Magnetic, electrical, and small-angle neutron-scattering studies of possible long-range order in the pyrochlores Tl2Mn2O7 and In2Mn2O7

Tl2Mn2O7 and In2Mn2O7 show spontaneous magnetization around 121 K indicating magnetic phase transitions. The related pyrochlores Y2Mn2O7 and Lu2Mn2O7 also exhibit spontaneous magnetization at their respective magnetic transition temperatures around 17 K. However, neutron-scattering studies do not provide any evidence for long-range order but suggest reentrant spin-glass behavior. In view of this we have performed small-angle neutron-scattering (SANS) investigations on Tl2Mn2O7 and In2Mn2O7. SANS results substantiate the long-range magnetic ordering in Tl2Mn2O7 as indicated by the magnetic behavior. Preliminary SANS data of In2Mn2O7, while more ambiguous, are not inconsistent with the presence of long-range order. Effective paramagnetic and saturation magnetic moments of Tl2Mn2O7 and In2Mn2O7 indicate the presence of Mn3+ ions on the Mn4+ sublattice as do the measured lattice constants. The occurrence of ferromagnetic ordering in these materials at high temperatures has been attributed to the double exchange interaction between Mn3+ and Mn4+ spins.