  We report the results of detailed investigations of magnetization,
heat-capacity, dielectric, pyrocurrent, and magneto(di)electric measurements on
Tb2BaCoO5, belonging to a hither-to-unexplored spin-chain cobaltates family,
R2BaCoO5 (R= Rare-earths). The magnetic measurements reveal that this compound
exhibits an antiferromagnetic transition at (T_N=) 18.8 K and there is a
spin-reorientation beyond 40 kOe below T_N. Dielectric and pyrocurrent data
measured as a function of temperature and magnetic-field establish that this
compound is a type-II multiferroic material. The most fascinating finding which
we would like to emphasize is that the observed value of the magnetodielectric
effect beyond the metamagnetic transition field is the largest (close to 55%,
below 10 K, for H= 100 kOe) ever reported for polycrystals of a compound in
bulk form, thereby offering a hope to find a single-phase polycrystalline
compound at room temperature to enable ease of applications.
