  Single phase (001)-oriented BiFeO3 (BFO) thin films grown by pulsed laser
deposition can only be obtained in a narrow window of deposition pressure and
temperature and have a low magnetic moment. Out of the stability window Fe- or
Bi-rich impurity phases form, which has a strong impact on the physical and
structural properties of the films, even for impurity concentrations hardly
detectable by standard X-ray diffraction measurements. By using more sensitive
tools such as X-Ray absorption spectroscopy and X-ray magnetic circular
dichroism and performing advanced X-ray diffraction characterization, we show
that in non-optimal conditions Fe forms ferrimagnetic gamma-Fe2O3 precipitates
that are responsible for virtually all the ferromagnetic signal measured on
such BFO films by standard magnetometry. This confirms that the BFO phase has a
very low intrinsic moment that does not depend on strain. We also study the
influence of film thickness on the nucleation of parasitic phases and find that
epitaxial strain can stabilize the pure BFO phase in slightly over-oxidizing
growth conditions.
