57Fe Mössbauer study of unusual magnetic structure of multiferroic 3R-AgFeO2. (7th June 2017) - Sobolev, A
Abstract: We report new results of a 57 Fe Mössbauer study of hyperfine magnetic interactions in the layered multiferroic 3 R -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 57 Fe the nuclei below T N1 ≈ 14 K indicates that 3 R -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 57 Fe 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 (Δ H 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 3 R -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.