ISOTROPIC SPIN-WAVE THEORY OF SHORT-RANGE MAGNETIC ORDER

We present an isotropic spin-wave theory of short-range order in Heise nberg magnets, and apply it to square lattice S = 1/2 and S = 1 antife rromagnets. Our theory has three identical (isotropic) spin-wave modes , whereas the conventional spin-wave theory has two transverse modes a nd one longitudinal mode. We calculate the temperature dependences of various thermodynamic observables analytically and find good (several percent) agreement with independently obtained numerical results in a broad temperature range.