Magnetoelectric coupling and multi-blocking effect in Ising-chain magnet Sr2Ca2CoMn2O9

We have demonstrated magnetoelectric (ME) coupling in an Ising-chain magnet Sr2Ca2CoMn2O9, via detailed investigation of ac susceptibility and dielectric constant as a function of temperature, magnetic field and frequency. Sr2Ca2CoMn2O9 consists of spin-chains of one CoO6 trigonal prismatic and two alternating MnO6 octahedra polyhedron. The (Co2+ Mn4+ Mn4+) distribution stabilizes a (up-down-up) spin-state along the chains which are distributed on a triangular lattice. This compound undergoes a partially disordered antiferromagnetic transition at TN~28 K. The dielectric constant exhibits a clear peak at TN only in presence of an external magnetic field (H>5 kOe), evidencing the presence of ME coupling, which is further confirmed by H-dependence dielectric measurements. The mechanism of this ME coupling is discussed as a result of exchange-striction in an Ising-chain magnet. In addition to this strong spin-lattice coupling, we report a dipolar relaxation phenomenon similar to spin-relaxation arising from the single-ion magnetism (spin-blocking effect). We term such phenomenon a multi-blocking effect.