Evidence for static alternating magnetic fields in the vortex cores of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ is reported. Muon spin rotation measurements of the internal magnetic field distribution of the vortex state of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.50}$ in applied fields of $H=1\mathrm{}\mathrm{T}$ and $H=4\mathrm{}\mathrm{T}$ reveal a feature in the high-field tail of the field distribution which is not present in optimally doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.95}$ and which fits well to a model with static magnetic fields in the vortex cores. The magnitude of the fields is estimated to be 18(2) G and decreases above $T=10\mathrm{}\mathrm{K}$. We discuss possible origins of the additional vortex core magnetism within the context of existing theories.

• Received 28 August 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.137002