  We study the effects of quantum fluctuations in magnetic properties of
quantum frustrated ferromagnets in a magnetic field. It is shown that a
non-classical phase or a phase separation appears due to quantum fluctuations
below the saturation field in a parameter range close to the classical
zero-field phase boundary between ferromagnetic and antiferromagnetic phases,
for the case that the classical antiferromagnetic state is not an eigenstate of
the quantum model. As an example to which this argument is applicable, we study
the S=1/2 J1-J2 Heisenberg model with ferromagnetic J1 (J1<0) on the bcc
lattice using a dilute Bose gas approach. For -1.50097 < J1/J2 < -1.389,
magnons form f-wave two-magnon bound states, leading to a spin nematic phase,
and for -1.389 < J1/J2 < -0.48 a canted coplanar antiferromagnetic phase
appears accompanied with a phase separation below the saturation field.
