Spin dynamics in a 3D Heisenberg ferromagnet. V. Finite fields and temperatures

Abstract

For pt.IV see ibid., vol.15, p.6411 (1982). An extension of the Resibois-De Leener model (1966) for spin correlation functions which includes the influence of external magnetic fields and finite temperatures is introduced. It is suggested here that, apart from the exchange constants Jij, the important parameter in this description is still the magnetisation rho (B, T). With this model, using the earlier experimentally determined spin-diffusion constant, a satisfactory description is obtained for experimental data on spectral densities of the spin autocorrelation functions in the paramagnetic state far from any spontaneous ordering effects. At low temperatures (large values of p) a satisfactory description of the experimental spectral densities of parallel autocorrelation functions is found within the spin-wave model of Vaks and co-workers (1968). Furthermore, the origin for the transverse nuclear spin relaxation time T2 of 1H in Rb2CuBr4.2H2O is also discussed. At low temperatures/high fields the dipolar interaction between like proton spins in the dominating interaction determining T2(1H). At high temperatures/low fields the influence of electron spin fluctuations is seen on the transverse relaxation times as well as the nuclear dipolar interaction.