Hydrodynamics of easy-axis antiferromagnets in a magnetic field: Tricritical behavior

Abstract

A linear hydrodynamic theory for the spin dynamics of easy-axis antiferromagnets in an applied magnetic field is outlined, with particular emphasis on the behavior along the second-order line including the region near a tricritical point. Expressions are obtained for the $\omega$,$q$-dependent spin susceptibilities. In the paramamagnetic phase, the dynamic staggered susceptibility is characterized by a single peak at $\omega =0\mathrm{}$. In the antiferromagnetic phase, at long wavelengths, a three-peak structure is found: two narrow peaks centered at $\omega =0\mathrm{}$ with widths proportional to $q^2$, which are associated with the coupled magnetization-energy fluctuations and a broad peak, also at $\omega =0\mathrm{}$, which characterizes the adiabatic, isomagnetic decay of the staggered moment. The relative weight of the broad peak is equal to the ratio of the adiabatic staggered susceptibility at constant magnetization to the isothermal staggered susceptibility at constant field. The coupling of the magnetization-energy fluctuations also affects the frequency dependence of the uniform field susceptibility. The temperature dependence of various parameters in the theory is discussed in light of static scaling laws and a brief comparison is made with the hydrodynamics of antiferromagnets in and near a spin-flop phase.