  We compute the Gaussian-fluctuation corrections to the saddle-point
Schwinger-boson results using collective coordinate methods. Concrete
application to investigate the frustrated J1-J2 antiferromagnet on the square
lattice shows that, unlike the saddle-point predictions, there is a quantum
nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by
considering the corrections to the spin stiffness on large lattices and
extrapolating to the thermodynamic limit, which avoids the infinite-lattice
infrared divergencies associated to Bose condensation. The very good agreement
of our results with exact numerical values on finite clusters lends support to
the calculational scheme employed.
