I.S. Glazkova

Mössbauer investigations of hyperfine interactions features of 57Fe nuclei in BiFeO3 ferrite

New results of 57Fe Mossbauer studies on BiFeO3 powder sample performed at various temperatures above and below magnetic phase transitions point TN ≈ 640K are reported. We have performed self-consistent calculations of the lattice contributions to the EFG tensor, taking into account dipole moments of the O2− and Bi3+ ions. Low-temperature 57Fe Mossbauer spectra recorded at T < TN were analyzed assuming an anharmonic cycloidal modulation of the Fe3+ magnetic moments. The cycloidal modulation of the iron spin was described with the elliptic Jacobi function sn[(±4K(m)/λ)x,m]. The good fit of the experimental spectra was obtained for the anharmonicity m = 0.44 ± 0.04 (T = 4.9K) resulting from the easy-axis magnetic anisotropy.

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.

Hyperfine Interactions of 57 Fe Nuclei in ScCo 1– x Fe x O 3 ( x = 0.05, 0.4) Substituted Cobaltites

The Mössbauer study of 57Fe nuclei in Sc1–yCo1–xFexO3–3/2y (x = 0.05, 0.4; y = 0.1, 0.2) cobaltites has been reported. Mössbauer spectra, as well as calculations of the parameters of the electric field gradient tensor, have shown that Fe3+ cations not only occupy characteristic for them positions with octahedral oxygen coordination, but also partially substitute Sc3+ cations in ScOn polyhedra with a large coordination number (n = 8–12). The character of distribution of iron cations over two positions depends on the composition of studied cobaltites. In contrast to Co3+ cations, Fe3+ cations in both positions are stabilized in a highspin state (S = 5/2). The analysis of the magnetic hyperfine structure of 57Fe spectra measured at T < TN has shown that magnetically ordered iron-rich microclusters can be formed in the octahedral sublattice of the Sc0.9Co0.6Fe0.4O2.85 and Sc0.8Co0.6Fe0.4O2.7 cobaltites. The most fraction of iron in Sc0.9Co0.9557Fe0.05O2.85 remains in the paramagnetic state down to the lowest temperatures.

57Fe Mössbauer investigation of multiferroics BiMn0.9657Fe0.04O3 and BiMn0.7Fe0.3O3

New results of 57Fe probe Mossbauer studies on powder BiMn0.9657Fe0.04O3 and BiMn0.7Fe0.3O12 samples are reported. These manganites attract a great interest due to their multiferroic properties as well as phenomenon of cooperative orbital ordering, caused by the Jahn-Teller Mn3+ ions in their structure and totally disappearing at high iron concentration. 57Fe Mossbauer spectra were measured and analyzed in a wide temperature range including critical points of magnetic (TN) and orbital ordering (TOO) phase transitions in terms of crystal structure features and phases coexistence.