Search for the breakdown of the spin-wave approximation in EuO (abstract)

Abstract

The spin-wave approximation in a Heisenberg ferromagnet is valid at low temperatures where the spin-wave interactions are negligible. At higher temperatures these interactions become highly nonlinear and the spin-wave approximation should break down. In a recent paper Fishman and Vignale1 predicted that this breakdown should occur near the temperature T̄=0.4zJs, where z is the number of nearest neighbors, J is the ferromagnetic coupling constant, and s is the spin of the ferromagnet. In another related paper Fishman and Liu2 have calculated that above this temperature T̄ a second pole appears in the correlation function at an energy close to 2zJs. The origin of this new mode is the strong coupling of transverse and longitudinal fluctuations that forces the spins to precess about their local mean field. When the ‘‘spin-wave’’ energies are close to the precessional mode energy 2zJs a splitting of the ‘‘spin-wave’’ branch is predicted. In order to test this prediction we have performed a high-resolution inelastic neutron scattering experiment to measure the spin waves in EuO, a prototypical Heisenberg ferromagnet, in the vicinity of the q-E region where this splitting should occur. Although our experiment was performed at T=20 K, which is above T̄≊12 K, our measurements failed to observe the predicted splitting of the ‘‘spin-wave’’ branch. Only a single resolution-limited spin-wave branch was observed, in agreement with earlier measurements by Passell et al.3