Magnetic proximity effect in perovskite superconductor/ferromagnet multilayers

Multilayers of superconducting/ferromagnetic (SC/FM) ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}∕{\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_{3}$ have been studied by neutron reflectometry. The occurrence of a structurally forbidden Bragg peak in the FM state highlights a significant difference between the nuclear and magnetic depth profiles. From comparison with simulated reflectivity curves we identify two possible magnetization profiles: (i) a magnetic moment within the SC layer antiparallel to the one in the FM layer (antiphase magnetic proximity effect) or (ii) a ``dead'' region in the FM layer with zero net magnetic moment. In addition, we observe an anomalous enhancement of the off-specular reflection in the SC state which signals a strong mutual interaction between SC and FM order parameters.