Three--body Correlation Effects on the Spin Dynamics of Double--Exchange Ferromagnets

TL;DR

This paper develops a variational approach to study spin wave excitations in double-exchange ferromagnets, revealing significant effects of three-body correlations on spin dynamics, phase stability, and dispersion, relevant to manganites and colossal magnetoresistance.

Contribution

It introduces an exact treatment of three-body correlations in spin dynamics, extending previous approximations and providing insights into ferromagnetic phase boundaries and spin wave behavior.

Findings

Three-body correlations significantly alter spin wave dispersion.

Ferromagnetic phase boundary is notably affected by correlations.

Spin wave softening and hardening depend on electron concentration.

Abstract

We present a variational calculation of the spin wave excitation spectrum of double--exchange ferromagnets in different dimensions. Our theory recovers the Random Phase approximation and 1/S expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three--body correlations between a Fermi sea electron--hole pair and a magnon excitation and show that they strongly affect the spin dynamics in the parameter range relevant to experiments in the manganites. The manifestations of these correlations are many-fold. We demonstrate that they significantly change the ferromagnetic phase boundary. In addition to a decrease in the magnon stiffness, we obtain an instability of the ferromagnetic state against spin wave excitations close to the Brillouin zone boundary.Within a range of intermediate concentrations, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness, which changes into hardening for other concentrations. We discuss the relevance of these results to experiments in colossal magnetoresistance ferromagnets.