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 ${\mathrm{LuCoGaO}}_{4}$, a quasi-two-dimensional dilute triangular lattice antiferromagnetic spin-glass material. This system is based on ${\mathrm{Co}}^{2+}$ ions that are randomly distributed on triangular bilayers within the ${\mathrm{YbFe}}_{2}{\mathrm{O}}_{4}$-type, hexagonal crystal structure. Antiferromagnetic short-range two-dimensional correlations at wave vectors $\mathbf{Q}=\left(1/3,1/3,L\right)$ develop within the bilayers at temperatures as high as $|{\mathrm{\ensuremath{\Theta}}}_{\mathrm{CW}}|\ensuremath{\sim}100$ K and extend over roughly five unit cells at temperatures below ${T}_{\mathrm{g}}=19$ 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 $\mathrm{\ensuremath{\Delta}}E\ensuremath{\sim}12\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ localized around $\mathbf{Q}=\left(1/3,1/3,L\right)$. 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 ${S}_{\mathrm{eff}}=1/2$ Ising-like doublet of the ${\mathrm{Co}}^{2+}$ moments.