Enhanced ferromagnetic moment in Co-doped BiFeO3 thin films studied by soft x-ray circular dichroism

BiFeO3 (BFO) shows both ferroelectricity and magnetic ordering at room temperature, but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferromagnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD) studies of BiFe1−xCoxO3 (x = 0 to 0.30) (BFCO) thin films grown on LaAlO3(001) substrates. The XAS results indicated that the Fe and Co ions are in the Fe3+ and Co3+ states. The XMCD results showed that the Fe ions show ferromagnetism, while the Co ions are antiferromagnetic at room temperature. The XAS and XMCD measurements also revealed that part of the Fe3+ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD signals of the octahedrally coordinated Fe3+ ions increase with Co content. The results suggest that an impurity phase such as the ferrimagnetic γ-Fe2O3 which exists at low Co concentration decreases with increasing Co concentration and that the ferromagnetic component of the Fe3+ ion in the octrahedral crystal fields increases with Co concentration, probably reflecting the increased canting of the Fe3+ ions.