  A hexamer-type spin excitation seen in spinel chromates $A$Cr$_2$O$_4$ ($A$ =
Mg, Zn, or Cd) is the representative spin-liquid-like state caused by
geometrical frustration. To clarify an origin of the state, we comparatively
studied spin excitations in an isomorphic material HgCr$_2$O$_4$ by inelastic
neutron scattering, and observed a different molecular-type excitation.
Numerical analyses performed using model Hamiltonians suggest that these two
types of spin excitations originate from a spin-3/2 molecular singlet hidden in
a magnetically ordered phase. The difference between the molecular types is
explained by the difference in the kind of exchange interactions occurring in
the chromates.
