Field-induced inter-planar correlations in the high-temperature superconductor La_1.88Sr_0.12CuO₄

We present neutron scattering studies of the inter-planar correlations in the high-temperature superconductor La_1.88Sr_0.12CuO₄ (T_c=27 K). The correlations are studied both in a magnetic field applied perpendicular to the CuO₂ planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field is to increase the magnetic scattering signal at all values of the out-of-plane wave vector L, indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only L~0 is probed. Interestingly, the results of quench-cooling the sample are similar to those obtained by applying a magnetic field. Finally, a small variation of the incommensurate peak position as a function of L provides evidence that the incommensurate signal is twinned with the dominating and sub-dominant twin displaying peaks at even or odd L, respectively.