Self-polarized terahertz magnon absorption in a single crystal of ${\mathrm{BiFeO}}_3$

We report the polarization dependence of terahertz magnon absorption in single crystals of ${\mathrm{BiFeO}}_3$ grown by a modified floating zone method. In a (111)${}_{\mathrm{pc}}$-oriented crystal, two major magnon absorption signals were observed for all terahertz polarizations, which indicates that magnetic domains were not aligned in one of the three allowed directions. In contrast, the absorption modes in a (001)${}_{\mathrm{pc}}$-oriented crystal showed significant polarization dependence, which was unchanged even after annealing the crystal at temperatures far above the Néel point to demagnetize it. This polarization dependence coincides with that of $E$ mode phonons. Thus, we conclude that magnon and phonon in ${\mathrm{BiFeO}}_3$ are strongly coupled and the selection rules for magnon absorption are governed by the activity of $E$ mode phonons, namely, the crystalline anisotropy originating from ferroelectric polarization.

Figure 1

(a) Relationship of the unit cell with the direction of ferroelectric polarization, $\mathit{P}$, and wave vectors of the magnetic order, $\mathit{q}$, in ${\mathrm{BiFeO}}_3$. Projection of the wave vectors of magnons on (b) the (111)${}_{\mathrm{pc}}$ plane and (c) the (001)${}_{\mathrm{pc}}$ plane. (d) and (e) Photographs of the surfaces of the (001)${}_{\mathrm{pc}}$-oriented crystal with the cross Nicol configuration. While in (d) ferroelectric polarization is parallel to the wire direction of one of the polarizers, in (e) both of the polarizers are rotated by ${45}^{\circ }$ from the former state.

Figure 2

Polarization dependence of the THz transmission spectra of the (111)${}_{\mathrm{pc}}$-oriented (upper) and (001)${}_{\mathrm{pc}}$-oriented (lower) ${\mathrm{BiFeO}}_3$ crystals at room temperature.

Figure 3

Temperature dependence of the THz transmission spectra of the (001)${}_{\mathrm{pc}}$-oriented single crystal of ${\mathrm{BiFeO}}_3$ at temperatures between 10 and 300 K for THz polarizations along the ${\left[1\overline{1}0\right]}_{\mathrm{pc}}$ (upper) and the ${\left[110\right]}_{\mathrm{pc}}$ (lower) axes.

Figure 4

Polarization dependence of the THz transmission spectra of the (120)${}_{\mathrm{hex}}$-oriented ${\mathrm{BiFeO}}_3$ crystal at room temperature. The gray and black curves show the spectra for THz polarizations parallel and perpendicular to the ${\left[001\right]}_{\mathrm{hex}}$ axis, respectively.