Spin Dynamics of the Magnetoresistive Pyrochlore Tl_2Mn_2O_7

TL;DR

This study uses neutron scattering to analyze the magnetic order and spin dynamics in the magnetoresistive pyrochlore Tl_2Mn_2O_7, revealing distinct behavior from similar manganite systems and suggesting a different underlying mechanism for magnetoresistance.

Contribution

First detailed neutron scattering analysis of spin dynamics in Tl_2Mn_2O_7, highlighting differences from manganite systems and implications for magnetoresistance mechanisms.

Findings

Magnetic correlations develop and diverge at T_C (123 K).

Spin waves are gapless and follow a Dq^2 dispersion in the ferromagnetic phase.

No significant diffusive component observed near T_C, contrasting with manganite systems.

Abstract

Neutron scattering has been used to study the magnetic order and spin dynamics of the colossal magnetoresistive pyrochlore Tl_2Mn_2O_7. On cooling from the paramagnetic state, magnetic correlations develop and appear to diverge at T_C (123 K). In the ferromagnetic phase well defined spin waves are observed, with a gapless ($\Delta <0.04$ meV) dispersion relation E=Dq^{2} as expected for an ideal isotropic ferromagnet. As T approaches T_C from low T, the spin waves renormalize, but no significant central diffusive component to the fluctuation spectrum is observed in stark contrast to the La$_{1-x}$(Ca,Ba,Sr)$_x$MnO$_3$ system. These results argue strongly that the mechanism responsible for the magnetoresistive effect has a different origin in these two classes of materials.