Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy.We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by magnetic field or Zn-doping. We propose that the observed electromagnon mode involves the collective precession of the spins with oscillating in-plane electric polarization through the mechanism of the linear magnetoelectric effect. Fe2Mo3O8 forms a hexagonal lattice belonging to a polar space group 4 P63mc ( Fig. 1(a)). There exist two types of magnetic site for Fe 2+ Alternatively, the FM state is stabilized also by substitution of more than 12.5 % of Fe with Zn [24,26,27]. Coexistence of spontaneous polarization and magnetic order below the transition temperature allows strong ME coupling and large linear ME coefficients in both the in-plane and out-ofplane components, which promises the characteristic spin wave excitation responding to ac electric/magnetic field of light.Single crystals of Fe2Mo3O8 and (Zn0.125Fe0.875)2Mo3O8 were grown by chemical vapor transport reaction as described in Refs. [28,29] from the stoichiometric mixture of MoO2, Fe, Fe2O3, and ZnO. Samples with ab-plane and ac-plane cut, whose dimensions are typically 2 x 2 mm 2 , were prepared.The time-domain terahertz spectroscopy was employed to measure the refractive indices in a frequency range of 0.5 -2.8 THz and the details about the experimental setup and procedures are described in Ref. [30]. Laser pulses with 100-fs duration from a Ti: sapphire laser were split into two paths 5 to generate and detect the wave form of terahertz pulses. A ZnTe (110) crystal and a dipole antenna were used for generation and detection of terahertz pulses, respectively. The Hdc was applied to the sample with a superconducting magnet in Voigt geometry, i.e., a light propagation vector k  perpendicular to Hdc. electromagnon, because it can be excited by the ⊥ (Fig. 1(e)) but not by ⊥ ( Fig. 1(f)), while MM1 is active for ⊥ (not with ⊥ ), indicating its magnetic-dipole (M1) active nature. To check the correlation between the mode profile and the magnetic order, we also measured the spectra for the collinear ferrimagnetic phase in the doped sample (y = 0.125) (Figs. 1(g)-(i)). This composition shows the FM state even at zero field ( Fig. 1(c)). A single resonance peak is observed around 2.6 THz (MM2) (Fig. 1(g) and 1(i)), while no discernible resonance structure is seen around 1.2 THz.Thus, the electromagnon resonance is absent ( Fig. 1(h)) in the current energy window, while the MM2 is active for ⊥ ( Fig. 1(i)) similarly to the MM1. Here, d is the thickness of the sample, t the transmittance, and  the angular frequency of light. t0 is assumed to be a background due to flat absorption. We def...

We report the polarization rotation of terahertz light resonant with the magnetoelectric (ME) spin excitation in the multiferroic (Fe,Zn)_{2}Mo_{3}O_{8}. This resonance reflects the frequency dispersion of the diagonal ME susceptibility (axion term), with which we quantitatively reproduce the thermal and magnetic-field evolution of the observed polarization rotation spectra. The application of the sum rule on the extrapolated dc value of the spectral weight of the ME oscillator provides insight into the dc linear ME effect. The present finding highlights a novel optical functionality of spin excitations in multiferroics that originates from diagonal ME coupling.

Elementary excitation characteristic of the magnetoelectric (ME) multiferroics is a magnon endowed with the electric activity, which is referred to as electromagnon. The electromagnon resonance mediated by the bilinear exchange coupling potentially exhibits the strong terahertz light-matter interaction with novel optical properties different from the conventional magnon excitation. Here we report the robust electromagnon resonance on the helimagnetic Y-type hexaferrites in a wide temperature range including room temperature. Furthermore, the efficient magnetic-field controls of the electromagnon are demonstrated on the flexible spin structure of these compounds, leading to the generation/annihilation of the resonance as well as the large resonance energy shift. These terahertz characteristics of the electromagnon exemplify the versatile magneto-optical functionality driven by the ME coupling in the multiferroics, paving a way for possible terahertz applications as well as terahertz control of a magnetic state of matter.

No abstract