Spin Waves throughout the Brillouin Zone of a Double-Exchange Ferromagnet

Abstract

We use inelastic neutron scattering to measure the spin wave dispersion throughout the Brillouin zone of the double-exchange ferromagnet L${\mathrm{a}}_{0.7}$P${\mathrm{b}}_{0.3}$Mn${\mathrm{O}}_{3}$. Magnons with energies as high as 95 meV are directly observed and an unexpectedly simple Heisenberg Hamiltonian, with solely a nearest-neighbor coupling of $8.79\ifmmode\pm\else\textpm\fi{}0.21\mathrm{meV}$, accounts for the entire dispersion relation. The calculated Curie temperature for this local moment Hamiltonian overestimates the measured Curie point (355 K) by only 15%. Raising temperature yields unusual broadening of the high frequency spin waves, even within the ferromagnetic phase.