A.M. Gapochka

57Fe Mössbauer study of unusual magnetic structure of multiferroic 3R-AgFeO2

We report new results of a 57Fe Mössbauer study of hyperfine magnetic interactions in the layered multiferroic 3R-AgFeO2 demonstrating two magnetic phase transitions at T N1 and T N2. The asymptotic value β *  ≈  0.34 for the critical exponent obtained from the temperature dependence of the hyperfine field H hf(T) at 57Fe the nuclei below T N1  ≈  14 K indicates that 3R-AgFeO2 shows quasi-3D critical behavior. The spectra just above T N1 (T N1  <  T  <  T  *  ≈  41 K) demonstrate a relaxation behavior due to critical spin fluctuations which indicates the occurrence of short-range correlations. At the intermediate temperature range, T N2  <  T  <  T N1, the 57Fe Mössbauer spectra are described in terms of collinear spin-density-waves (SDW) with the inclusion of many high-order harmonics, indicating that the real magnetic structure of the ferrite appears to be more complicated than a pure sinusoidally modulated SDW. Below T  <  T N2  ≈  9 K, the hyperfine field H hf reveals a large spatial anisotropy (ΔH anis  ≈  30 kOe) which is related with a local intra-cluster (FeO6) spin-dipole term that implies a conventional contribution of the polarized oxygen ions. We proposed a simple two-parametric formula to describe the dependence of H anis on the distortions of the (FeO6) clusters. Analysis of different mechanisms of spin and hyperfine interactions in 3R-AgFeO2 and its structural analogue CuFeO2 points to a specific role played by the topology of the exchange coupling and the oxygen polarization in the delafossite-like structures.

Spatially modulated magnetic structure of AgFeO2: Mössbauer study on 57Fe nuclei

The results of the Mössbauer study of ferrite AgFeO2 manifesting multiferroic properties (at T ≤ TN2) have been presented. The hyperfine interaction parameters of 57Fe nuclei have been analyzed in a wide temperature range including the points of two magnetic phase transitions (TN2 ≈ 7–9 K and TN1 ≈ 15–16 K). It has been shown that the Mössbauer spectra of the 57Fe nuclei are sensitive to the variations of the character of the magnetic ordering of Fe3+ ions in the studied ferrite. The results of the model identification of a series of spectra (4.7 K ≤ T ≤ TN2) under the assumption of the cycloid magnetic structure of ferrite AgFeO2 have been presented. The analysis of the results has been performed in comparison with the literature data for other oxide multiferroics.

Mössbauer study of compounds of Cu3 − xFexSnS4 and Cu2Fe1 −xZnxSnS4 systems

A Mössbauer study of the structural and charge states of 57Fe and 119Sn atoms in the compounds of Cu3 −xFexSnS4 and Cu2Fe1 − xZnxSnS4 systems was performed. It was shown that the iron atoms in the compounds of both systems were in the divalent and trivalent states occupying the tetrahedral positions of the structure. The character of the changes of the degree of covalency of the Fe2+-S, Fe3+-S and Sn4+-S bonds during the isomorphic substitution in the systems was established.

Mössbauer study of isomorphous substitutions in Cu2Fe1-xCuxSnS4 and Cu2Fe1-xZnxSnS4 series

The investigation of isomorphous substitutions in minerals of stannite group was carried out for two series Cu2FeSnS4 – Cu3SnS4 (stannite – kuramite) and Cu2FeSnS4 – Cu2ZnSnS4 (stannite – kesterite) by 57Fe and 119Sn Mossbauer spectroscopy. The calculation of the lattice contribution to the electric field gradient tensor at the 57Fe nuclei and an estimation of the quadrupole shift for the determination of Fe3+ atom positions in the structure were carried out. The charge and structural states of Fe and Sn atoms in compounds of the two systems were determined. The schemes of isomorphous substitutions were found for both series in all range of iron concentrations. The changes in the covalence degree of the bonds Fe2+ − S, Fe3+ − S and Sn4+ − S in the process of the substitution were found.

Mössbauer studies of BiFe1–xScxO3 (x = 0, 0.05) Multiferroics

The effect replacing Fe atoms with Sc atoms has on the spatial spin-modulated structure and electrical and magnetic hyperfine interactions of 57Fe nuclei in BiFeO3 multiferroic is studied by means of Mössbauer spectroscopy. The temperature dependences of the parameter of anharmonicity and hyperfine parameters of the Mössbauer spectrum of 57Fe nuclei are obtained for all of the investigated ferrites in the temperature range of 5–300 K.

Hyperfine interactions of 57Fe impurity nuclei in TmNiO3 and YbNiO3 nickelates in the range of magnetic and structure phase transitions

Hyperfine interactions of 57Fe impurity nuclei are studied by probe Mössbauer spectroscopy in TmNiO3 and YbNiO3 perovskite-like nickelates in the ranges of temperature transitions of an insulator (T < TIM) ↔ metal (T > TIM) and antiferromagnetic (T < TN) ↔ paramagnetic (T > TN). The changing behavior of hyperfine interaction parameters of 57Fe nuclei in the ranges of phase transition temperatures (TIM and TN) is analyzed. The results are interpreted in the context of the charge disproportionation of Ni3+ cations associated with the electronic localization in monoclinic-distorted nickelates at T < TIM.

Mössbauer Investigations of Cu3−xFexSnS4 and Cu2Fe1−xZnxSnS4 Systems

Two isomorphous series Cu3SnS4−Cu2FeSnS4 (kuramite–stannite) and Cu2FeSnS4−Cu2ZnFeSnS4 (stannite–kesterite) have been studied by 57Fe and 119Sn Mossbauer spectroscopy. It has been found for the first series that at the Fe atom concentration below x0 = 0.45±0.02, iron atoms are only in the trivalent state and the concentration of Fe3+ and Cu1+ increases with an increase in x. The following scheme of isomorphous substitution of Cu for Fe may be proposed for this range of concentration: 2Cu2+(Td)→Cu1+(Td)+Fe3+(Oh). When x exceeds the boundary value x0, the Fe2+ ions appear. In the concentration range 0.45<x<1, the scheme of isomorphous substitution is Cu1+(T4d)+Fe3+(Oh)→2Fe2+(Td). Unlike the first series with the heterovalent substitution, for the second series, the isovalent substitution is observed: Fe2+(Td)→Zn2+(Td). For the Cu2Fe1−xZnxSnS4 system, Fe3+ atoms appear for all compounds. The noticeable changes of Fe2+ and Fe3+ atom concentration were not observed with an increase in x.

Spatial spin-modulated structure and hyperfine interactions of 57Fe nuclei in multiferroics BiFe1–xTxO3 (T = Sc, Mn; x = 0, 0.05)

The results of the Mössbauer studies on 57Fe nuclei in multiferroics BiFe1–xTxO3 (T = Sc, Mn; x = 0, 0.05) in the temperature range of 5.2–300 K have been presented. The Mössbauer spectra have been analyzed in terms of the model of an incommensurate spatial spin-modulated structure of cycloid type. Information has been obtained about the effect of the substitution of Sc and Mn atoms for Fe atoms on the hyperfine parameters of the spectrum: the shift and the quadrupole shift of the Mössbauer line, the isotropic and anisotropic contributions to the hyperfine magnetic field, and also the parameter of anharmonicity of the spatial spin-modulated structure.

Hyperfine interactions of 57Fe impurity nuclei in multiferroic CuCrO2

Hyperfine interactions of 57Fe nuclei in multiferroic CuCrO2 are studied by means of Mössbauer probe spectroscopy in a wide range of temperatures, including that of the magnetic phase transition. The temperature dependences of the parameters of electric and magnetic hyperfine interactions and the anharmonicity parameter of the spin wave are obtained as part of the spatial anharmonic spin-modulated structure.

Magnetic Exchange Interactions and Supertransferred Hyperfine Fields at 119Sn Probe Atoms in CaCu3Mn4O12 Manganite

The hyperfine magnetic interactions of 119Sn probe atoms in the CaCu3Mn3.96Sn0.04O12 double manganite by Mössbauer spectroscopy using magnetic measurements have been investigated. A consistent description of the results obtained in terms of the Weiss molecular field model by taking into account the peculiarities of the local environment of tin atoms has allowed the indirect Cu2+OMn4+ (JCuMn 51 ± 1 K) and Mn4+OMn4+ (JMnMn 0.6 ± 0.6 K) exchange interaction integrals to be estimated. Based on the KanamoriGoodenoughAnderson model, we show that the magnitude and sign of the intrasublattice exchange integral JMnMn correspond to both the electronic configuration of the Mn4+ cations and the geometry of their local crystallographic environment in the compound under study.