  Reflected neutron beam originating from a crystal monochromator contains
higher order wavelength contribution. Creating multilayer mirror structures
with custom reflectivity curves including a monochromatic polarized neutron
beam is a challenge in modern neutron optics. In this work we present the study
of three types of magnetron-sputtered aperiodic Fe/Si layer structures with the
purpose of higher harmonic suppression. First, an approximate sinusoidal
profile was achieved directly by carefully controlling the evaporation
parameters during sputtering process. Second, we implemented a random
distribution of the layer thicknesses in which the layer structure of the
sample was derived from a prescribed simulated spectrum. Third, a
quasi-sinusoidal rounded bilayer scattering length profile was attempted to
achieve by 0, 5, 10, 27 and 270x1e15 /cm2 fluence of 350 keV neon irradiation
starting from a sharp bilayer profile. Besides the highest fluence, which
considerably decreased the overall bilayer contrast, we found an improvement of
the monochromatisation with increasing irradiation fluence by the decrease of
the total intensity of higher order reflections from 11.1% to 2.2% while the
intensity of the first order Bragg peak decreased from 80% to 70%. The
polarizing efficiency also increased with increasing fluence of Ne+ irradiation
from 78.8% to 90.7%. We were able to achieve 92% and 82% reflectivity of the
random multilayer structure and sinusoidal profile sample with 87.6% and 97.1%
polarizing efficiency, respectively.
