Colossal Magnetoresistance Without Mn3+/Mn4+ Double Exchange in the Stoichiometric Pyrochlore Tl_2Mn_2O_7
Abstract
Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl_2Mn_2O_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_2Mn_2O_7 indicate oxidation states very close to Tl3+_2Mn4+_2O_7. Thus, Tl_2Mn_2O_7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn3+, both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl_2Mn_2O_7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.
- Publication:
-
Science
- Pub Date:
- July 1996
- DOI:
- 10.1126/science.273.5271.81
- Bibcode:
- 1996Sci...273...81S