Critical behavior of the magnetoresistive pyrochlore T12Mn2O7

We report measurements of the ac susceptibility and de magnetization of the colossal magnetoresistive pyrochlore Tl2Mn2O7 from which we determine the critical temperature T-c = 120.65 +/- 0.15 K and critical exponents delta = 4.65 +/- 0.15 (from the critical isotherm), gamma + beta = 1.75 +/- 0.03 (from the temperature dependence of the crossover line), and gamma = 1.31 +/- 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 Mn3+ ions in this system.