Colossal Magnetoresistance Without Mn ³⁺ /Mn ⁴⁺ Double Exchange in the Stoichiometric Pyrochlore Tl ₂ Mn ₂ O ₇

TLDR

The authors aim to propose an alternate mechanism for colossal magnetoresistance in Tl₂Mn₂O₇ based on magnetic ordering driven by superexchange and strong spin‑fluctuation scattering above the Curie temperature. They suggest that magnetic ordering via superexchange and spin‑fluctuation scattering above the Curie temperature underlies CMR in this stoichiometric pyrochlore. Structural analysis shows Tl₂Mn₂O₇ is perfectly stoichiometric with Mn⁴⁺, lacking mixed valence or Jahn‑Teller Mn³⁺, indicating CMR occurs without the conventional Mn³⁺/Mn⁴⁺ double‑exchange mechanism.

Abstract

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl 2 Mn 2 O 7 pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstroms) observed in phases based on LaMnO 3 perovskites that exhibit CMR. Both results in Tl 2 Mn 2 O 7 indicate oxidation states very close to Tl 3+ 2 Mn 4+ 2 O 7 . Thus, Tl 2 Mn 2 O 7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn 3+ , both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl 2 Mn 2 O 7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.