Electromagnon resonance in a collinear spin state of the polar antiferromagnet Fe2Mo3O8

Abstract

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...

Confirming the trilinear form of the optical magnetoelectric effect in the polar honeycomb antiferromagnet Co$_{2}$Mo$_3$O$_8$

“…Similarly to the cases of the antiferromagnetic Fe 2 Mo 3 O 8 [24], and ferrimagnetic Zn-doped Fe 2 Mo 3 O 8 [25,56] and Mn 2 Mo 3 O 8 [57], in the lowenergy range (<16 meV) we observe two strong magnetic excitations in zero field both in the terahertz absorption and the inelastic neutron scattering data. In contrast to the sister compounds, both of these modes in Co 2 Mo 3 O 8 are doubly degenerate and show a V-shape splitting in magnetic fields applied along the c axis, as expected for a four-sublattice easy-axis collinear antiferromagnet.…”

Imaging antiferromagnetic antiphase domain boundaries using magnetic Bragg diffraction phase contrast

“…[16] Fe2Mo3O8 and several isostructural compounds have recently attracted significant attention due to the plethora of unusual effects originating from coupling of magnetism to the crystal lattice. They include giant magnetoelectricity, [17,18] unconventional electromagnon excitations, [19] giant thermal Hall effect, [20] exotic axion-type magnetoelectric susceptibility, [21] as well as nonreciprocal spectroscopic effects in the THz range. [22] AFM domain behavior is clearly relevant to all these intriguing properties.…”

Structure, phonons, and orbital degrees of freedom in Fe2Mo3O8

“…In Fe 2 Mo 3 O 8 , the ferrimagnetic spin arrangement can be induced by applying external magnetic field [7] or substituting Fe by nonmagnetic Zn ions [4]. As a clear manifestation of the multiferroic character in the optical properties, magnetoelectric spin excitations were reported in Fe 2 Mo 3 O 8 [8] and in Zn-doped Fe 2 Mo 3 O 8 , exhibiting an optical diode effect [9] and nonreciprocal gyrotropic birefringence [10], respectively.…”