We demonstrate that long-range interaction in a system can lead to a very strong interaction between long-wavelength quasiparticles and make them heavily damped. In particular, we discuss magnon spectrum using $1/S$ expansion in three-dimensional Heisenberg ferromagnet (FM) with arbitrary small dipolar forces at $T⪡T_C$. We obtain that a fraction of long-wavelength magnons with energies ${ϵ}_{\mathbf{k}}<T$ has anomalously large damping ${\Gamma }_{\mathbf{k}}$ (${\Gamma }_{\mathbf{k}}/{ϵ}_{\mathbf{k}}$ reaches 0.3 for certain $\mathbf{k}$). This effect is observed both in quantum and classical FMs. Remarkably, this result contradicts expectation of the quasiparticle concept according to which a weakly excited state of a many-body system can be represented as a collection of weakly interacting elementary excitations. Particular materials are pointed out which are suitable for corresponding experiments.

• Received 7 July 2010

DOI:https://doi.org/10.1103/PhysRevB.82.024432