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 $J_{\mathrm{eff}}=1/2$. Several types of nondispersive short-range magnetic excitations were discovered. The scattering intensity maps in $\mathit{Q}$ space are well reproduced by dynamical structure factor analyses using molecular model Hamiltonians. The results of analyses suggest that the molecular excitations below $T_N$ arise from molecular ground states, one of which consists of antiferromagnetically coupled ferromagnetic subunits. The quasielastic excitations above $T_N$ are interpreted as its precursor. A combination of frustration and $J_{\mathrm{eff}}=1/2$ might induce these quantum phenomena.

• Received 21 February 2011

DOI:https://doi.org/10.1103/PhysRevB.84.054405