Interpretation of neutron scattering and μSR data on paramagnetic Gd in terms of magnetic clusters and coherent dissipative structures

Some qualitative considerations of spin correlation effects in rare earth systems above the Curie temperature Tc are given. In particular we refer to recent measurements of the magnetic excitation spectra of Gd by inelastic neutron scattering (Cable and Nicklow) displaying persistent spin correlations at temperatures up to more than T=2.5Tc, and to recent μSR data. Neutron data show that the cross-over wave vector qc at which the system changes from spin diffusion to spin wave behaviour and the corresponding damping parameter for βq,c are only slowly varying with temperature in the range from T=320 to T=850 K (Tc=293 K). A quantum correlations theory, based on the fermionic density matrix for the spin system in connection with complex scaling and thermalisation, has been applied to this experimental context. Our theory reveals that qc and βq,c vary in such a way that the spatial extension of the quantum correlations is independent of temperature in this range. We will demonstrate here that the concept of domain wall motion of short-lived magnetic clusters in conjunction with the recent theory of quantum correlations can give further insight into the complicated picture of high temperature spin fluctuation in condensed matter. (orig.)