SPIN-WAVES AND STABILITY OF MAGNETIC ORDER IN FRUSTRATED MAGNETS

We study the quantum renormalization of the spin-wave spectrum and of the magnetization in the 2d antiferromagnetic J1-J2 model in the large-S limit. We compare linear spin-wave, first-order spin-wave and Schwinger boson mean-field theories and show that the last two agree to order 1/S for any J2/J1, whereas linear spin-wave theory becomes qualitatively wrong in the region of strong frustration (J2/J1 almost-equal-to 1/2). The main consequence, namely the possibility of stabilizing long-range order by quantum fluctuations, is emphasized in a related example, the ferromagnetic J1-J2-J3 model. The compatibility of these results with numerical studies of the S = 1/2 case is discussed.