The Neel and collinear order parameters of the/r/2 model are calculated by the quantum Monte Carlo simulation with the reweighting method. Systems of sizes AT=4 x 4, 4 x 8 and 8 x 8 are treated. The thermodynamic quantities obtained by the size extrapolation suggest that the Neel order remains finite for a 0<0.4 and vanishes for a 0>0.6, where a is defined by J2Uu and that the collinear order remains for ce0>0.7 and vanishes for a 0<0.6. The present result is consistent with the result of the modified spin-wave calculation, conflicting with that of the naive spin-wave theory. JrJ2 model, quantum Monte Carlo method, reweighting method.
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