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dc.contributor.authorHou, Zhuofei
dc.description.abstractMonte Carlo and spin dynamics techniques have been used to perform large-scale simulations of the dynamic behavior of a nanoscale, classical, Heisenberg antiferromagnet on a simple cubic lattice. Systems are of size $Ltimes Ltimes D$ with linear sizes $Lleqslant40$ and $Dleqslant40$ at a temperature below the N'{e}el temperature. Nanoparticles are modeled with completely free boundary conditions, i.e., six free surfaces, and nanofilms are modeled with two free-surfaces in the spatial z-direction and periodic boundaries parallel to the surfaces in the x-,y-directions. Results are compared to those for the textquotedblleft infinitetextquotedblright system with fully periodic boundary conditions. The temporal evolutions of spin configurations were determined numerically from coupled equations of motion for individual spins using a fast spin dynamics algorithm based on the fourth-order Suzuki-Trotter decomposition of exponential operators, with initial spin configurations generated by Monte Carlo simulations. The local dynamic structure factor $S(mathbf{r}_{0},mathbf{q},omega)$ was calculated from the local space- and time-displaced spin-spin correlation function, where $mathbf{r}_{0}$ denotes the starting point from which the correlation function is calculated. Multiple excitation peaks for wave vectors within the first Brillouin zone appear in the spin-wave spectra for the transverse component of the dynamic structure factor $S^T(mathbf{r}_{0},mathbf{q},omega)$ in the classical Heisenberg antiferromagnetic nanofilms and nanoparticles, which are lacking if periodic boundary conditions are used. With the assumption of $q$-space spin-wave reflection with broken momentum conservation due to free-surface confinements, we successfully explained the locations of those excitations quantitatively in the linear dispersion region. Meanwhile, we also observed two novel quantized spin-wave excitation modes in the spatial z-direction in nanofilms for $S^T(mathbf{r}_{0},mathbf{q},omega)$. Results of this study indicate the presence of new forms of spin-wave excitation behavior which have yet to be observed experimentally but could be directly tested through neutron scattering experiments on nanoscale RbMnF$_3$ particles or films.
dc.subjectspin waves
dc.subjectcorrelation function
dc.subjectdynamic structure factor
dc.subjectneutron scattering experiment
dc.subjectdispersion relationship
dc.subjectHybrid Monte Carlo
dc.subjectspin dynamics
dc.subjectSuzuki-Trotter decomposition
dc.subjectclassical Heisenberg model
dc.subjectcompletely free boundary conditions
dc.subjectpartially free boundary conditions
dc.titleSpin dynamics simulation studies of nanoscale classical heisenberg antiferromagnets
dc.description.departmentPhysics and Astronomy
dc.description.advisorDavid P. Landau
dc.description.committeeDavid P. Landau
dc.description.committeeShan-Ho Tsai
dc.description.committeeBernd Schuttler
dc.description.committeeWilliam Dennis

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