Dielectric and multiferroic behavior in Sm2BaNiO5, a Haldane spin-chain compound

The Haldane spin-chain (S = 1) insulating compound, Sm2BaNiO5, which has been proposed to order antiferromagnetically around (TN = ) 55 K, was investigated for its complex dielectric permittivity, magnetodielectric and pyrocurrent behavior as a function of temperature (T). In order to enable meaningful discussions, the results of ac and dc magnetization and heat-capacity studies are also reported. We emphasize on the following findings: (i) There is a pyrocurrent peak near TN, but it is shown not to arise from ferroelectricity, but possibly due to 'thermally stimulated depolarization current', unlike in many other members of this rare-earth series in which case ferroelectric features were reported at or above TN; (ii) however, the pyrocurrent measured in the presence of a bias electric field (after cooling in zero electric field) as well as dielectric constant reveal a weak peak with increasing T around 22 K-the temperature around which population of the exchange-split excited state of Kramers doublet has been known to occur. This finding suggests that this compound presents a novel situation in which multiferroicity is induced by an interplay between crystal-field effects and exchange interaction. No multiglass features could be observed down 2 K unlike in many other members of this family.