Local magnetic field distributions in superconducting niobium at 4.2 K by neutron diffraction

Neutron diffraction experiments on flux line lattices in superconducting Nb are reported. From the integral reflectivities of the (10), (11), (20), (21) and (31) reflections the absolute values of the formfactors were obtained as a function of the flux density. Considering certain properties of the flux line lattice it is also possible to determine the phases of the formfactors. From this the local field distribution in the mixed state could be calculated by Fourier-transformation. The dependence of the resulting maximum, minimum, and saddle point fields on the flux density and on the impurity parameter is discussed and compared with theory.