Experimental evidence of symmetry breaking in the multiferroic Ba3NbFe3Si2O14 using sound velocity measurements

Using high-resolution sound velocity measurements, we determined the temperature dependence of the principal elastic constants of the multiferroic compound ${\mathrm{Ba}}_{3}{\mathrm{NbFe}}_{3}{\mathrm{Si}}_{2}{\mathrm{O}}_{14}$ at different magnetic fields. All four elastic constants revealed an anomaly at ${T}_{N}=27.1(1)$ K, which coincides very well with the emergence of the magnetic chiral state. More importantly, we carried out velocity measurements as a function of a field rotating in the basal trigonal plane at different fields and temperatures. In particular, in the magnetically ordered state, we observed an angular hysteresis of $\mathrm{\ensuremath{\Delta}}{C}_{11}/{C}_{11}$, which might be associated with the field-induced ferroelectric polarization suddenly switching from the ${b}^{*}$ axis to the $a$ direction, due to the magnetoelectric effect previously observed in ${\mathrm{Ba}}_{3}{\mathrm{NbFe}}_{3}{\mathrm{Si}}_{2}{\mathrm{O}}_{14}$. The data analysis also leads to an estimate for the lattice distortion $({e}_{1}\ensuremath{-}{e}_{2}\ensuremath{\le}{10}^{\ensuremath{-}5})$ accounting for a symmetry reduction in the magnetic chiral state possibly associated with the induced polarization along the $a$ axis in zero field.