Polarized Neutron Reflectivity and Electron Microscopy Analysis of the Magnetic Microstructure in Antiparallel-Coupled Co Multilayers

Antiparallel exchange-coupled thin films are a convenient way to provide a ferromagnetic surface in situations where zero net magnetization is required, for example, when studying superconducting-ferromagnetic proximity effects with spin-triplet superconducting correlations.\footnote{T. Khaire, etal. Phys. Rev. Lett. 104, 137002 (2010)} We use the complementary techniques of polarized neutron reflectivity (PNR) and scanning electron microscopy with polarization analysis (SEMPA) to characterize the magnetic structure of such an antiferromagnetically coupled Co/Ru/Co multilayer. We find that, although the average macroscopic magnetization follows the simple antiparallel coupling picture, at the nanoscale the 3-dimensional magnetic structure is much more complex. The films are mostly antiparallel, but the magnetization directions fluctuate by as much as $\pm $ 40\r{ } over lengths as small as 100 nm. This structure has significant implications when trying to understand the local spin-dependent transport properties at the ferromagnetic interface. Applying magnetic fields further complicates the structures, leading to spin-flop related magnetic arrangements.