  We describe powder and single-crystal inelastic neutron scattering
experiments on a spinel-type antiferromagnet GeCo$_2$O$_4$, represented by an
effective total angular momentum J_eff = 1/2. Several types of non-dispersive
short-range magnetic excitations were discovered. The scattering intensity maps
in $\vec{Q}$ space are well reproduced by dynamical structure factor analyses
using molecular model Hamiltonians. The results of analyses strongly suggest
that the molecular excitations below T_N arise from a hidden molecular-singlet
ground state, in which ferromagnetic subunits are antiferromagnetically
coupled. The quasielastic excitations above T_N are interpreted as its
precursor. A combination of frustration and J_eff = 1/2 might induce these
quantum phenomena.
