  We have examined the magnetic properties of superconducting
YBa_2(Cu_0.96Co_0.04)_3O_y (y ~ 7, T_sc = 65 K) using elastic neutron
scattering and muon spin relaxation (muSR) on single crystal samples. The
elastic neutron scattering measurements evidence magnetic reflections which
correspond to a commensurate antiferromagnetic Cu(2) magnetic structure with an
associated Neel temperature T_N ~ 400 K. This magnetically correlated state is
not evidenced by the muSR measurements. We suggest this apparent anomaly arises
because the magnetically correlated state is dynamic in nature. It fluctuates
with rates that are low enough for it to appear static on the time scale of the
elastic neutron scattering measurements, whereas on the time scale of the muSR
measurements, at least down to ~ 50 K, it fluctuates too fast to be detected.
The different results confirm the conclusions reached from work on equivalent
polycrystalline compounds: the evidenced fluctuating, correlated Cu(2) moments
coexist at an atomic level with superconductivity.
