Orientation dependence of the magnetic phase diagram of Yb2Ti2 O7

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb2Ti2O7 as a putative U(1) QSL but a new class of ultrapure single crystals make it abundantly clear that the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for B∥(100), while a field parallel to (110) or (111) enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb2Ti2O7 in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of TC and a reentrant phase boundary under applied magnetic fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.