Anisotropic rare-earth spin-glasses

We present magnetic measurements (ac and dc susceptibilities and magnetization) on $Y\mathrm{Er}$ and $Y\mathrm{Gd}$ single crystals. The longitudinal (i.e., along the $c$ axis) susceptibility of $Y\mathrm{Er}$ shows a typical spin-glass-like cusp, whereas no cusp is observed in the transverse directions. This suggests that only the longitudinal spin components are frozen, in agreement with the theoretical predictions of Cragg and Sherrington for a spin-glass with a large uniaxial crystal field. The $Y\mathrm{Er}$ system appears to represent an almost perfect Ising type of spin-glass. Surprisingly the $Y\mathrm{Gd}$ system is also definitely anisotropic. A spin-glass-like cusp is observed only for the transverse susceptibility. This anisotropy cannot be ascribed to the crystal field and we discuss its possible origins. Finally, we present some data on the remanence properties of the $Y\mathrm{Er}$ and $Y\mathrm{Gd}$ alloys.