Paramagnetic critical scattering by a dilute incommensurate antiferromagnet YEr

We have studied the critical scattering of neutrons by an YEr single crystal containing 3 at% Er ( T_N=3.25 K). The experiment was performed between 3.309 and 4.39 K. The isointensity contours in the ( a, c) plane are ellipses elongated along the a direction, with an anisotropy ratio of about 2.3, independent of the temperature. This ratio, probably specific to yttrium, is much stronger than for pure Ho and Dy. The thermal variation of the diffraction width is compatible with v=0.8±0.2, and the intensity variation with γ=2±0.5.