This work studies the influence of oxygen flow and substrate temperature on the physical properties of reactive sputtered thin films of iron oxide spinels. We have focused on the variations of the magnetic moment, which can be as low as 20% of the bulk value, for certain experimental conditions. We present a model which attributes the missing moment to a mispopulation of Fe moments on the A and B sites. Direct experimental evidence to prove this model is obtained from conversion electron Mössbauer spectroscopic experiments, which correlate the missing moment with increased A‐site population compared with the “textbook” population (only 8 A sites out of 64 are filled). For example, samples deposited at room temperature have always a low moment and are found to have a large A‐site population. When the samples are annealed in vacuum under specific conditions to maintain the oxygen ratio and to give kinetic energy to the atoms to reach their spinel positions, an increase of the magnetic moment is observed. © 1988, American Institute of Physics. All rights reserved.