Spatial and time scales of spin-density fluctuations (SDFs) were analyzed in 3d ferromagnets using ab initio linear-response calculations of complete wave-vector and energy dependence of the dynamic spin susceptibility tensor. We demonstrate that SDFs are spread continuously over the entire Brillouin zone and while the 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, the on-site spin correlator and the related effective fluctuating moment were properly evaluated and their universal dependence on the 3d band population is further discussed.
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This work was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy (DOE). V.P.A. acknowledges support from the Office of Basic Energy Science, Division of Materials Science and Engineering. V.N.V. and A.I.L. acknowledge support from the Hamburg Centre for Ultrafast Imaging (CUI). The research was partially performed at Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under Contract No. DE-AC02-07CH11358.
© 2017 American Physical Society.