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 L(III) edge. A lower bound for this enhancement is of order 20. The magnetic modulation wave vector tau(m) decreases continuously with decreasing temperature over most of the range between the Neel and Curie points, in agreement with prior neutron-diffraction measurements. However, no evidence was found for any lock-in behavior above T(c), as has been observed in other rare earths. A very strong hysteresis effect is present in Tb below T(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 --> T(c)+ while that of the charge peak at (002) remains constant. No conclusive evidence was found for scattering at higher harmonics.