The microwave properties of sheets containing silica-coated NiFeMo flakes are investigated and discussed according to the sheet structure, the silica layer uniformity and thickness. Commercial NiFeMo flakes are coated using the Stoeber method and embedded in polymer using the doctor blade process. The silica layer thickness is estimated by transmission electron microscopy (TEM) and the coating quality is probed by scanning electron microscopy (SEM). The in-plane orientation of magnetic moments within the sheet is evidenced by 57Fe Mössbauer spectrometry. As a result, the permittivity of composites is strongly reduced when the NiFeMo flakes are completely coated, whereas the permeability level is weakly and progressively altered when the silica thickness increases. We show that the frequency range for microwave absorption can be tuned according to the silica thickness and the flake loading. Reflection losses of −13 dB are obtained in the 1–2 GHz range for a normal incident electromagnetic wave interacting with 3 mm sheets backed by a perfect conductor and loaded with silica (90 nm width)-coated NiFeMo flakes.