Magnetic phases of the mixed-spin J1-J2 Heisenberg model on a square lattice -: art. no. 014421

We study the zero-temperature phase diagram and the low-energy excitations of a mixed-spin (S-1>S-2) J(1)-J(2) Heisenberg model defined on a square lattice by using a spin-wave analysis, the coupled cluster method, and the Lanczos exact-diagonalization technique. As a function of the frustration parameter J(2)/J(1) (>0), the phase diagram exhibits a quantized ferrimagnetic phase, a canted spin phase, and a mixed-spin collinear phase. The presented results point towards a strong disordering effect of the frustration and quantum spin fluctuations in the vicinity of the classical spin-flop transition. In the extreme quantum system (S-1, S-2)=(1, 1/2), we find indications of a different quantum spin state in the region 0.46<J(2)/J(1)<0.5.