Open Access

The term multiferroics mostly refers to materials exhibiting concomitant ferroelectric and magnetic order. Due to the coupling between microscopic electric and magnetic dipoles, these compounds exhibit magnetoelectric effects. Such materials are promising candidates for technical applications and therefore have attracted intense research interest. In the present work, excitations of spin, lattice and charge degrees of freedom in the multiferroic transition metal molybdenum oxides M2Mo3O8 (M = Co, Fe) and lacunar spinels have been investigated by broadband optical spectroscopy, covering frequencies from the terahertz up to the visible range. The study of the low-energy spin excitations in the transition metal molybdenum oxides reveals rich spectra of conventional magnons, electromagnons and magnetoelectric resonances. Due to the dynamical magnetoelectric effect, strong unidirectional transmission is observed in these compounds in the terahertz range. The investigation of lattice dynamics and electronic transitions in Fe2Mo3O8 reveals signatures of short-range magnetic ordering in the optical spectra. Besides, the temperature dependent measurements suggest a coupling of the phonons to orbital degrees of freedom. A comparative infrared study of a large series of lacunar spinels allows to assign the observed phonon modes to the different structural units of these compounds. Besides, for these narrow-gap semiconductors the systematics of the size of the electronic band gap of could be derived.