NEUTRON-SCATTERING STUDIES OF THE 2 MAGNETIC CORRELATION LENGTHS IN TERBIUM

Extensive neutron-scattering experiments have been performed in order to characterized the nature of the two correlation lengths observed in Tb, a phenomenon common to Ho as well as SrTiO3. In the vicinity of the transition temperature, 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, delta) magnetic satellite 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 (theta almost-equal-to 1.35-degrees) 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 approximately 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 SrTiO3, 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 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 (approximately 2 mueV 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.