Magnetic Field Distribution in Type II Superconductors by Neutron Diffraction

Abstract

In 1964 de Gennes and Matricon1 suggested that the periodic structure of the mixed state in type II superconductors could be investigated by neutron diffraction. A first experiment of this kind was performed by Cribier et al. 2 which proved the existence of a triangular flux line lattice in Nb. In the following years no other neutron scattering experiments were reported, mainly because of the development of the decoration technique by Essmann and Trauble3 which provides a much more direct means for the investigation of flux line arrangements. However, this method is not able to determine the microscopic magnetic structure of the mixed state. In the following we show that small-angle scattering of subthermal neutrons reveals the magnetic structure via the experimentally determined form factors.4,5 The form factors have been determined from integral reflectivities of flux line Bragg reflections of different order. Various types of flux line lattices have been investigated, ranging from a polycrystalline state up to single crystals of less than 30′ mosaic spread. The growth of different types of flux line lattices depends mainly on the crystalline state and purity of the superconducting metal. For type II/1 superconductors6 (K ≃ 1) experimental results7 and theoretical considerations8,9. suggest an attraction between flux lines. This assumption has been verified by our measurements.