Macroscopic Anisotropy and Symmetry Breaking in the Pyrochlore Antiferromagnet Gd${}_2$Ti${}_2$O${}_7$}

In the Heisenberg antiferromagnet $G{d}_{2}T{i}_2{O}_7$, 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 $G{d}^{3+}$ 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.