Low-frequency spin dynamics of a 3D Heisenberg ferromagnet in the critical region

Nuclear spin-lattice relaxation rates T₁^-1 have been measured on ¹H and ⁷⁹Br(I) nuclei in Rb ₂CuBr ₄ · 2H ₂O, a weakly anisotropic 3D S = {1}/{2} Heisenberg ferromagnet with Tc = 1.874 K, in the critical region in zero field. A good quantitative description of the experimental results has been obtained with a semi-phenomenological model for the frequency dependent spin correlation functions in the hydrodynamic (spin wave) regime. The relaxation rates are expressed in the experimentally known static susceptibilities. The exchange anisotropy and the non-zero nuclear Larmor frequency have been taken into account, and appear to determine strongly the divergent behavior of the relaxation rates near T_c.