Multiferroicity and magneto-electric effect in Gd${}_2$BaNiO${}_5$

We report the observation of electric polarization in the magnetically ordered state of the Haldane chain compound, Gd${}_2$BaNiO${}_5$, with strongly correlated magnetic and dielectric properties. The results of dc magnetic susceptibility and heat capacity measurements indicate two magnetic transitions,one corresponding to the anti-ferromagnetic order at T${}_N$ around 55 K and the other to spin-reorientation transition at T${}_{SR}$ around 24 K. The dielectric permittivity (${ϵ}^{\mathrm{\prime }}$) and loss (tan$\delta$) also exhibit anomalies in the vicinity of T${}_{SR}$ and T${}_N$ respectively. Below the spin-reorientation transition, concurrently magnetic-field-induced spin-flop and the meta-electric transitions are observed at a critical magnetic field in isothermal magnetization and magneto-dielectric results respectively. Another interesting finding is that the change in dielectric constant changes its sign at the critical magnetic field. The origin of the observed magneto-electric effect is discussed on the basis of spin-phonon coupling.