We investigated the microstructural and magnetic properties of nanocrystalline FeCuNbSiB thin films produced through magnetron sputtering followed by heat treatment. Our research focused on film thicknesses (80 nm and 160 nm) and heat treatment temperatures (490 °C–520 °C) that resulted in samples characterized by low coercivity and high effective magnetization. Conducting a comprehensive microstructural examination, we employed X-ray diffraction and transmission electron microscopy techniques, including selected area electron diffraction, high-resolution imaging and Fourier transform analysis. Magnetic properties were investigated using an alternating gradient field magnetometer and broadband ferromagnetic resonance. Structural analysis revealed, for all our heat-treated films, a well-defined microstructure characterized by nanograins, with a Fe3Si ferromagnetic phase, embedded within an amorphous matrix. Our films exhibit sub-Oe coercivity, reaching as low as 0.51 Oe, along with an increase in effective magnetization from 1170 emu/cm3 for the amorphous phase to a maximum of 1260 emu/cm3 in the nanocrystalline phase. •FeCuNbSiB thin films were produced through magnetron sputtering, then heat treated.•The Fe3Si phase was obtained with crystallite sizes ranging from 11 nm to 15 nm.•Crystallization resulted in sub-Oe coercivity and increased effective magnetization.•Magnetic properties were investigated using AGFM and broadband-FMR techniques.