This study investigated the preparation and particle size control of nanocrystalline magnetite (Fe(3)O(4)) containing a small amount of Ge. Thin films were prepared by radio-frequency sputtering with a composite target of Ge chips set on a Fe(3)O(4) compound target in a mixed atmosphere of Ar and O(2). X-ray diffraction revealed that the diffraction peak of magnetite gradually broadened as the oxygen ratio increased, with the mean grain size ranging from 26 to 2 nm. Transmission electron microscopy also revealed that the magnetite structurally changed from polycrystalline single phase to isolated granular nanocrystals. Magnetization at 8 x 10(5) A/m was monotonically reduced from 0.32 to 0.04T, and coercivity was monotonically reduced from 4.2 x 10(4) to 2.1 x 10(3) A/m with increasing the oxygen ratio from 0 to 0.4%. (C) 2010 Elsevier B.V. All rights reserved.