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

Abstract

${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ 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

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