Neutron-scattering studies of the two magnetic correlation lengths in terbium.

Abstract

Extensive neutron-scattering experiments have been performed in order to characterize the nature of the two correlation lengths observed in Tb, a phenomenon common to Ho as well as ${\mathrm{SrTiO}}_{3}$. In the vicinity of the transition temeprature, each of those crystals exhibits an anomalous two-component q profile in the critical scattering; the usual broad peak and an unexpected additional narrow peak. In order to clarify the spatial origin of the narrow component, the (0,0,\ensuremath{\delta}) magnetic statellite peak of Tb has been closely examined using a very narrow neutron beam realized by a high spatial resolution reflectometer at the National Institute of Standards and Technology, which can produce well-defined beam widths of 0.3 mm and less. The small scattering angle (\ensuremath{\theta}\ensuremath{\approxeq}1.35 \ifmmode^\circ\else\textdegree\fi{}) and the narrow beam width result in an extraordinarily fine q and E resolution. As recently reported, we have confirmed that the intensity of the narrow component is enhanced near the edge of the crystal; it demonstrates that the major part is located within the near surface volume or ``skin'' of the crystal. A skin thickness of \ensuremath{\sim} 0.2 mm full width at half maximum (FWHM) is obtained by fitting the results to a model scatterer distribution which convolutes the beam profile. In contrast to the case of the central peak observed in ${\mathrm{SrTiO}}_{3}$, this unusual spatial distribution establishes the narrow component as a distinct entity in the critical fluctuations. We have shown that the narrow component possesses a different temperature dependence of \ensuremath{\delta} from that of the broad one. Moreover, high E resolution scans show that the narrow component has a distinct energy width which is smaller than our resolution limit (\ensuremath{\sim}2 \ensuremath{\mu}eV FWHM), and that it is essentially temperature independent. We believe that this quasistatic character of the narrow component is the important key to understanding its physical origin.