Magnetic x-ray-scattering study of Tb

The temperature and energy dependence of the magnetic scattering in the ferromagnetic spiral phase of Tb has been studied between 220 and 235 K by high-resolution x-ray-scattering techniques using synchrotron radiation. In order to see the magnetic scattering it was necessary to exploit the resonant enhancement of the magnetic x-ray cross section by tuning the incident photon energy to the Tb ${\mathit{L}}_{\mathrm{III}}$ edge. A lower bound for this enhancement is of order 20. The magnetic modulation wave vector ${\mathrm{\tau }}_{\mathit{m}}$ decreases continuously with decreasing temperature over most of the range between the Néel and Curie points, in agreement with prior neutron-diffraction measurements. However, no evidence was found for any lock-in behavior above ${\mathit{T}}_{\mathit{c}}$, as has been observed in other rare earths. A very strong hysteresis effect is present in Tb below ${\mathit{T}}_{\mathit{N}}$, with the value of the modulation wave vector apparently depending on the rate of cooling from the paramagnetic regime. In addition, the radial linewidth of the magnetic satellite at (002${)}^{+}$ is not resolution-limited and increases as T→${\mathit{T}}_{\mathit{c}}^{+}$ while that of the charge peak at (002) remains constant. No conclusive evidence was found for scattering at higher harmonics.