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 dive rgence at the Curie point T-C=293 K. However, chi' does diverge to the dema gnetization-limited value 1/N approximate to 70 at the spin reorientation t ransition temperature T-sr=225 K, where the first anisotropy constant K-1 c hanges sign. The c-axis susceptibility chi(parallel to)'(T) shows a step to reach a value of 10 at T-c which is essentially independent of measuring f ield in the ranges 10 Hz-1 kHz and 8-800 A/m. However, there is a peak in t he perpendicular susceptibility chi(perpendicular to)'(T) near T-C, which r eaches 45. A superimposed 50 Hz depinning field does not influence the ac r esponse. It is proposed that gadolinium exhibits long-period sinusoidally m odulated c-axis order below T-C, analogous to that of erbium. The period of the modulation is estimated as 70 Angstrom. (C) 2000 American Institute of Physics. [S0021- 8979(00)70508-1].