  We present a neutron-scattering study of the static and dynamic spin
correlations in the underdoped high-temperature superconductor
YBa(2)Cu(3)O(6.45) in magnetic fields up to 15 T. The field strongly enhances
static incommensurate magnetic order at low temperatures and induces a
spectral-weight shift in the magnetic-excitation spectrum. A reconstruction of
the Fermi surface driven by the field-enhanced magnetic superstructure may thus
be responsible for the unusual Fermi surface topology revealed by recent
quantum-oscillation experiments.
