POLARIZED NEUTRON STUDY OF HIGH-T(C)SUPERCONDUCTORS

The magnetization density induced by an applied magnetic field in the high-T(c) superconductor YBa2Cu3O6+x has been measured by polarized neutron diffraction. Measurements have been carried out on single crystals intercalated with different oxygen compositions: x = 0.52 and x = 1.00, at different fields up to 10 tesla and different temperatures. The magnetic density is located mainly on the copper sites, and the behaviour of these magnetic sites depends strongly on temperature. On the copper of the CuO2 planes, the superconductivity is associated with a quenching of the spin part of the density: this quenching begins below T(c) for YBa2Cu3O7 (T(c) = 90 K) but far above T(c) for YBa2Cu3O6.52 (T(c) = 48 K). This is in accordance with the variations of the Knight shift measured by NMR. On the copper of the chains, the magnetization measured by neutrons shows a combination of Curie and Pauli paramagnetism, which practically does not depend on the onset of superconductivity and reflects the order of the chain fragments.