  BiFeO$_3$ (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
BiFe$_{1-x}$Co$_x$O$_3$ ({\it x} = 0 to 0.30) (BFCO) thin films grown on
LaAlO$_3$(001) substrates. The XAS results indicated that the Fe and Co ions
are in the Fe$^{3+}$ and Co$^{3+}$ 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
Fe$^{3+}$ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD
signals of the octahedrally coordinated Fe$^{3+}$ ions increase with Co
content. The results suggest that an impurity phase such as the ferrimagnetic
$\gamma$-Fe$_2$O$_3$ which exists at low Co concentration decreases with
increasing Co concentration and that the ferromagnetic component of the
Fe$^{3+}$ ion in the octrahedral crystal fields increases with Co
concentration, probably reflecting the increased canting of the Fe$^{3+}$ ions.
