Spin-wave excitations and perpendicular susceptibility of a diluted antiferromagnet near percolation threshold

The long-wavelength excitations of a diluted antiferromagnet near the percolation threshold p c are studied. Within the hydrodynamic theory, the excitation frequency depends on two parameters, A and χ t . A is the stiffness associated with the spatial variation in the staggered magnetization and χ t is the perpendicular susceptibility in the ordered state of the antiferromagnet. The critical behavior of A near p c is known. We develop a field-theoretic formalism to calculate χ t . We ex- plicitly calculate χ t in the mean-field approximation and find that it diverges as ||ln(p-p c )||, as the concentration p approaches p c . Some further scaling arguments yield a scaling relation relating the divergence exponent of χ t with other known exponents at the percolation critical point.