The Magnetic Structure of Er₂Ti₂O₇

We employ the previously published neutron scattering experiments\cite{1,2,3} to suggest that Er$_2$Ti$_2$O$_7$ has a broken-symmetry multiple-{\bf q} state with tetragonal magnetic symmetry\cite{4}. The ordered moments do not appear to lie close to the crystal-field anticipated directions\cite{5} and we suggest that the low energy gapless mode, visible in specific-heat measurements\cite{6}, is not of the usual transverse isotropic Goldstone-mode type, but is longitudinal and is associated with the internal transfer of magnetism between distinct magnetic Bragg spots.