Field-driven phase transitions generally arise from competition between Zeeman energy and exchange or crystal-field anisotropy. Here we present the phase diagram of a frustrated pyrochlore magnet ${\mathrm{G}\mathrm{d}}_2{\mathrm{T}\mathrm{i}}_2{\mathrm{O}}_7$, where crystal-field splitting is small compared to the dipolar energy. We find good agreement between zero-temperature critical fields and those obtained from a mean-field model. Here, dipolar interactions couple real space and spin space, so the transitions in ${\mathrm{G}\mathrm{d}}_2{\mathrm{T}\mathrm{i}}_2{\mathrm{O}}_7$ arise from field-induced “cooperative anisotropy,” reflecting the broken spatial symmetries of the pyrochlore lattice.

• Received 6 August 2001

DOI:https://doi.org/10.1103/PhysRevLett.89.067202