The dynamics of longitudinal spin fluctuations

The dynamical processes underlying the longitudinal spin fluctuation spectra in Heisenberg systems are investigated in two distinct physical regimes. Very recent polarized-neutron data in the one-dimensional ferromagnet CsNiF3 have supported our previous prediction of a regime in which longitudinal spectra are dominated by a coupling between spin and exchange energy densities. A quantum-mechanical analysis is found to give results in good quantitative agreement with the experimental line shapes. The second regime investigated deals with much smaller (and less easily accessible) wave vectors. Here, a new hydrodynamic mode ("second magnon") may in principle be supported at not too high temperatures. The observability of this feature in real systems (in particular, EuO) is examined.