Spin and orbital frustration in FeSc2S4 probed by 45Sc NMR

We report on Sc-45 NMR experiments in the geometrically frustrated spinels FeSc2S4 and MnSc2S4. The temperature dependences of line shift K(T), linewidth Delta(T), spin-spin relaxation rate 1/T-2(T), and spin-lattice relaxation rate 1/T-1(T) 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(T) of FeSc2S4 exhibits a cusplike maximum, a hallmark of spin frustration. Orbital fluctuations strongly enhance 1/T-1(T) at elevated temperatures. Toward lowest temperatures these fluctuations slow down resulting in an increase of 1/T-2(T). The exponential decrease of 1/T-1(T) results from the opening of a spin gap Delta=0.2 meV, a characteristic feature of a spin liquid.