Exploring low-temperature dynamics in triple perovskite ruthenates using nonlinear dielectric susceptibility measurements

We report the nonlinear dielectric properties of three triple-perovskite ruthenates: BaCoRuO, BaBiRuO, and SrCaRuO. These compounds exhibit notable correlations among their spin, charge, lattice, and polar degrees of freedom. BaCoRuO displays a pronounced frequency-dependent relaxation in χ just above the magnetoelastic transition, occurring around 100 K, followed by an abrupt loss of polarization within the ordered phase. In BaBiRuO, we encounter the possibility of multiple coexisting relaxation behaviors, indicating a complex phase strongly influenced by the spin-gap opening at 175 K. Lastly, SrCaRuO displays anomalies with strong dispersion effects close to its magnetic transitions, indicating a robust coupling between magnetic and dipolar orders in the system. These measurements highlight the significance of higher-order (hyper-)susceptibilities in providing profound insight into the dynamics of a system, offering information otherwise inaccessible through the linear polarization response alone.