Rare-earth metals: Is gadolinium really ferromagnetic?

Gadolinium is accepted to be one of the four ferromagnetic elements, along with iron, cobalt and nickel, although its Curie point, T_C (the temperature above which ferromagnetism is lost), is only 292 K. Ferro-magnets exhibit a characteristic divergence of their susceptibility at T_C, but no such divergence in any direction is found for needle-shaped crystals of gadolinium. Instead, the susceptibility diverges at a lower spin-reorientation temperature, T_sr, of 225 K, where the anisotropy changes sign. We propose that the magnetic order between T_sr and T_C is not truly ferromagnetic, but is akin to the incommensurate order found in erbium.