X-ray- and neutron-scattering measurements of two length scales in the magnetic critical fluctuations of holmium

The short-ranged correlations associated with magnetic ordering in the rare-earth antiferromagnet holmium have been characterized in high-resolution x-ray- and neutron-scattering studies. For temperatures within about 2 K above TN, the short-ranged magnetic fluctuations exhibit two length scales, instead of the single one expected in an ideal system. Well above TN, the shorter of the two length scales exhibits power-law behavior consistent with normal critical fluctuations; the line shape in momentum space is well described by a Lorentzian, and the measured critical exponents are ν=0.54±0.04 and γ=1.24±0.15. The longer of the two length scales is well described by a squared-Lorentzian line shape, and exhibits a power-law temperature dependence. Both the shorter and the longer length-scale fluctuations are approximately spatially isotropic. We propose that the longer length-scale fluctuations are related to random strain fields which are localized at or near the sample surfae. These results are reminiscent of behavior observed at the cubic-to-tetragonal structural phase transitions of some perovskite materials.