Certain Gd alloys, such as Gd-Y, are known to exhibit an apparent “double ferromagnetism” in which the $c$ axis component orders on cooling from the paramagnetic phase $\left(T_{C1}\right)$, while the basal-plane component remains effectively paramagnetic. Subsequently the basal-plane component orders at a lower temperature $\left(T_{C2}\right)$. Our measurements of low-field ac susceptibility and magnetization for high-quality Gd and Gd-Y single crystals suggest that either the behavior originally used to identify ferro I, namely, the concave-upward growth of the $a$ axis moment in the approach on cooling to $T_{\text{SR}}$, is not necessarily an indicator of the phase, or that ferro I is also present in pure Gd. Of the several models that exist to describe ferro I, the results from this study favor the random-cone model.

• Received 25 September 2008

DOI:https://doi.org/10.1103/PhysRevB.79.104401