Orbital-order driven ferroelectricity and dipolar relaxation dynamics in multiferroic GaMo4S8

Abstract

We present the results of broadband dielectric spectroscopy of $\mathrm{GaM}{\mathrm{o}}_{4}{\mathrm{S}}_{8}$, a lacunar spinel system that recently was shown to exhibit non canonical, orbitally driven ferroelectricity. Our study reveals complex relaxation dynamics of this multiferroic material, both above and below its Jahn-Teller transition at ${T}_{\mathrm{JT}}=47\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Above ${T}_{\mathrm{JT}}$, two types of coupled dipolar-orbital dynamics seem to compete: relaxations within cluster-like regions with short-range polar order as in relaxor ferroelectrics and critical fluctuations of only weakly interacting dipoles, the latter resembling the typical dynamics of order-disorder type ferroelectrics. Below the Jahn-Teller transition, the onset of orbital order drives the system into long-range ferroelectric order and dipolar dynamics within the ferroelectric domains is observed. The coupled dipolar and orbital relaxation behavior of $\mathrm{GaM}{\mathrm{o}}_{4}{\mathrm{S}}_{8}$ above the Jahn-Teller transition markedly differs from that of the skyrmion host $\mathrm{Ga}{\mathrm{V}}_{4}{\mathrm{S}}_{8}$, which seems to be linked to differences in the structural distortions of the two systems on the unit-cell level.