Spin dynamics of aJ1−J2antiferromagnet and its implications for iron pnictides

Abstract

Motivated by the recent observation of antiferromagnetic correlations in the paramagnetic phase of iron pnictides, we study the finite-temperature spin dynamics of a two-dimensional ${J}_{1}\ensuremath{-}{J}_{2}$ antiferromagnet. We consider the paramagnetic phase in the regime of a $(\ensuremath{\pi},0)$ collinear ground state, using the modified spin wave theory. Below the mean-field Ising transition temperature, we identify short-range anisotropic antiferromagnetic correlations. We show that the dynamical structure factor $\mathcal{S}(\mathbf{q},\ensuremath{\omega})$ contains elliptic features in the momentum space, and determine its variation with temperature and energy. Implications for the spin-dynamical experiments in the iron pnictides are discussed.