Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn 7 O 12 Using 57 Fe Probe Mössbauer Spectroscopy.

Here, we report the results of a Mössbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn 7 O 12 doped with 57 Fe probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein BiMn 6.96 57 Fe 0.04 O 12 undergoes a cascade of structural ( T 1 ≈ 590 K, T 2 ≈ 442 K, and T 3 ≈ 240 K) and magnetic ( T N1 ≈ 57 K, T N2 ≈ 50 K, and T N3 ≈ 24 K) phase transitions. The analysis of the electric field gradient (EFG) parameters, including the dipole contribution from Bi 3+ 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 Mn 3+ ions. The parameters of the hyperfine interactions of 57 Fe in the phases with non-zero spontaneous electrical polarization ( P s ), including the P 1 ↔ Im transition at T 3 , were analyzed. On the basis of the structural data and the quadrupole splitting Δ( T ) derived from the 57 Fe Mössbauer 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 ↔ I 2/ 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.