Spin dynamics in a 3D Heisenberg ferromagnet in the paramagnetic state II

In this paper experimental results on electron spin dynamics in a 3D Heisenberg ferromagnet are presented. These experimental results, the spectral densities phi 0 + ( omega ), phi 0 z ( omega ) and phi 01 + ( omega ) of the spin auto- and pair correlation functions, have been measured in Rb 2 CuBr 4 .2H 2 O at relatively high temperatures (T=20K, 77K, so that kT>>JS 2 +g mu B B). Comparison of these results with some current theoretical models on spin correlations shows a good high-frequency/short-time agreement. The low-frequency part/diffusion region, shows a remarkable discrepancy, which can be explained by a too high theoretical diffusion constant. The authors' experimental diffusion constant is determined to be D exp =0.31+or-0.03 J 1 a 2 /h(cross).