Alternating current susceptibility of a gadolinium crystal

The complex ac susceptibility of long bar-shaped crystals of gadolinium cut parallel or perpendicular to the c axis does not exhibit the expected divergence at the Curie point TC=293 K. However, χ′ does diverge to the demagnetization-limited value 1/N≈70 at the spin reorientation transition temperature Tsr=225 K, where the first anisotropy constant K1 changes sign. The c-axis susceptibility χ∥′(T) shows a step to reach a value of 10 at Tc which is essentially independent of measuring field in the ranges 10 Hz–1 kHz and 8–800 A/m. However, there is a peak in the perpendicular susceptibility χ⊥′(T) near TC, which reaches 45. A superimposed 50 Hz depinning field does not influence the ac response. It is proposed that gadolinium exhibits long-period sinusoidally modulated c-axis order below TC, analogous to that of erbium. The period of the modulation is estimated as 70 Å.