Molecular spin-orbit excitations in the J_eff=12 frustrated spinel GeCo₂O₄

We describe powder and single-crystal inelastic neutron scattering experiments on a spinel-type antiferromagnet GeCo₂O₄, represented by an effective total angular momentum J_eff=1/2. Several types of nondispersive short-range magnetic excitations were discovered. The scattering intensity maps in 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 _eff=1/2 might induce these quantum phenomena.