Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn₇O₁₂ Using ⁵⁷Fe Probe M?ssbauer Spectroscopy

Here, we report the results of a M & ouml;ssbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn7O12 doped with Fe-57 probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein (BiMn6.96Fe0.04O12)-Fe-57 undergoes a cascade of structural (T-1 approximate to 590 K, T-2 approximate to 442 K, and T-3 approximate to 240 K) and magnetic (T-N1 approximate to 57 K, T-N2 approximate to 50 K, and T-N3 approximate to 24 K) phase transitions. The analysis of the electric field gradient (EFG) parameters, including the dipole contribution from Bi3+ ions, confirmed the presence of the local dipole moments p(Bi), which are randomly oriented in the paraelectric cubic phase (T > T-1). The unusual behavior of the parameters of hyperfine interactions between T-1 and T-2 was attributed to the dynamic Jahn-Teller effect that leads to the softening of the orbital mode of Mn3+ ions. The parameters of the hyperfine interactions of Fe-57 in the phases with non-zero spontaneous electrical polarization (P-s), including the P1 <-> Im transition at T-3, were analyzed. On the basis of the structural data and the quadrupole splitting Delta(T) derived from the Fe-57 M & ouml;ssbauer spectra, the algorithm, based on the Born effective charge model, is proposed to describe P-s(T) dependence. The P-s(T) dependence around the Im <-> I2/m phase transition at T-2 is analyzed using the effective field approach. Possible reasons for the complex relaxation behavior of the spectra in the magnetically ordered states (T < T-N1) are also discussed.