Study of Inverse Proximity Effect in Ferromagnet/ Superconductor Bilayers Using a Sagnac Interferometer

It was recently proposed theroretically that ferromagnetic order can be induced in the superconductor in a ferromagnet/superconductor ($S/F$) bilayer structure through a so called inverse proximity effect. The proposal predicts a sizable magnetic moment in the ``S" layer that couples antiferromagnetically to the moment in the ``F" layer due to Cooper pairs near the interface formed with one electron in the F layer and one in the S layer. The induced magnetic moment is expected to penetrate the superconductor over a size of the Cooper pairs, i.e. $\xi_s$. In order to directly test this interesting scenario, we fabricated Ni/Pb and Ni/Al bilayer samples and probed the possible induced magnetic moments in the ``S" layer through high resolution Surface Magneto Optical Polar Kerr Effect (PKE) measurements on the ``S" layer side through the bilayer's $T_C$ using a Sagnac interferometer. The thickness of the ``S" layer was fabricated to be larger than the optical skin depth in order to make sure that our experiment doesn't pickup up any magnetic moment from the ``F" layer. $\xi_s$ dependence of the effect is studied by comparing the results in Ni/Pb and Ni/Al samples.