Neutron scattering study on La_1.9Ca_1.1Cu₂O_6+δ and La_1.85Sr_0.15CaCu₂O_6+δ

We present neutron scattering data on two single crystals of the high temperature superconductor La_2−x(Ca,Sr)_xCaCu₂O_6+δ. The Ca_0.1-doped crystal exhibits a long-range antiferromagnetically ordered ground state. In contrast, the Sr_0.15-doped crystal exhibits short-range antiferromagnetic order as well as weak superconductivity. In both crystals antiferromagnetic correlations are commensurate; however, some results on the Ca_0.1-doped crystal resemble those on the spin-glass phase of La_2−xSr_xCuO₄, where magnetic correlations became incommensurate. In addition, both crystals show a structural transition from tetragonal to orthorhombic symmetry. Quite remarkably, the temperature dependence and correlation length of the magnetic order is very similar to that of the orthorhombic distortion. We attribute this behavior to an orthorhombic strain-induced interbilayer magnetic coupling, which triggers the antiferromagnetic order. The large size of the crystals made it also possible to study the magnetic diffuse scattering along rods perpendicular to the CuO₂ planes in more detail. For comparison we show x-ray diffraction and magnetization data. In particular, for the Ca_0.1-doped crystal these measurements reveal valuable information on the spin-glass transition as well as a second anomaly associated with the Néel transition.