Low-magnetic field control of electromagnon

Since the 1960s, there has been intense debate concerning the presence of the spin excitation driven by the electric field of light $E^\omega$ in ferroelectric magnets. This is recently revived by the observation of the absorption peak structure around 3 meV in a ferroelectric magnet TbMnO$_3$ [1]. As an origin of this excitation, the hybridized spin excitation with electric polarization (now called electromagnon) was considered [2]. However, this issue remains controversial by our THz spectroscopic studies on a family of $R$MnO$_3$ [3] ($R$ = Tb, Dy, and Gd$_{0.7}$Tb$_{0.3}$) in a variety of spin phases tuned by temperature and magnetic field. Here we report an optical investigation of the low-energy (2$-$10 meV) spin dynamics for other ferroelectric magnets, hexaferrite, by using THz time-domain spectroscopy. We find the signature of the genuine electromagnon at THz frequencies. As a manifestation of the strong magnetoelectric coupling inherent to the electromagnon, we demonstrate the low-magnetic field ($\sim$ 100 Oe) control of the optical constants at THz frequencies. \noindent [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [2] D. Senff {\it et al.}, PRL {\bf 98}, 137206 (2007). [3] N. Kida {\it et al.}, PRB {\bf 78}, 104414 (2008); Y. Takahashi {\it et al.}, PRL {\bf 101}, 187201 (2008); N. Kida {\it et al.}, J. Phys. Soc. Jpn. Dec. issue (2008).