A neutron study of the spin reorientation transition in Gd

Abstract

The temperature dependence of the spin orientation in Gd has been reexamined with particular emphasis on the spin-reorientation phase transition near 235 K. The spin orientation and the relative magnetization were determined in the 9–310 K region from Bragg intensity measurements on the (100) and (002) reflections from an isotopic single crystal of 160Gd. At low temperatures, the angle between the spin and the c axis is 40°. This angle ϑ increases with increasing temperature up to 71° at 130 K and then remains constant out to 220 K where it begins a precipitous decrease on approaching the reorientation transition at Tr⩾235 K. Unexpected intensity behavior was observed both below Tr, where the relative magnetization dips about 3% below the smooth curve in the 220–234 K region, and above Tr, where Bragg intensity remains at the (002) position out to about 285 K. Since the anisotropy constants of Gd indicate that ϑ = 0 above Tr, we attribute this (002) intensity to long wavelength orientation fluctuations and assume that only the average ϑ is zero above Tr. In spite of this complication, the transition exhibits Landau behavior in that the order parameter sin ϑ follows a (Tr−T)1/2 dependence in the range 223–234 K.