  We have studied spin excitations in a single-domain crystal of
antiferromagnetic LiCoPO4 by THz absorption spectroscopy. By analyzing the
selection rules and comparing the strengths of the absorption peaks in the
different antiferromagnetic domains, we found electromagnons and
magnetoelectric spin resonances besides conventional magnetic-dipole active
spin-wave excitations. Using the sum rule for the magnetoelectric
susceptibility we determined the contribution of the spin excitations to all
the different off-diagonal elements of the static magnetoelectric
susceptibility tensor in zero as well as in finite magnetic fields. We conclude
that the magnetoelectric spin resonances are responsible for the static
magnetoelectric response of the bulk when the magnetic field is along the
x-axis, and the symmetric part of the magnetoelectric tensor with zero diagonal
elements dominates over the antisymmetric components.
