Probing magnon dynamics and interactions in a ferromagnetic spin-1 chain

Abstract

NiNb${}_2$O${}_6$ is an almost ideal realization of a 1D spin-1 ferromagnetic Heisenberg chain compound with weak unidirectional anisotropy. Using time-domain THz spectroscopy, we measure the low-energy electrodynamic response of NiNb${}_2$O${}_6$ as a function of temperature and external magnetic field. At low temperatures, we find a magnon-like spin-excitation, which corresponds to the lowest energy excitation at $q\sim 0$. At higher temperatures, we unexpectedly observe a temperature-dependent renormalization of the spin-excitation energy, which has a strong dependence on field direction. Using theoretical arguments, exact diagonalizations and finite temperature dynamical Lanczos calculations, we construct a picture of magnon-magnon interactions that naturally explains the observed renormalization. This unique scenario is a consequence of the spin-1 nature and has no analog in the more widely studied spin-1/2 systems.

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