SPIN DYNAMICS IN 2-DIMENSIONAL PERCOLATING HEISENBERG ANTIFERROMAGNETS

Inelastic neutron-scattering experiments were performed on two-dimensi onal Heisenberg antiferromagnets, Rb2MncMg1-cF4, with the magnetic con centrations (c = 0.598 and 0.516) just above and below the percolation threshold (c(p) = 0.593 for the square lattice). Also, the magnetic r esponse (chi '' (q, E)) was investigated over a wide temperature range . We observed an overdamped chi '' (q, E), which was well fitted to a damped harmonic-oscillator scattering function. At low temperatures, c hi '' (q, E) is dispersive and can be described by the predicted funct ional form of fracton excitations. As the temperature increases, the q -dependence gradually becomes weaker, and chi '' (q, E) almost becomes independent of q at temperatures higher than 150 K. The incoherent ch aracter of chi '' (q, E) at high temperatures indicates that collectiv e motions do not exist due to very small thermal correlation lengths. Fracton excitations exist only at low temperatures.