Neutron diffraction study of the magnetic structure of erbium

Abstract

The magnetic structure of high-purity single crystals of erbium has been studied by neutron-diffraction techniques. Although the general characteristics of the magnetic structure of erbium have been found to be in agreement with earlier measurements, several interesting new features have been observed. At 84.4 °K the $c$-axis moment orders in a sinusoidally modulated magnetic structure with wave vector along the $c$ axis, and develops higher-order harmonics as the temperature is decreased below the Néel point. The third- and fifth-order modulations of the $c$-axis moment were of measurable intensity at temperatures as high as 75 and 55 °K, respectively. Higher-order harmonics up to the 17th order have been observed as the temperature was decreased down to 22 °K. At 52.4 °K the basal-plane moment was found to order in a spiral with wave vector equal to that of the $c$-axis moment. Third- and fifth-order harmonics of the basal-plane moment were observed between approximately 50 and 18 °K. at 18 °K a transition to a ferromagnetic spiral structure was observed; significant hysteresis effects were observed in the vicinity of this transition temperature. Significant short-range-order effects have been observed above the Néel and the basal-plane-ordering temperatures. The temperature dependence of the wave vector of the magnetic structure was found to exhibit a number of anomalies associated with commensurability of the magnetic periodicity with that of the lattice. The basic features of the temperature dependence of the wave vector are in good agreement, in the (84-18) °K temperature region, with the Elliott-Wedgewood theory. The temperature dependence of the $c$- and $a$-axis lattice constants has been measured and found to exhibit significant magnetostriction effects.