SINGLE VALENCE AND STRUCTURAL ORDER IN THE COLOSSAL-MAGNETORESISTANT PYROCHLORE TL2MN2O7

Without Mn3+/Mn4+ mixed-valence or Jahn-Teller distortions in the MnO6 octahedra, the origin of colossal magnetoresistance (CMR) in the pyro chlore Tl2Mn2O7 is thought to arise instead from superexchange with la rge incoherent scattering near the ferromagnetic transition. Using x-r ay-absorption spectroscopy, we have shown that the electronic state of T1 is nearly identical to that in Tl2O3, indicating doping into the T 1 6s bands, that there is no appreciable Mn mixed valence, and that th e MnO6 octahedra are well ordered. This further supports the proposed mechanism for CMR in Tl2Mn2O7 based on our earlier structural studies.