Magnetic order inYBa2Cu3O6+xsuperconductors

Polarized and unpolarized neutron diffraction has been used to search for magnetic order in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ superconductors. Most of the measurements were made on a high quality crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}.$ It is shown that this crystal has highly ordered ortho-II chain order, and a sharp superconducting transition. Inelastic scattering measurements display a very clean spin-gap and pseudogap with any intensity at 10 meV being 50 times smaller than the resonance intensity. The crystal shows a complicated magnetic order that appears to have three components. A magnetic phase is found at high temperatures that seems to stem from an impurity with a moment that is in the $a-b$ plane, but disordered on the crystal lattice. A second ordering occurs near the pseudogap temperature that has a shorter correlation length than the high temperature phase and a moment direction that is at least partly along the c axis of the crystal. Its moment direction, temperature dependence, and Bragg intensities suggest that it may stem from orbital ordering of the d-density wave type. An additional intensity increase occurs below the superconducting transition. The magnetic intensity in these phases does not change noticeably in a 7 T magnetic field aligned approximately along the c axis. Searches for magnetic order in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ show no signal while a small magnetic intensity is found in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.45}$ that is consistent with the c axis directed magnetic order. The results are contrasted with other recent neutron measurements.