  We have demonstrated magnetodielectric (MD) 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, made of the regular stacking of one CoO6
trigonal prism with two MnO6 octahedra. The (Co2+ Mn4+ Mn4+) unit 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 (above 5 kOe), evidencing the
presence of MD coupling, which is further confirmed by field-dependent
dielectric measurements. We argue that spatial inversion symmetry can be broken
as a result of exchange-striction along each spin chain, inducing uncompensated
local dipoles. At low temperatures, a dipolar relaxation phenomenon is
observed, bearing strong similarities with the blocking effect typical of the
spin dynamics in this compound. Such a spin-dipole relationship is referred to
as a multi-blocking effect, in relation with the concept of magnetodielectric
multiglass previously introduced for related materials.
