Structural phase transitions and weak ferromagnetism in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Nd}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\delta }}$

When cooled, ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Nd}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ undergoes structural transformations involving tilts of the ${\mathrm{CuO}}_6$ octahedra which can be controlled by varying x and δ. Using synchrotron x-ray and neutron powder diffraction we observe that the transformation from Bmab to Pccn space-group symmetry is accompanied by a 90° copper spin reorientation in the basal plane. Furthermore, a second magnetic transition at lower temperatures yields weak ferromagnetism. These observations may have important implications for the suppression of superconductivity in the P$4_2$/ncm phase of ${\mathrm{La}}_{1.875}$ ${\mathrm{Ba}}_{0.125}$ ${\mathrm{CuO}}_4$.