  We study the zero-temperature phase diagram of the frustrated square-lattice
S=1/2 antiferromagnet in an external magnetic field numerically with the
Lanczos algorithm. For strong frustration, we find disordered phases at high
(and low) magnetic fields. Between these two disordered phases we find a
plateau in the magnetization curve at half of the saturation magnetization
which corresponds to a state with up-up-up-down (uuud) spin order. This and
other considerations [cond-mat/0003343] suggest an unusual ordering scenario:
There are an ordered phase with a spin gap (the plateau) and disordered
magnetically gapless phases above and below.
  The transition to saturation is studied in further detail and problematic
conclusions in earlier investigations of this region are pointed out.
