence of the magnetic and nonmagnetic components was observed in the temperature span of about 50 K. A thermal hysteresis (of about 25 K) of the magnetic component confirms the first-order type magnetic transition. Temperature dependence of the hyperfine field of the magnetic component could be interpreted in terms of spin-fluctuation theory.     

Magnetic interaction in Tm3Fe5O12 studied by means of169Tm and57Fe Mössbauer spectroscopy

The temperature dependence of the hyperfine parameters of thulium iron garnet (Tm₃Fe₅O₁₂) powder was studied from 90 to 550 K using¹⁶⁹Tm and⁵⁷Fe Mössbauer spectroscopy (MB). The spectra were analyzed by least mean square fits to the transmission function. The temperature dependence of the magnetic fields of the thulium nuclei is well described by the mean field model. The coupling constants between the magnetic lattice occupied by the thulium atoms and the magnetic lattices occupied by the iron atoms were derived.     

Mössbauer emission spectroscopy of metastable electronic states

In⁵⁷Fe Mössbauer emission spectroscopy, the parent nucleus⁵⁷Co is incorporated in the matrix by diffusion, implantation or co-precipitation. Mössbauer emisssion spectra often differ from the corresponding absorption spectra in several aspects. The “anomalous” features (denoted as “after-effects” of nuclear transformation) comprise mainly (a) anomalous charge states, (b) anomalous spin states, and (c) metastable electronic populations within these charge states. The first two classes of effects were extensively studied during the sixties and seventies. More recently, several investigations have shown that, under certain circumstances, the electronic systems of Fe(II) and Fe(III) have not reached the thermal equilibrium within the Mössbauer time window. These effects have been evidenced (i) within the crystal field ground-state manifolds, but also (ii) as spectral contributions from excited crystal field states. In the present paper, an overview of these experiments will be given, together with their theoretical interpretation and in comparison to results obtained with other methods (e.g. optical spectroscopy). Special emphasis will be given to the systems⁵⁷Co/LiNbO₃ and⁵⁷Co/MgO, where nonequilibrium populations within the⁶A₁ ground manifold of Fe(III) and Γ_5g ground manifold of Fe(II) have been observed in an external magnetic field.     

Magnetic field and temperature dependence of the anomalous emission line intensities in LiNbO3:57Co-initial population and relaxation

The magnetic field dependence of the anomalous Fe³⁺ emission line intensities in LiNbO₃:⁵⁷Co cannot be due to a direct spin-lattice relaxation process in the⁶S ground state. Raman and Orbach processes in the ground state are ruled out by the temperature independent behaviour of the spectra. The observed line intensities are proportional to the initial populations of the corresponding Zeeman levels.     

Site occupation in57Fe doped NdCo12B6: Mössbauer analysis

The analysis of room temperature and4.2 K Mössbauer spectra of NdCo₁₂B₆ doped with⁵⁷Fe is presented. A clear preference of the⁵⁷Fe atoms for the 18h sites, of increased number of Co near neighbour atoms compared with the 18g site, is obtained in good agreement with recent studies of related 1:12:6 compounds.     

CEMS-study of the interface in Fe/Cr multilayer systems

The magnetic hyperfine (hf.) fields at the Fe/Cr interface were analyzed in epitaxial Fe/Cr thin film structures of (100)- and (110)-orientation with monolayer resolution by means of in-situ⁵⁷Fe Conversion Electron Mössbauer Spectroscopy (CEMS). The hf. field (300 K) in the 1st Fe-monolayer (ML) at the interface has been found to be strongly reduced to 22.0/20.9 T for (110)-/(100)-orientation, whereas the 2nd and 3rd ML reveal a slightly increased hf. field of 33.7 T as compared with the Fe-bulk value of 33.4 T. The temperature dependence of the hf. field at the interface shows aT ^3/2 spin wave law. The spin wave parameters are enlarged with respect to the bulk value indicating a reduced exchange interaction. A discontinuity in theT ^3/2-dependence is interpreted by the onset of magnetic order (Néel-temperature) of the Cr layers adjoining the⁵⁷Fe probe layer.     

A Mössbauer effect study of Y(57Fe:Mn)12

The intermetallic compound, YMn₁₂, doped with 0.5at.%⁵⁷Fe has been investigated using the⁵⁷Fe Mössbauer resonance. The data show that the Fe impurities occupy preferentially the f-site at the expense of the i- and j-sites. Whereas the i- and j-sites are expected to carry identical local magnetic moments in the ordered state, low temperature hyperfine fields of 50.08(±0.05)T and 2.90(±0.03)T are measured for the i- and j-sites respectively. It is proposed that large transferred hyperfine fields are responsible for this disparity.     

Mössbauer study of Tl containing highT c superconductors

Mössbauer spectroscopy, X-ray diffractometry, ESR and electrical resistivity measurements, were used to study highT _c (above 100 K) superconducting materials in order to get information about the effect of the preparation circumstances as well as about the anomalous behaviour in⁵⁷Fe and¹¹⁹Sn doped TlBaCaCu(⁵⁷Fe)O_4.5+δ and TlBaCuCu(¹¹⁹Sn)O_4.5+δ superconductors. We have found that the Mössbauer parameters strongly depend on the preparation circumstances. In many cases the presence of Mössbauer lines of nonsuperconducting oxide phases indicated that the iron and tin could not entirely built in into the lattice of superconductor. Four valence state tin and four valence state iron sites were identified in the superconducting phases. We have found anomalous temperature dependent changes in the⁵⁷Fe Mössbauer spectra of TlBaCaCu(⁵⁷Fe)O_4.5+δ samples around theT _c between 105–135 K. In this temperature range the total area of the spectrum temporary increases.     

Iron transfer between ferritin molecules

When ferritin solutions containing 8–12⁵⁷Fe atoms/molecule are mixed after iron loading, with ferritin containing a small⁵⁶Fe core (⁵⁶Fe-ferritins) or with an equal volume of NaCl solution (controls) and the mixtures are frozen in liquid nitrogen (LN) at specified times, the spectra of the⁵⁶Fe-ferritins show that almost all⁵⁷Fe is attached to large clusters, whereas in the controls all⁵⁷Fe is present as isolated atoma,⁵⁷Fe dimers or small clusters and no large⁵⁷Fe containing clusters are observed. This shows that⁵⁷Fe present as solitary or dimer species in ferritin migrates to ferritin molecules containing larger⁵⁶Fe clusters.     

Single crystalline elastic constants of Fe-Ni invar alloys

The single crystalline elastic constants of Fe-Ni alloys containing 30 – 50 wt.% Ni are reported. The anomalous temperature dependence in the ferromagnetic state is due to a large magnetic contribution to the elastic constants.     

In-beam M?ssbauer spectroscopy of 57 Mn implanted into lithium hydride

We measured the temperature dependence of ⁵⁷Fe M?ssbauer spectra obtained after ⁵⁷Mn implantation into polycrystalline LiH with an extremely low implantation dose. Density functional calculations suggested that the Fe atoms were predominantly implanted into both Li and H substitutional sites of the LiH crystal.     

Mössbauer emission study of57Co: YBa2Cu3O7−y HTSC

Mössbauer emission spectroscopy is performed on⁵⁷Co:YBa₂Cu₃O_7–y oxides in a temperature range from 300 K to 77 K. The spectra show, at least two different location for the⁵⁷Co(⁵⁷Fe) impurities. The isomer shift and quadrupole splitting values of the⁵⁷Fe daughter are coincident with those observed in⁵⁷Fe absorption experiments. From the analysis of theIS andQS values as well as from the relative location of Fe impurity levels in the HTSC matrix it is argued that:a) Co impurities enters into the lattice mainly in the Cu1 sites, but some of them have a higher coordination number.b) The daughter⁵⁷Fe exists as localized Fe⁴⁺ state.c) The parent⁵⁷Co enter as a localized Co³⁺ state. These conclusions appear consistent with the observed increase ofN(0) on doping with Fe or Co ions and with the existence of localized magnetic moments as determined from paramagnetic susceptibility measurements.     

57Co doped oxides as57Fe Mössbauer single line sources

The preparation of several⁵⁷Fe Mössbauer single line sources is reported. It is also discussed the mechanisms involved in the appearance of anomalous charge states for the⁵⁷Fe daughter. The⁵⁷CoO:Li, MgO:⁵⁷Co and Ti⁵⁷CoO₃ appear to be very good candidates to be used as strong sources.     

Chemical disorder of La and Sr ions in the block layer of LaSrFeO4

⁵⁷Fe Mössbauer spectroscopic studies were carried out on the single layered perovskite, LaSrFeO₄, in the temperature range 15-380 K. The observed line width asymmetries in the line shapes of the spectra were understood to arise from a chemical disorder of La³⁺ and Sr²⁺ ions in the block layer of the perovskite structure. The temperature dependence of the hyperfine magnetic field, quadrupole interaction, and isomer shift observed for the system was also analyzed in terms of the local environment of Fe atoms in this structure.