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Nematic phase and phase separation near saturation field in frustrated ferromagnets

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.