X-RAY-SCATTERING AND NEUTRON-SCATTERING MEASUREMENTS OF 2 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-re solution x-ray- and neutron-scattering studies. For temperatures withi n about 2 K above T(N), the short-ranged magnetic fluctuations exhibit two length scales, instead of the single one expected in an ideal sys tem. Well above T(N), the shorter of the two length scales exhibits po wer-law behavior consistent with normal critical fluctuations; the lin e shape in momentum space is well described by a Lorentzian, and the m easured critical exponents are nu=0.54+/-0.04 and gamma=1.24+/-0.15. T he longer of the two length scales is well described by a squared-Lore ntzian line shape, and exhibits a power-law temperature dependence. Bo th the shorter and the longer length-scale fluctuations are approximat ely spatially isotropic. We propose that the longer length-scale fluct uations are related to random strain fields which are localized at or near the sample surfae. These results are reminiscent of behavior obse rved at the cubic-to-tetragonal structural phase transitions of some p erovskite materials.