Breakdown of long-wavelength magnons in cubic antiferromagnets with dipolar forces at small temperature

TL;DR

This paper analyzes how dipolar forces affect the magnon spectrum in cubic antiferromagnets at low temperatures, revealing a gap, anisotropic effects, and conditions under which long-wavelength magnons may become overdamped, challenging quasiparticle assumptions.

Contribution

It provides a detailed theoretical study of dipolar effects on magnon spectra in cubic antiferromagnets, including gap formation, spectrum splitting, and damping mechanisms, with implications for experimental observations.

Findings

Dipolar forces induce a gap in the magnon spectrum.

Magnon spectrum splits into two branches due to dipolar interactions.

Long-wavelength magnons can become overdamped under certain conditions.

Abstract

Using $1/S$ expansion, we discuss the magnon spectrum of Heisenberg antiferromagnet (AF) on a simple cubic lattice with small dipolar interaction at small temperature $T\ll T_N$, where $T_N$ is the Neel temperature. Similar to 3D and 2D ferromagnets, quantum and thermal fluctuations renormalize greatly the bare gapless spectrum leading to a gap $\Delta\sim \omega_0$, where $\omega_0$ is the characteristic dipolar energy. This gap is accompanied by anisotropic corrections to the free energy which make the cube edges easy directions for the staggered magnetization (dipolar anisotropy). In accordance with previous results, we find that dipolar forces split the magnon spectrum into two branches. This splitting makes possible two types of processes which lead to a considerable enhance of the damping compared to the Heisenberg AF: a magnon decay into two other magnons and a confluence of two magnons. It is found that magnons are well defined quasiparticles in quantum AF. We demonstrate however that a small fraction of long-wavelength magnons can be overdamped in AFs with $S\gg1$ and in quantum AFs with a single-ion anisotropy competing with the dipolar anisotropy. Particular materials are pointed out which can be suitable for experimental observation of this long-wavelength magnons breakdown that contradicts expectation of the quasiparticle concept.