Non-Planar Magnetic Structures and Trigonal Interactions in Erbium

The magnetic structures of erbium metal have been studied using a high-resolution elastic-scattering technique. The results show that the elliptical cycloidal structures between 55 K and 18 K are not planar. A comparison of the experimental intensities with the predictions of a mean-field model shows that these structures are distorted by two-ion interactions of trigonal symmetry, which distinguish between the two h.c.p. sublattices. The resulting structures are wobbling cycloids, in which there is a b -axis moment perpendicular to the cycloidal ( a,c )-plane, oscillating with a period different from that of the basic structure. In the cone phase below 18 K, the trigonal interactions introduce bunching effects around alternating a directions in a threefold symmetric pattern.