Enhanced magnetic moment in ultrathin Fe-doped CoFe 2O 4 films

Abstract

The effect of film thickness on the magnetic properties of ultrathin Fe-doped cobalt ferrite (Co 1-xFe 2+xO 4) grown on MgO (001) substrates is investigated by superconducting quantum interference device magnetometry and x-ray magnetic linear dichroism, while the distribution of the Co2 + cations between the octahedral and tetrahedral lattice sites is studied with x-ray absorption spectroscopy. For films thinner than 10 nm, there is a large enhancement of the magnetic moment; conversely, the remanent magnetization and coercive fields both decrease, while the magnetic spin axes of all the cations become less aligned with the [001] crystal direction. In particular, at 300 K the coercive fields of the thinnest films vanish. The spectroscopy data show that no changes occur in the cation distribution as a function of film thickness, ruling this out as the origin of the enhanced magnetic moment. However, the magnetic measurements all support the possibility that these ultrathin Fe-doped CoFe 2O 4 films are transitioning into a superparamagnetic state, as has been seen in ultrathin Fe 3O 4. A weakening of the magnetic interactions at the antiphase boundaries, leading to magnetically independent domains within the film, could explain the enhanced magnetic moment in ultrathin Fe-doped CoFe 2O 4 and the onset of superparamagnetism at room temperature. © 2012 American Physical Society.