  There is strong evidence that magnetic interactions play a crucial role in
the mechanism driving high-temperature superconductivity in cuprate
superconductors. To investigate this further we have done neutron scattering
experiments on the simplest high-temperature superconductor La(2-x)Sr(x)CuO(4)
(LSCO) in an applied magnetic field. Below the superconducting transition
temperature (Tc), the field penetrates the material via an array of normal
state inclusions or vortices while phase coherent superconductivity
characterized by zero resistance is suppressed to the lower field-dependent
irreversibility temperature. The measurements described here were performed on
underdoped LSCO (x=0.10), which develops static incommensurate order below Tc
in zero field. Our results show that application of a magnetic field enhances
this response without changing the onset temperature. For H=5T the
field-induced signal saturates to three times the zero-field signal and phase
coherent superconductivity is established within the antiferromagnetic phase.
