  We investigate magnetic excitations in an $S=3/2$ Heisenberg model
representing two-dimensional antiferromagnet ${\rm Ba_2CoGe_2O_7}$. In
terahertz absorption experiment of the compound, Goldstone mode as well as
novel magnetic excitations, conventional magnetic resonance at 2 meV and both
electric- and magnetic-active excitation at 4 meV, have been observed. By
introducing a hard uniaxial anisotropy term $\Lambda (S^z)^2$, three modes can
be explained naturally. We also indicate that, via the spin-dependent
metal-ligand hybridization mechanism, the 4 meV excitation is an
electric-active mode through the coupling between spin and electric-dipole.
Moreover, at 4 meV excitation, an interference between magnetic and electric
responses emerges as a cross correlated effect. Such cross correlation effects
explain the non-reciprocal linear directional dichroism observed in ${\rm
Ba_2CoGe_2O_7}$.
