J. Phys. IV France 07 (1997) C1-625-C1-628
Structure, Magnetism and Colossal Magnetoresistance Behavior in A2Mn2O7 Pyrochlores (A = Dy-Lu, Y, Sc, In or Tl)M.A. Subramanian1, J.E. Greedan2, N.P. Raju2, A.P. Ramirez3 and A.W. Sleight4
1 Dupont Central Research and Development, Experimental Station, Wilmington, DE 19880-0328, U.S.A.
2 Institute for Materials Research, McMaster University, Hamilton, L85 4M1, Canada
3 Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974, U.S.A.
4 Department of Chemistry, Oregon State University, Corvallis, OR 97331, U.S.A.
This paper presents a bird's eye view of the structural, electrical and magnetic properties of A2Mn2O7 (A = rare earth, Y, Sc, In, Tl) phases crystallizing in pyrochlore-related structure. Electrical measurements indicate the compounds containing rare earth, Y, Sc or In are insulators whereas Tl2Mn2O7 is highly conducting. All the compounds show spontaneous magnetization at low temperatures indicating ferromagnetic transitions. Detailed ac and dc susceptibility as well as neutron diffraction studies do not provide any evidence for long-range order in A2Mn2O7 phases where A is a rare earth, Y or Sc, but suggest a spin-glass like behavior. However, powder neutron diffraction and small-angle neutron scattering investigations substantiate the long-range magnetic ordering in Tl2Mn2O7, which is also now known to exhibit colossal magnetoresistance (CMR) behavior. Structural analysis of this phase shows no deviations from ideal stoichiometry and gives an Mn-O distance of ~1.90 Å, significantly shorter than the Mn-O distances in the range 1.94 to 2.00 Å observed in phases hased on LaMnO3 perovskites which show CMR. The above observation indicates that oxidation state of Mn in Tl2Mn2O7 should be very close to 4+ and has neither mixed valency for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn3+. Both were thought to play an essential role in CMR materials. We propose that CMR in Tl2Mn2O7 is based on magnetic ordenng driven by superexchange and strong spin fluctuation scattering above Tc.
© EDP Sciences 1997