RESONANCE-ENHANCED MAGNETIC-X-RAY DIFFRACTION FROM A RARE-EARTH ALLOY

A single crystal of a rare-earth random-site alloy, Ho0.5Er0.5, has be en studied with x-ray diffraction. We have used the technique of reson ance-enhanced scattering at the Ho and Er L(III) absorption edges to s tudy element-specific effects in the temperature dependence of the mag netic structure below the Neel temperature. In the highest-temperature phase, a c-axis modulated structure in which the Ho and Ef moments ta ke different tilt angles from the basal plane, measurements at the two edges show identical satellite positions. The lowest-temperature phas e, a conical spiral which locks to a structure with wavevector 9/40 c below 20 K, is accompanied by a basal-plane lattice modulation that i s related to the magnetic structure. The modulation changes the symmet ry of the crystal lattice; as a consequence we suggest that the magnet ic structure locks to one with equidistant spin-slip discommensuration s. In addition, we find an unusual energy and polarization dependence in the satellite intensity in this phase that is not seen at higher te mperatures. We present arguments to demonstrate that the unusual behav iour is not accounted for by multiple-scattering events.