Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets: I. Theory

TL;DR

This theoretical study analyzes how uniform magnetic fields influence the magnetic spectrum of anisotropic and Dzyaloshinskii-Moriya layered quantum Heisenberg antiferromagnets, predicting field-dependent magnon gaps and spin-flop transitions.

Contribution

It develops a comprehensive field-theory approach to describe magnetic field effects on magnon spectra in layered antiferromagnets, linking low-energy spectrum models with spin-wave theory.

Findings

Magnetic field dependence of magnon gaps and spectral intensities calculated.

Identification of various ground states and their temperature evolution.

Prediction of spin-flop transitions for different field orientations.

Abstract

We consider theoretically the effects of an applied uniform magnetic field on the magnetic spectrum of anisotropic two-dimensional and Dzyaloshinskii-Moriya layered quantum Heisenberg antiferromagnets. The first case is relevant for systems such as the two-dimensional square lattice antiferromagnet Sr(2)CuO(2)Cl(2), while the later is known to be relevant to the physics of the layered orthorhombic antiferromagnet La(2)CuO(4). We first establish the correspondence betwenn the low-energy spectrum obtained within the anisotropic non-linear sigma model and by means of the spin-wave approximation for a standard easy-axis antiferromagent. Then, we focus on the field-theory approach to calculate the magnetic field dependence of the magnon gaps and spectral intensities for magnetic fields applied along the three possible crystallographic directions. We discuss the various possible ground states and their evolution with temperature for the different field orientations, and the occurrence of spin-flop transitions for fields perpendicular to the layers (transverse fields) as well as for fields along the easy axis (longitudinal fields). Measurements of the one-magnon Raman spectrum in Sr(2)CuO(2)Cl(2) and La(2)CuO(4) and a comparison between the experimental results and the predictions of the present theory will be reported in part II of this research work [L. Benfatto et al., cond-mat/0602664].