Reentrant Phase Diagram of Yb2Ti2O7 in a ⟨111⟩ Magnetic Field

We present a magnetic phase diagram of rare-earth pyrochlore Yb2Ti2O7 in a < 111 > magnetic field. Using heat capacity, magnetization, and neutron scattering data, we show an unusual field dependence of a first-order phase boundary, wherein a small applied field increases the ordering temperature. The zero-field ground state has ferromagnetic domains, while the spins polarize along < 111 > above 0.65 T. A classical Monte Carlo analysis of published Hamiltonians does account for the critical field in the low T limit. However, this analysis fails to account for the large bulge in the reentrant phase diagram, suggesting that either long-range interactions or quantum fluctuations govern low field properties.