Effective field theory and the scattering process for magnons in the ferromagnet, antiferromagnet, and ferrimagnet

TL;DR

This paper develops a unified effective field theory framework for magnons in ferromagnets, antiferromagnets, and ferrimagnets, analyzing their particle states, scattering processes, and effects of symmetry breaking.

Contribution

It introduces a low-energy effective Lagrangian applicable to all three spin systems, clarifies the nature of gapped and gapless modes, and calculates scattering amplitudes and lengths, highlighting the unique gapped mode in ferrimagnets.

Findings

Gapped modes appear only in ferrimagnets.

Scattering length of gapped mode is finite and proportional to the gap.

External magnetic fields do not affect scattering amplitudes, but anisotropy does.

Abstract

We discuss that a low-energy effective Lagrangian relying on SO(3) $\rightarrow$ SO(2) is applicable for a ferrimagnet as well as a ferromagnet and an antiferromagnet. The analysis of the particle states shows that there exist not only massless modes with the dispersion relations $\omega \propto |\bm{k}|,\, |\bm{k}|^2$, i.e., the so-called type-I and type-II Nambu-Goldstone modes, respectively, but also gapped modes with $\omega \propto m^2+|\bm{k}|^2$. We clarify how the coefficients of the terms with one time derivative and those with two time derivatives in the effective Lagrangian determine the order parameters specifying whether the system is in a ferromagnetic, antiferromagnetic or ferrimagnetic state; we stress that the gapped mode related to the spontaneous symmetry breaking appears only in the ferrimagnetic system and not in the ferromagnetic and antiferromagnetic systems. We also establish the power counting scheme and calculate the scattering amplitudes and thereby the scattering lengths between the two Nambu-Goldstone bosons. We show that the scattering length of the gapped mode is finite and proportional to the gap. This characteristic property of the gapped NG mode can be used to discriminate it from gapped excitations which originate in other mechanisms. Finally, we study the effects of the explicit symmetry breaking that are given by an external magnetic field and a single-ion anisotropy, and show that the external magnetic fields do not have any effects on the scattering amplitudes in all the spin systems as was known for the ferromagnet system. In contrast, the anisotropy does affect the scattering amplitudes, the phase shift, and the scattering length except for spin 1/2 systems. This result supports the possibility of the Efimov effect in spin systems discussed in previous studies.