USE OF A F-19 NMR PROBE FOR THE STUDY OF THE LOCAL MAGNETIC-STRUCTURE IN THE AF AND SC HTC YBACUO SERIES - EXPERIMENTAL LIGHTS ON THE HTC SUPERCONDUCTIVITY MECHANISM

Three series of YBaCuOF powdered samples have been derived from the characteristic compositions O6, O6.7 and O7 by a soft fluorination. The fluorine atoms occupy at first the O vacancies in the chains, then the bridging sites between the chains. F-19 NMR shows 4 types of lines corresponding respectively to a classical AF state, a very peculiar paramagnetic state in a large temperature range around T(N) with a very high local AF correlation. This structure is seen again in the superconducting samples and reveals an equilibrium between two magnetic structures at T(C) with slow magnetic fluctuations and lineshift. When put in the bridges, the F atoms seem to harden the magnetic lattice and push the sample out of the SC domain.