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

Abstract

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 TN1 and TN2. The asymptotic value β* ≈ 0.34 for the critical exponent obtained from the temperature dependence of the hyperfine field Hhf(T) at 57Fe the nuclei below TN1 ≈ 14 K indicates that 3R-AgFeO2 shows quasi-3D critical behavior. The spectra just above TN1 (TN1 < 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, TN2 < T < TN1, 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 < TN2 ≈ 9 K, the hyperfine field Hhf reveals a large spatial anisotropy (ΔHanis ≈ 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 Hanis 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.