Single-ion anisotropy in the gadolinium pyrochlores studied by electron paramagnetic resonance

The electron paramagnetic resonance is used to measure the single-ion anisotropy of ${\mathrm{Gd}}^{3+}$ ions in the pyrochlore structure of ${({\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x})}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. A rather strong easy-plane-type anisotropy is found. The anisotropy constant $D$ is comparable to the exchange integral $J$ in the prototype ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, $D\ensuremath{\simeq}0.75J$, and exceeds the dipolar energy scale. Physical implications of an easy-plane anisotropy for a pyrochlore antiferromagnet are considered. We calculate the magnetization curves at $T=0$ and discuss phase transitions in a magnetic field.