Magnetic and vibronic THz excitations in Zn doped Fe${}_2$Mo${}_3$O${}_8$

Abstract

We report on optical excitations in the magnetically ordered phases of multiferroic Fe${}_{1.86}$Zn${}_{0.14}$Mo${}_3$O${}_8$ in the frequency range from 10-130 cm${}^{-1}$ (0.3-3.9 THz). In the collinear easy-axis antiferromagnetic phase below $T_N=50$~K eleven optically active modes have been observed in finite magnetic fields, assuming that the lowest-lying mode is doubly degenerate. The large number of modes reflects either a more complex magnetic structure than in pure Fe${}_2$Mo${}_3$O${}_8$ or that spin stretching modes become active in addition to the usual spin precessional modes. Their magnetic field dependence, for fields applied along the easy axis, reflects the irreversible magnetic-field driven phase transition from the antiferromagnetic ground state to a ferrimagnetic state, while the number of modes remains unchanged in the covered frequency region. We determined selection rules for some of the AFM modes by investigating all polarization configurations and identified magnetic- and electric-dipole active modes as well. In addition to these sharp resonances, a broad electric-dipole active excitation band, which is not influenced by the external magnetic field, occurs below $T_N$ with an onset at 12 cm${}^{-1}$. We are able to model this absorption band as a vibronic excitation related to the lowest-lying Fe${}^{2+}$ electronic states in tetrahedral environment.

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Cited by in corpus (9)

• Direct observation of topological magnon polarons in a multiferroic material
• Magnetization reversal through an antiferromagnetic state
• Band-Mott mixing hybridizes the gap in Fe${}_2$Mo${}_3$O${}_8$
• Prethermal nematic order and staircase heating in a driven frustrated Ising magnet with dipolar interactions
• Thermodynamic determination of the equilibrium first-order phase-transition line hidden by hysteresis in a phase diagram
• Disorder-induced excitation continuum in a spin-1/2 cobaltate on a triangular lattice
• Optical magnetoelectric effect in the polar honeycomb antiferromagnet Fe2Mo3O8
• Magnetic Interactions in the Polar Ferrimagnet with a Bipartite Structure
• Optical phonons as a testing ground for spin group symmetries