7th INTERNATIONAL CONFERENCE ON FERRITES

J. Phys. IV France 07 (1997) C1-625-C1-628
DOI: 10.1051/jp4:19971258

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. Sleight<sup>4

1</sup>  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.

Abstract
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 T_c.

© EDP Sciences 1997