Antiferromagnetism in the Vortex Cores of YBa$_2$Cu$_3$O$_{7-\delta}$
  We report spatially resolved nuclear magnetic resonance measurements on a high temperature superconductor that indicate the presence of correlated antiferromagnetic fluctuations in the vortex core. The nuclear spin-lattice relaxation rate, $1/^{17}T_1$, of planar $^{17}O$, in near-optimally doped YB$_2$Cu$_3$O$_{7-\delta}$ (YBCO), was measured. Outside of the core, $(^{17}T_1T)^{-1}$ is independent of temperature consistent with theoretical predictions for a {\it d}-wave superconductor. In the vortex core $(^{17}T_1T)^{-1}$ increases with decreasing temperature following an antiferromagnetic Curie-Weiss law. 
V. F. Mitrovi{\'c}, E. E. Sigmund, W. P. Halperin, A. P. Reyes, P.   Kuhns, W. G. Moulton