We report detailed measurements of the low temperature magnetic phase diagram of ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Heat capacity and time-of-flight neutron scattering studies of single crystals reveal unconventional low-energy states. ${\mathrm{Er}}^{3+}$ magnetic ions reside on a pyrochlore lattice in ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, where local $XY$ anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long-range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of noncollinear phases, divided by a zero temperature phase transition at ${\mu }_0{H}_c\sim 1.5\mathrm{T}$. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.

• Received 6 August 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.147205