Experimental evidence from measurements of the ac and dc susceptibility, and heat-capacity data show that the pyrochlore structure oxide,

{Gd}_{2} {Ti}_{2} O_{7},

exhibits short-range order that starts developing at 30 K, as well as long-range magnetic order at

T \sim 1

K. The Curie-Weiss temperature,

θ_{CW} = - 9.6

K, is largely due to exchange interactions. Deviations from the Curie-Weiss law occur below

\sim 10

K while magnetic heat-capacity contributions are found at temperatures above 20 K. A sharp maximum in the heat capacity at

T_{c} = 0.97

K signals a transition to a long-range-ordered state, with the magnetic specific accounting for only

\sim 50 %

of the magnetic entropy. The heat capacity above the phase transition can be modeled by assuming that a distribution of random fields acts on the

^{8} S_{7 / 2}

ground state for

{Gd}^{3 +} .

There is no frequency dependence to the ac susceptibility in either the short-range- or long-range-ordered regimes, hence suggesting the absence of any spin-glass behavior. Mean-field theoretical calculations show that no long-range-ordered ground state exists for the conditions of nearest-neighbor antiferromagnetic exchange and long-range dipolar couplings. At the mean-field level, long-range order at various commensurate or incommensurate wave vectors is found only upon inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar coupling. The properties of

{Gd}_{2} {Ti}_{2} O_{7}

are compared with other geometrically frustrated antiferromagnets such as the

{Gd}_{3} {Ga}_{5} O_{12}

gadolinium gallium garnet,

R_{2} {Ti}_{2} O_{7}

pyrochlores where

R = Tb,

Ho, and Tm, and Heisenberg-type pyrochlore such as

Y_{2} {Mo}_{2} O_{7},

{Tb}_{2} {Mo}_{2} O_{7},

and spinels such as

{ZnFe}_{2} O_{4} .

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