  We study the spin-wave spectrum and the spin-flop transitions in La_2CuO_4 in
a uniform magnetic field at zero temperature. Using the non-linear sigma-model,
we show that a field applied along the orthorhombic b direction leads to a
two-step rotation of the staggered magnetization, first in the bc and then in
the ac plane, until the order parameter is completely aligned along the c axis.
In contrast, for a perpendicular magnetic field, we find a conventional
spin-flop transition induced by the competition between the field and the
interlayer coupling. A comparison with recent measurements of the
field-dependence of the in-plane spin-wave gap shows a beautiful agreement
between theory and experiments.
