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Fe70Co30N thin films with thickness from 20 to 1100 Å were prepared by radio- frequency reactive sputtering in an N2--Ar mixture. The FeCoN films prepared in a low nitrogen flow rate percentage (<6%) and sputtering pressure (<8 mTorr) have a high Bs of about 24.0 kG, but a moderate hard-axis coercivity Hch of 5-30 Oe. With further increase in N2 percentage or sputtering pressure, films become significantly softer, with Hch of about 0.1-0.6 Oe, and have a higher resistivity of up to about 160 μΩ·cm. The change in the magnetic properties with nitrogen flow rate percentage and sputtering pressure can be attributed to the formation of an ultrafine grain size nanocrystalline FeCoN thin film as observed by high-resolution transmission electron microscope. The soft properties of FeCoN films with nano-sized crystallites remain stable even after being annealed at 270°C.