  We have investigated the structural and magnetic properties of BiFeO$_3$
(BFO) thin films grown over (100)-oriented Si substrates by rf magnetron
sputtering in a new route under O$_2$ free low pressure Ar atmosphere.
Single-phase BFO films were deposited in a heated substrate and post-annealed
in situ. The new routed allows high deposition rate and produce polycrystalline
BFO pure phase, confirmed by high resolution X-ray diffraction. Scanning
electron and atomic force microscopy reveal very low surface roughness and mean
particle size of 33 nm. The BFO phase and composition were confirmed by
transmission electron microscopy and line scanning energy-dispersive X-ray
spectroscopy in transmission electron microscopy mode. The surface chemistry of
the thin film, analyzed by X-ray photoelectron spectroscopy, reveals the
presence of Fe$^{3+}$ and Fe$^{2+}$ in a 2:1 ratio, a strong indication that
the film contains oxygen vacancies. An hysteretic ferromagnetic behavior with
room temperature high saturation magnetization $\sim 165 \times 10^3$ A/m was
measured along the film perpendicular and parallel directions. Such high
magnetization, deriving from this new route, is explained in the scope of
oxygen vacancies, the break of the antiferromagnetic cycloidal order and the
increase of spin canting by change in the surface/volume ratio. Understanding
the magnetic behavior of a multiferroic thin films is a key for the development
of heterogeneous layered structures and multilayered devices and the production
of multiferroic materials over Si substrates opens new possibilities in the
development of materials that can be directly integrated into the existent
semiconductor and spintronic technologies.
