Antiferromagnetism in the vortex cores of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$

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 ${}^{17}\mathrm{O},$ in near-optimally doped ${\mathrm{YB}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta },$ was measured. Outside of the core, ${(}^{17}{T}_1{T)}^{-1}$ is independent of temperature consistent with theoretical predictions for a d-wave superconductor. In the vortex core, ${(}^{17}{T}_1{T)}^{-1}$ increases with decreasing temperature following an antiferromagnetic Curie-Weiss law.