NMR Evidence of Antiferromagnetic Spin Fluctuations in Nd_0.85Sr_0.15NiO_2

Despite the recent discovery of superconductivity in Nd_1-xSr_xNiO_2 thin films, the absence of superconductivity and antiferromagnetism in their bulk materials remains a puzzle. Here we report the ^1H NMR measurements on powdered Nd_0.85Sr_0.15NiO_2 samples by taking advantage of the enriched proton concentration after hydrogen annealing. We find a large full width at half maximum of the spectrum, which keeps increasing with decreasing the temperature T and exhibits an upturn behavior at low temperatures. The spin-lattice relaxation rate ^1T_1^-1 is strongly enhanced when lowering the temperature, developing a broad peak at about 40 K, then decreases following a spin-wave-like behavior ^1T_1^-1\propto T^2 at lower temperatures. These results evidence a short-range glassy antiferromagnetic ordering of magnetic moments below 40 K and dominant antiferromagnetic fluctuations extending to much higher temperatures. Our findings reveal the strong electron correlations in bulk Nd_0.85Sr_0.15NiO_2, and shed light on the mechanism of superconductivity observed in films of nickelates.