Magnons in CMR pyrochlore Tl2Mn2O7

TL;DR

This paper presents a theoretical calculation of magnon dispersion in Tl2Mn2O7, a pyrochlore compound with colossal magnetoresistance, showing good agreement with experimental spin wave observations and highlighting the effects of conduction electron coupling.

Contribution

The authors develop a perturbative approach to derive the effective spin-wave Hamiltonian for Tl2Mn2O7, incorporating conduction electron interactions, which aligns with experimental neutron scattering data.

Findings

Calculated magnon dispersion matches experimental results

Coupling with conduction electrons renormalizes ferromagnetic spin waves

Agreement with neutron scattering measurements

Abstract

Well defined spin waves were observed when the spin dynamics of Tl2Mn2O7, the first pyrochlore compound found to exhibit colossal magnetoresistance, was measured [J.W.Lynn et al., Phys.Rev.Lett. 80,4582(1998)], in stark contrast with the experimental results on the larger family of magnetoresistive manganites with perovskite structure. In this work, we present our calculation for the spin waves in Tl2Mn2O7, which we described using the microscopic generic model proposed recently for this compound [C.I.Ventura and M.A.Gusmao, Phys.Rev.B 65, 14422(2002)]. We have employed a canonical transformation to determine perturbatively the effective spin-wave Hamiltonian, obtaining therefrom the renormalization of the ferromagnetic spin waves related to the localized Mn$^{4+}$ spins, due to their coupling with the conduction electrons present. We have calculated the magnon dispersion relations along different paths in the first Brillouin zone, comparing them with those which are obtained for an ideal isotropic ferromagnet. This comparison evidences an agreement between the ferromagnetic magnons obtained from the generic model and the bare spin waves, such as had been found in neutron scattering experiments.