  In the Heisenberg antiferromagnet $Gd_2Ti_2O_7$, the exchange interactions
are geometrically frustrated by the pyrochlore lattice structure. This ESR
study reveals a strong temperature dependent anisotropy with respect to a [111]
body diagonal below a temperature $T_A=80$ K, despite the spin only nature of
the $Gd^{3+}$ ion. Anisotropy and symmetry breaking can nevertheless appear
through the superexchange interaction. The presence of short range planar
correlation restricted to specific Kagom\'{e} planes is sufficient to explain
the two ESR modes studied in this work.

Strongly Correlated Electrons