rf-sputtered Fe/Ge multilayers for neutron-polarizing monochromators

State of the art diode rf-sputtering is used to fabricated high quality polarizing monochromators for neutrons. Optimization of the deposition parameters is achieved using in- situ kinetic ellipsometry and a great number of ex-situ characterization techniques such as grazing x-ray reflection, x-ray diffusion, alternating field gradient magnetometry. Mossbauer spectrometry, and electron microscopy. A precise picture of the structural characteristics of the system is deduced and related to the neutron performance as measured by polarized neutron reflectometry. We show that the structural behavior is controlled by crystallization of the iron layers and by the occurrence of amorphous interdiffused layers at each interface. As a consequence, the polarizing efficiency of these mirrors depends directly on the amount of iron involved in the interdiffusion. Using optimized deposition conditions, the flipping ratio is found to be around 40 for Fe/Ge mirrors with a medium period value of 120 angstroms. Positive spin-state reflectivity at the first Bragg peak is close to 100% when 150 bilayers are included and a controlled graded layer thickness allows the angular acceptance to be enhanced up to 39%.