Quantum ferrimagnets

An instability of a ferromagnetic state in quantum spin systems induced by a two-particle bound-state condensation is shown to produce, for S>2, a ferrimagnetic ground state with a quantized (S_Z=S-1) value of spontaneous magnetization per site. For low S, instead, the ground state below the instability has zero net magnetization, while a spontaneous symmetry breaking reveals itself in quadratic, cubic, or both quadratic and cubic correlations for S=1, 2 or 3/2, respectively. The role of quantum fluctuations and the reorientation process in a magnetic field are examined.