Crystal Symmetry Lowering in Chiral Multiferroic Ba${}_3$TaFe${}_3$Si${}_2$O${}_{14}$ observed by X-Ray Magnetic Scattering

Abstract

Chiral multiferroic langasites have attracted attention due to their doubly-chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft X-ray diffraction study of the multiferroic Ba${}_3$TaFe${}_3$Si${}_2$O${}_{14}$ at the Fe $L_{2,3}$ and oxygen $K$ edges. Below $T_N$ ($\approx 27K$) we observe the satellite reflections $(0,0,\ddot{I}„)$, $(0,0,2\ddot{I}„)$, $(0,0,3\ddot{I}„)$ and $(0,0,1-3\ddot{I}„)$ where $\ddot{I}„\approx 0.140\pm 0.001$. The dependence of the scattering intensity on X-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane ($\hat{\mathbf{c}}$-axis) magnetic dipole while the $(0,0,2\ddot{I}„)$ originates from the electron density distortions accompanying magnetic order. We observe dissimilar energy dependences of the diffraction intensity of the purely magnetic odd-harmonic satellites at the Fe $L_3$ edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase.