Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn7O12 Using 57Fe Probe Mössbauer Spectroscopy.

Abstract

Here, we report the results of a Mössbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn7O12 doped with 57Fe probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein BiMn6.9657Fe0.04O12 undergoes a cascade of structural (T1 ≈ 590 K, T2 ≈ 442 K, and T3 ≈ 240 K) and magnetic (TN1 ≈ 57 K, TN2 ≈ 50 K, and TN3 ≈ 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 pBi, which are randomly oriented in the paraelectric cubic phase (T > T1). The unusual behavior of the parameters of hyperfine interactions between T1 and T2 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 57Fe in the phases with non-zero spontaneous electrical polarization (Ps), including the P1 ↔ Im transition at T3, were analyzed. On the basis of the structural data and the quadrupole splitting Δ(T) derived from the 57Fe Mössbauer spectra, the algorithm, based on the Born effective charge model, is proposed to describe Ps(T) dependence. The Ps(T) dependence around the Im ↔ I2/m phase transition at T2 is analyzed using the effective field approach. Possible reasons for the complex relaxation behavior of the spectra in the magnetically ordered states (T < TN1) are also discussed.