Two-dimensional planar ferromagnetic coupling in LaMnO3

Neutron-scattering experiments have been performed on a LaMnO3 single crystal. The spin waves exhibit two-dimensional anisotropic dispersion, that is, strong planar ferromagnetic coupling and weak antiferromagnetic interplane coupling with a finite gap. The critical exponent beta of the order parameter is 0.25 +/- 0.01, which is much smaller than values obtained for the three-dimensional Ising (0.326) and Heisenberg (0.367) models. Ferromagnetic spin-wave-like dispersion remains for omega > 10 meV at T-c approximate to 140 K, though the spectrum near the zone center is entirely overdamped around omega = 0.