Magnetic Form Factor of Terbium

The magnetic form factor of Tb at 4.2°K has been measured to $\frac{\left(sin\theta \right)}{\lambda }=1.1\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$ by the use of both polarized and unpolarized neutrons. The crystal was magnetized along the $b$ and $a$ directions by the use of a split-coil superconducting magnet providing a field of 40 kOe. At lower scattering angles where the magnetic amplitude $p$ is greater than the nuclear amplitude $b$, the main source of experimental error is uncertainty in the extinction correction, which was estimated by changing wavelength as well as sample thickness. At higher scattering angles where $p<b$, the polarized-beam experiments yielded accurate form-factor values. Except for a scale factor, the measurements are in good agreement with theoretical values, suggesting a contribution from the conduction electrons which has been borne out by Fourier synthesis of the magnetization density. A theoretical calculation has been made of the aspherical contribution to the form factor.