Spin dynamics of strongly-doped La_{1-x}Sr_xMnO_3

Abstract

Cold neutron triple-axis measurements have been used to investigate the nature of the long-wavelength spin dynamics in strongly-doped La${}_{1-x}$Sr${}_x$MnO${}_3$ single crystals with $x$=0.2 and 0.3. Both systems behave like isotropic ferromagnets at low T, with a gapless ($E_0<0.02$ meV) quadratic dispersion relation $E=E_0+D{q}^2$. The values of the spin-wave stiffness constant $D$ are large ($D_{T=0}$ = 166.77 meV${\mathring{A}}^2$ for $x$=0.2 and D${}_{T=0}$ = 175.87 meV${\mathring{A}}^2$ for $x$=0.3), which directly shows that the electron transfer energy for the $d$ band is large. $D$ exhibits a power law behavior as a function of temperature, and appears to collapse as T -> T_C. Nevertheless, an anomalously strong quasielastic central component develops and dominates the fluctuation spectrum as T -> T_C. Bragg scattering indicates that the magnetization near $T_C$ exhibits power law behavior, with $\beta \simeq 0.30$ for both systems, as expected for a three-dimensional ferromagnet.

4 pages (RevTex), 3 figures (encapsulated postscript)