  We study the alpha = J_2/J_1-dependence of the magnetization process in the
J_1-J_2 model on a square lattice with frustrating couplings J_2 along the
diagonals. Perturbation expansions around alpha=J_2/J_1=0 and 1/alpha=0$ yield
an adequate description of the magnetization curve in the antiferromagnetic and
collinear antiferromagnetic phase, respectively. The transition from one phase
to the other (0.5 < alpha < 0.7) leaves pronounced structures in the
longitudinal and transverse structure factors at p=(pi,pi) and p=(0,pi).
