  We measured two magnetic modes with finite and discrete energies in an
antiferromagnetic ordered phase of a geometrically frustrated magnet MgCr2O4 by
single-crystal inelastic neutron scattering, and clarified the spatial spin
correlations of the two levels: one is an antiferromagnetic hexamer and the
other is an antiferromagnetic heptamer. Since these correlation types are
emblematic of quasielastic scattering with geometric frustration, our results
indicate instantaneous suppression of lattice distortion in an ordered phase by
spin-lattice coupling, probably also supported by orbital and charge. The
common features in the two levels, intermolecular independence and discreteness
of energy, suggest that the spin molecules are interpreted as quasiparticles
(elementary excitations with energy quantum) of highly frustrated spins, in
analogy with the Fermi liquid approximation.
