Spins in the vortices of a high-temperature superconductor

Neutron scattering is used to characterize the magnetism of the vortices fo r the optimally doped high-temperature superconductor La2-xSrxCuO4 (x = 0.1 63) in an applied magnetic field. As temperature is reduced, Low-frequency spin fluctuations first disappear with the loss of vortex mobility, but the n reappear. We find that the vortex state can be regarded as an inhomogeneo us mixture of a superconducting spin fluid and a material containing a near ly ordered antiferromagnet. These experiments show that as for many other p roperties of cuprate superconductors, the important underlying microscopic forces are magnetic.