CRITICAL BEHAVIOR OF THE MAGNETORESISTIVE PYROCHLORE TL2MN2O7

We report measurements of the ac susceptibility and dc magnetization of the colossal magnetoresistive pyrochlore ${\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ from which we determine the critical temperature ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}120.65\ifmmode\pm\else\textpm\fi{}0.15\mathrm{K}$ and critical exponents $\ensuremath{\delta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4.65\ifmmode\pm\else\textpm\fi{}0.15$ (from the critical isotherm), $\ensuremath{\gamma}+\ensuremath{\beta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.75\ifmmode\pm\else\textpm\fi{}0.03$ (from the temperature dependence of the crossover line), and $\ensuremath{\gamma}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.31\ifmmode\pm\else\textpm\fi{}0.05$ (from the temperature dependence of the susceptibility along the same line). Unlike recently reported values for perovskite systems, these exponent values are very close to those predicted by the near-neighbor, 3D Heisenberg model. This pyrochlore system, however, exhibits a marked regular (i.e., noncritical) component in the response near ${T}_{c}$, which is unusual in view of the absence of ${\mathrm{Mn}}^{3+}$ ions in this system.