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

Abstract

We report new results of a 57 Fe Mössbauer study of multiferroic 3R-AgFeO 2 powder samples performed in a wide temperature range, including two points, T N1 » 14 K and T N2 » 9 K, of magnetic phase transitions. At the intermediate temperature range, T N2 < T < T N1 , the 57 Fe Mössbauer spectra can be described in terms of collinear spin-density-waves (SDW) with the inclusion of many high-order harmonics, indicating that the real magnetic structure of this ferrite appears to be more complicated than a pure sinusoidally modulated SDW. The spectra at low temperatures, T < T N2 , consist of a Zeeman pattern with line broadenings and sizeable spectral asymmetry. It has been shown that the observed spectral shape is consistent with a transition to the elliptical cycloidal magnetic structure. An analysis of the experimental spectra was carried out under the assumption that the electric hyperfine interactions are modulated when the Fe 3+ magnetic moment rotates with respect to the principal axis of the EFG tensor and emergence of the strong anisotropy of the magnetic hyperfine field H hf at the 57 Fe nuclei. The large and temperatureindependent anharmonicity parameter, m » 0.78, of the cycloidal spin structure obtained from the experimental spectra results from easy-axis anisotropy in the plane of rotation of the iron spin.Analysis of different mechanisms of spin and hyperfine interactions in 3R-AgFeO 2 and its structural analogue CuFeO 2 points to a specific role played by the topology of the exchange coupling and the oxygen polarization in the delafossite structures.

Modulated non-collinear magnetic structure of (Co$_{0.97}$$^{57}$Fe$_{0.03}$)$_{4}$Nb$_{2}$O$_{9}$ as revealed by Mössbauer spectroscopy

“…Clearly, at ground state, the helicoidal structure are isotropic for Co/ 57 Fe(1) and anisotropic for Co/ 57 Fe(2). With increasing temperature, both of them become anisotropic but reduce in their magnitude, and the magnetic structure eventually becomes almost collinear near the transition temperature, ∼ 30 K. The spatial anisotropy of the determined hyperfine magnetic field, B h f ∝ A • µ eff , is largely due to the angular dependency of the hyperfine coupling tensor A and/or anisotropy µ eff as were discussed in other systems 23,24 .…”

“…Therefore, to describe the low temperature spectra satisfactorily, we used a more sophisticated model by taking into account the features associated with an incommensurate helicoidal structure as were used to described the complex Mössbauer spectra of BiFeO 3 22 , 3R-AgFeO 2 23 , Fe 3 PO 7 24 , FeP 25 and many others with modulated magnetic structures. In this case, the experimental spectrum is approximated as a superposition of two sets of Zeeman patterns corresponding to the two crystal sites.…”

“…The angle, Θ n , between the normal direction of the rotation plane and the c-axis/V zz was fitted as a free parameter. To take into account of possible anharmonicity (bunching) of the spatial distribution of the magnetic moments of Co 2+ / 57 Fe 2+ , a Jacobian elliptic function was used 23,25…”

Crystal structure and hyperfine interactions of delafossite (CuFeO2) synthesized hydrothermally

“…4) is a superposition of the sextet and doublet in which the intensity of the first line is larger than the second (the I 2 /I 1 ratio is 0.84). This asymmetry, opposite to the doublet asymmetry visible at 16, 20 K and RT, can be explained by the spin relaxation effects or short-range magnetic interactions between Fe 3+ ions, which are often observed in materials with frustrated exchange interactions (Sobolev et al, 2017).…”

Modulated Magnetic Structures in BaRFeO4 (R = Y and Dy): Magnetic and 57Fe Mössbauer Investigations

“…To describe the influence of moment modulation on the spectral shape, we have fitted the spectra for T N2 < T < T N1 using a Fourier expansion along the propagation vector q of the IC modulation. 13,26 Assuming that the hyperfine field Bh f,Fe(i) (z i ) at each particular iron position (z i ) is parallel and proportional to the magnetic moment μ Fe(i) , the amplitude modulation of hyperfine field can be defined as:…”