SPIN DYNAMICS SIMULATIONS OF COLLECTIVE EXCITATIONS
IN MAGNETIC LIQUIDS
Author(s): I.P.Omelyan (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Str., 79011 Lviv, Ukraine), I.M.Mryglod (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Str., 79011 Lviv, Ukraine; Institute for Theoretical Physics, University of Linz, A-4040 Linz, Austria), R.Folk (Institute for Theoretical Physics, University of Linz, A-4040 Linz, Austria)
A novel approach is developed for computer simulation studies of dynamical properties of spin liquids. It is based on the Liouville operator formalism of Hamiltonian dynamics in conjunction with Suzuki-Trotter-like decompositions of exponential propagators. As a result, a whole set of symplectic time-reversible algorithms has been introduced for numerical integration of the equations of motion at the presence of both translational and spin degrees of freedom. It is shown that these algorithms can be used in actual simulations with much larger time steps than those inherent in standard predictor-corrector schemes. This has allowed one to perform direct quantitative measurements for spin-spin, spin-density and density-density dynamical structure factors of a Heisenberg ferrofluid model for the first time. It was established that like pure liquids the density spectrum can be expressed in terms of heat and sound modes, whereas like spin lattices in the ferromagnetic phase there exists one primary spin in the shape of spin-spin dynamic structure factors describing the longitudinal and transverse spin fluctuations. As it was predicted in our previous paper [Mryglod I., Folk R. et al., Physica A277 (2000) 389] we found also that a secondary wave peak appears additionally in the longitudinal spin-spin dynamic structure factor. The frequency position of this peak coincides entirely with that for a sound mode reflecting the effect of the liquid subsystem on spin dynamics. The possibility of longitudinal spin wave propagation in magnetic liquids at sound frequency can be considered as a new effect which has yet to be tested experimentally.
Comments: Figs. 6, Refs. 29, Tabs. 0.
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