${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ show spontaneous magnetization around 121 K indicating magnetic phase transitions. The related pyrochlores ${\mathrm{Y}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{Lu}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ 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 ${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$. SANS results substantiate the long-range magnetic ordering in ${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ as indicated by the magnetic behavior. Preliminary SANS data of ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$, while more ambiguous, are not inconsistent with the presence of long-range order. Effective paramagnetic and saturation magnetic moments of ${\mathrm{Tl}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ and ${\mathrm{In}}_2$${\mathrm{Mn}}_2$${\mathrm{O}}_7$ indicate the presence of ${\mathrm{Mn}}^{3+}$ ions on the ${\mathrm{Mn}}^{4+}$ 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 ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ spins.

• Received 24 August 1993

DOI:https://doi.org/10.1103/PhysRevB.49.1086