On the magnetic structure of CeAl 2: a 3-d neutron polarization analysis investigation

Abstract

The magnetic structure of the Kondo compound CeAl 2 is characterized by a propagation vector k = ( 1 2 1 2 − δ, 1 2 + δ) . It was initially described by a single k propagation vector, which implies an antiferromagnetic modulated structure. It has then been shown that the structure in fact was double k , which results for the two cerium sites of the asymmetric unit cell in two helices with opposite chiralities. These helices, built from two nonperpendicular basis vectors, are elliptic. We have measured with a high accuracy the angle between the two basis vectors of the helices, angle which determines their ellipticity. To do this, we have used the ILL Cryopad to analyze the rotation of the polarization vector for the three main different incident polarizations. We have found that when the temperature is lowered below T N down to T = 1.15 K, the helices are less and less elliptical, without however, becoming completely circular.