Macroscopic Anisotropy and Symmetry Breaking in the Pyrochlore   Antiferromagnet Gd$_{2}$Ti$_{2}$O$_{7}$}
  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. 
A. K. Hassan (1), L. P. L\'evy (1,2), C. Darie (3), P. Strobel (3)   ((1) Grenoble High Magnetic Field Laboratory (2) Instit...