Incommensurate geometry of the elastic magnetic peaks in superconductingLa1.88Sr0.12CuO4

We report magnetic neutron-scattering measurements of incommensurate magnetic order in a superconducting single crystal of ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4.$ We find that the incommensurate wave vectors which describe the static magnetism do not lie along high-symmetry directions of the underlying ${\mathrm{CuO}}_2$ lattice. The positions of the elastic magnetic peaks are consistent with those found in excess-oxygen doped ${\mathrm{La}}_2{\mathrm{CuO}}_{4+y}.$ This behavior differs from the precise magnetic order found in the low-temperature tetragonal ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ material for which stripes of spin and charge have been observed. These observations have clear implications for any stripe model proposed to describe the static magnetism in orthorhombic ${\mathrm{La}}_2{\mathrm{CuO}}_4$-based superconductors.