2-DIMENSIONAL FRUSTRATED HEISENBERG-MODEL - VARIATIONAL STUDY

The stability of the ferromagnetic phase of the two-dimensional (2D) q uantum spin-1/2 model with nearest-neighbor ferro- and next-nearest ne ighbor antiferromagnetic interactions is studied. It turns out that va lues of exchange integrals at which the ferromagnetic state becomes un stable with respect to a creation of one and two magnons are different . This difference shows that the classical approximation is inapplicab le to the study of the transition from the ferromagnetic to the single t state in contrast with 1D case. This problem is investigated using a variational function. It is based on the boson representation of spin operators which is different from the Holstein-Primakoff approximatio n. This allows us to obtain an accurate estimate of the transition poi nt and to study the character of the phase transition.