  Spatial and time scales of spin density fluctuations (SDF) were analyzed in
3d ferromagnets using ab initio linear response calculations of complete
wavevector and energy dependence of the dynamic spin susceptibility tensor. We
demonstrate that SDF are spread continuously over the entire Brillouin zone and
while majority of them reside within the 3d bandwidth, a significant amount
comes from much higher energies. A validity of the adiabatic approximation in
spin dynamics is discussed. The SDF spectrum is shown to have two main
constituents: a minor low-energy spin wave contribution and a much larger
high-energy component from more localized excitations. Using the
fluctuation-dissipation theorem (FDT), the on-site spin correlator (SC) and the
related effective fluctuating moment were properly evaluated and their
universal dependence on the 3d band population is further discussed.

Materials Science