Quasi-two-dimensional spin correlations in the triangular lattice bilayer spin glass LuCoGaO4

We present a single-crystal time-of-flight neutron scattering study of the static and dynamic spin correlations in LuCoGaO4, a quasi-two-dimensional dilute triangular lattice antiferromagnetic spin-glass material. This system is based on Co2+ ions that are randomly distributed on triangular bilayers within the YbFe2O4-type, hexagonal crystal structure. Antiferromagnetic short-range two-dimensional correlations at wave vectors Q=1/3,1/3,L develop within the bilayers at temperatures as high as |ΘCW|∼100 K and extend over roughly five unit cells at temperatures below Tg=19 K. These two-dimensional static correlations are observed as diffuse rods of neutron scattering intensity along c* and display a continuous spin freezing process in their energy dependence. Aside from exhibiting these typical spin-glass characteristics, this insulating material reveals a novel gapped magnetic resonant spin excitation at ΔE∼12meV localized around Q=1/3,1/3,L. The temperature dependence of the spin gap associated with this two-dimensional excitation correlates with the evolution of the static correlations into the spin-glass state ground state. We associate it with the effect of the staggered exchange field acting on the Seff=1/2 Ising-like doublet of the Co2+ moments.