Transition to long-range magnetic order in the highly frustrated insulating pyrochlore antiferromagnet Gd2Ti2O7

Experimental evidence from measurements of the ac and de susceptibility, an d heat-capacity data show that the pyrochlore structure oxide, Gd2Ti2O7, ex hibits short-range order that starts developing at 30 K, as well as long-ra nge magnetic order at T similar to 1 K. The Curie-Weiss temperature, theta( CW) = -9.6 K, is largely due to exchange interactions. Deviations from the Curie-Weiss law occur below -10 K while magnetic heat-capacity contribution s are found at temperatures above 20 K. A sharp maximum in the heat capacit y at T-c = 0.97 K signals a transition to a long-range-ordered state, with the magnetic specific accounting for only similar to 50% of the magnetic en tropy. The heat capacity above the phase transition can be modeled by assum ing that a distribution of random fields acts on the S-8(7/2) ground state for Gd3+. There is no frequency dependence to the ac susceptibility in eith er the short-range- or long-range-ordered regimes, hence suggesting the abs ence of any spin-glass behavior. Mean-field theoretical calculations show t hat no long-range-ordered ground state exists for the conditions of nearest -neighbor antiferromagnetic exchange and long-range dipolar couplings. At t he mean-field level, long-range order at various commensurate or incommensu rate wave vectors is found only upon inclusion of exchange interactions bey ond nearest-neighbor exchange and dipolar coupling. The properties of Gd2Ti 2O7 are compared with other geometrically frustrated antiferromagnets such as the Gd3Ga5O12 gadolinium gallium garnet, R2Ti2O7 pyrochlores where R = T b, Ho, and Tm, and Heisenberg-type pyrochlore such as Y2Mo2O7, Tb2Mo2O7, an d spinels such as ZnFe2O4. [S0163-1829(99)03321-4].