Structure, Magnetism and Colossal Magnetoresistance Behavior in A2Mn2O7 Pyrochlores (A = Dy-Lu, Y, Sc, In or Tl)

This paper presents a bird's eye view of the structural, electrical and magnetic properties of A 2 Mn 2 O 7 (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 Tl 2 Mn 2 O 7 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 A 2 Mn 2 O 7 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 Tl 2 Mn 2 O 7 , 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 A, significantly shorter than the Mn-O distances in the range 1.94 to 2.00 A observed in phases based on LaMnO 3 perovskites which show CMR. The above observation indicates that oxidation state of Mn in TI 2 Mn 2 O 7 should be very close to 4+ and has neither mixed valency for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn 3+ . Both were thought to play an essential role in CMR materials. We propose that CMR in Tl 2 Mn 2 O 7 is based on magnetic ordering driven by superexchange and strong spin fluctuation scattering above T c .