Journal of Magnetism and Magnetic Materials 104-107 (1992) 630-632 North-Holland Spin density in the high T c superconductor YBa2Cu307 J.X. Boucherle ", J.Y. Henry ~, R. Papoular b, j. Rossat-Mignod a,b, j. Schweizer ~' and F. Tasset ~ " DRFMC/SPSMS/MDN, Centre d'Etudes Nucldaires de Grenoble, 85X, 38041 Grenoble Cedex, France b Laboratoire L&m Brillouin, Centre d'Etudes Nucldaires de Saclay, 91191 Gif-sur-Yt,ette, France '" Institut Laue Langet,in, 156X, 38042 Grenoble, France Spin density has been measured at various temperatures on a single crystal of the high T~ superconductor YBa2Cu30 7. Polarized neutrons and a 10 T applied magnetic field were necessary for this experiment. In spite of the very weak susceptibility of the compound, it is clear that both copper sites contribute almost equally to the spin density in the normal state but the spin density due to the copper of the planes is quenched in the superconducting state. The knowledge of magnetic susceptibility is essen- tial to understand the behaviour of electrons and holes in high Tc superconductors. In this view, the use of polarized neuton diffraction, compared with standard methods, presents two tremendous advantages: (i) it allows a microscopic measurement, giving the magnetization density induced on each atom of the compound by the application of a field; (ii) it allows the possibility to measure this magneti- zation even below T~ in the superconducting state, when supercurrents are screening the applied magnetic field. We have already reported [1] the spin densities measured on a single crystal of YBa2Cu~O6+ x for three concentrations: x = 0.15, 0.37, 0.9. The induced magnetization densities were carried by both sites of copper. The corresponding moments were less than 10 3p~ B per atom for an applied field of 4.6 T at 30 K, but increased at temperatures lower than 5 K, particu- larly for the copper site of the chains, which became, according to the composition, 3 to 5 times larger at 1.5 K. We report here the results of measurements per- formed in a much higher field, above and below the transition temperature for a YBa2Cu30~,+x crystal with a high oxygen concentration. The magnetic susceptibility of YBa2Cu306+ x com- pounds being very weak (some 10 4 emu/mol), it was necessary to improve all the parameters of the mea- surements. These were performed on the polarized neutron spectrometer D3b on the hot source of ILL, with a wavelength A = 0.84 ,~, and a neutron flux considerably enhanced by the use of an Heusler monochromator. We have installed a 10 T cryomagnet in order to induce the highest possible magnetization. We have used the same 200 mm 3 single crystal as in the previous experiment, intercalated with a high oxy- gen concentration, close to 6.99 (critical temperaturc T~ = 90 K). The axis a + b of the crystal was set vertical and parallel to the field, so that reflections (hhl) could be measured in the horizontal plane without concern for the existence of the twin. The sample was field cooled in such a way that the field inside and outside the sample is about the same [2,3]. The flipping ratios of all the Bragg reflections (hhl) up to sin 0/A = 0.37 ,~ i were measured at three temperatures: 1.5, 30 and 120 K. Flipping ratios are directly connected to nuclear and magnetic structure factors [FN(hkl ) + FM(hkl)] 2 FM(hkl) R(hkl) = [ Fy(hkl) - FM(hkl)] 2 ~- 1 + 4 FN(hkl ~ The magnetic structure factors FM(hkl) are calculated from the flipping ratios R(hkl) knowing the nuclear structure factors FN(hkl) from the crystal structure. A correction for the diamagnetism of the core electrons [4] is then applied. Results can be represented either by magnetization density maps or by individual magnetic moments. We have reported in fig. l the projection of the magnetiza- tion density onto the plane (110) for the two different temperatures 30 and 120 K. Instead of using the classi- cal inverse Fourier transformation, which suffers from truncation effects and does not consider the uncertain- ties of measured FM'S, we preferred to employ a maxi- mum entropy method [5] to retrieve the most probable magnetization map compatible with the data. As for the previous experiments, the magnetization appears mainly on the copper atoms, but also at 120 K on a ridge in the CuO 2 plane including the oxygen. It is possible to refine, by a least square method, the magnetic moments induced on these atoms, using a spherical 3d form factor for thc copper and a spherical 2p one for the oxygen. The results are reported in table 1. There are different points which are worth dis- cussing in the above results: the normal state, the quenching of the magnetization in the superconducting 0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved