Quantum Effects in Uniform and Staggered Moment of Frustrated Quasi-2D Antiferromagnets

We investigate the frustrated two-dimensional S=1/2 next nearest neighbor Heisenberg antiferromagnet on a square lattice. This model is relevant for a variety of V oxides and Cu compounds and its anisotropic version for the Fe pnictides. We use spin-wave theory and exact diagonalization in a magnetic field for finite tiles with a new method for the finite size scaling procedure. The induced uniform and the staggered moment in the antiferromagnetically ordered phases are calculated. They deviate strongly from classical behavior depending on frustration ratio J (2)/J (1) and the exchange anisotropy. The former may be determined by comparison with experimental saturation fields. Applying a magnetic field up to one half of the saturation field stabilizes the staggered moment in the striped columnar (CAF) and N,el (NAF) antiferromagnetic phases, in particular close to the phase boundaries. The field dependence of the staggered moment uniquely determines the exchange parameters. This allows to derive the frustration ratio J (2)/J (1) also from the field dependence of neutron diffraction data.