A neutron diffraction study of the magnetic field dependence of the ma
gnetic Bragg reflections of single crystals of YBa2Cu3O6+x (x = 0.05 a
nd 0.34) has shown that the magnetic moments originally along the [100
] (or [010]) axis turn toward the [110] direction perpendicular to the
magnetic field applied along [1-10]. The very small in-plane anisotro
py is evaluated to Delta(xy) = 5 x 10(-5) meV. (C) 1998 Elsevier Scien
ce B.V.