We report on ${}^{45}\mathrm{Sc}$ NMR experiments in the geometrically frustrated spinels ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ and ${\mathrm{MnSc}}_2{\mathrm{S}}_4$. The temperature dependences of line shift $K\left(T\right)$, linewidth $\Delta \left(T\right)$, spin-spin relaxation rate $1∕T_2\left(T\right)$, and spin-lattice relaxation rate $1∕T_1\left(T\right)$ are deduced. The manganese compound reveals the behavior of a long-range ordered antiferromagnet, but with the onset of critical spin fluctuations at unusually high temperatures of 40 $T_N$. $K\left(T\right)$ of ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ exhibits a cusplike maximum, a hallmark of spin frustration. Orbital fluctuations strongly enhance $1∕T_1\left(T\right)$ at elevated temperatures. Toward lowest temperatures these fluctuations slow down resulting in an increase of $1∕T_2\left(T\right)$. The exponential decrease of $1∕T_1\left(T\right)$ results from the opening of a spin gap $\Delta =0.2\mathrm{meV}$, a characteristic feature of a spin liquid.

• Received 24 March 2005

DOI:https://doi.org/10.1103/PhysRevB.73.132409