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Colossal Magnetoresistance Without Mn3+/Mn4+ Double Exchange in the Stoichiometric Pyrochlore Tl2Mn2O7
1M. A. Subramanian and W. J. Marshall, Dupont Central Research and Development, Experimental Station, Wilmington, DE 19880-0328, USA. B. H. Toby, Reactor Radiation Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA. A. P. Ramirez, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974, USA. A. W. Sleight, Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA. G. H. Kwei, Chemistry and Materials Sciences Department, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
Abstract:
Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl2Mn2O7 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 LaMnO3 perovskites that exhibit CMR. Both results in Tl2Mn2O7 indicate oxidation states very close to Tl23+Mn24+O7. Thus, Tl2Mn2O7 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 Tl2Mn2O7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.