Anisotropic photo-control of spontaneous polarization in BiFeO$_{3}$ thin films: Indications of photo-assisted magnetoelectric effect

The terahertz (THz) emission from multiferroic BiFeO$_{3 }$(BFO) due to ultra-fast depolarization of electric order helps in evaluating its ferroelectric behavior [1]. In this work, we investigate THz-emission in BFO films on (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ (LSAT) substrates along $(100)$, $(110)$ and $(111) $directions. THz emission amplitude (E$_{THz})$ and its electric field dependence are similar in BFO $(100)$ and BFO $(110)$; E$_{THz}$ as function of electric field translates to ferroelectric-like hysteresis loop, and sign and amplitude of THz-emission are commensurate with the applied electric field. On the contrary, in BFO $(111)$ the hysteretic loop is absent, E$_{THz}$ varies linearly with electric field and the polarity of THz-emission is opposite to that of the electric field. These results suggest that THz-emission is mainly a result of ultrafast depolarization but with a superimposed character of ultrafast reorientation or demagnetization of antiferromagnetic (AFM) order. Though AFM order in BFO makes the determination of magnetoelectric (ME) coupling rather difficult, the photo-assisted electric field control of AFM order shown in the present work is suggestive of photo-assisted ME effect. 1. K. Takahashi \textit{et al.}, Phys. Rev. Lett. 96, 117402 (2006).