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 $T_c=120.65\mathrm{}\pm 0.15\mathrm{K}$ and critical exponents $\delta =4.65\mathrm{}\pm 0.15$ (from the critical isotherm), $\gamma +\beta =1.75\mathrm{}\pm 0.03$ (from the temperature dependence of the crossover line), and $\gamma =1.31\mathrm{}\pm 0.05$ (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.

• Received 10 March 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.219