SQUARE-LATTICE FRUSTRATED HEISENBERG-ANTIFERROMAGNET OF SPIN 1 2 IN ASTRONG MAGNETIC-FIELD/

The properties of the two-dimensional antiferromagnet of spin 1/2 on a square lattice with nearest- and next-nearest-neighbors interactions in a strong magnetic field close to saturation are studied in terms of the equivalent Bose-gas problem. A phase with a gap in the elementary excitations spectrum, short-range-ordered ground state, and a corresp onding plateau in the magnetization curve is shown to exist in the int erval of fields between the first and second critical fields h0 < h < h01. Its existence is closely related to the two-particle attraction i n the vicinity of the wave vector p0u over arrow pointing right = (pi, pi). In the interval between the second and third critical fields h01 < h < h(cf) the system behaves as a quasi-one-dimensional magnetic ph ase with long-range order in the ground state and a gapless mode. The magnetization depends on field linearly, with logarithmic corrections.