We performed inelastic neutron-scattering experiments with high energy resolution, in order to investigate the critical spin dynamics in the two-dimensional (2D) Ising antiferromagnet, Rb2CocMg1-cF4, with c = 0.6, 0.65, 0.7 and 1, above the percolation threshold cp (cp = 0.593 for a square lattice). First, we have confirmed the dynamical scaling for the homogeneous system (c = 1), and the observed dynamical critical exponent (z = 2.01 ± 0.05) was very close to that obtained from recent numerical studies. Next, the observed spin dynamics in the dilute systems could be described by the temperature dependence expected by the standard scaling theory, the power of the reduced temperature, however, the values of the exponents deviate from those predicted by the 2D Ising model and increase as c approaches to cp from the homogenous limit. Also, the observed spin dynamics for c close cp was in a quantitatively good agreement with that predicted by the model for critical spin dynamics in a percolating Ising system, in which the nature of the spin dynamics is determined by the requirement to overcome, via thermal excitation, energy barriers to the overturning of the spins in domains resulting from the inhomogeneous geometry of the percolating network. Although we could not directly detect the difference in the functional form between that expected by the standard scaling theory and that predicted by the percolation model in the present experimental condition, we conclude, from the quantitative agreement, that the cirtical spin dynamics in Rb2CocMg1-cF4 with c close to and just above cp is well described by the percolation model.
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