We report a temperature-dependent high-resolution x-ray diffraction investigation of 200-nm epitaxial BiFeO 3 thin films grown on ͑001͒ SrTiO 3 . We find that BiFeO 3 undergoes two high-temperature transitions: a first-order ␣-␤ phase transition between 745 and 780°C and a more diffuse transition toward the ␥ phase at 860°C. Reciprocal space maps reveal that thin films remain monoclinic crossing the ␣-␤ phase transition. Linear extrapolation of the in-plane lattice parameters to higher temperatures appears to rule out cubic symmetry for the ␥ phase.

We have used pulsed laser deposition to grow BiFeO 3 /BaTiO 3 multilayers on SrTiO 3 substrates. The samples are characterized by Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM), and by X-ray diffraction measurements and modelling. The two multilayers investigated here have modulation periods A = 90Å and 180Å, while the total thickness of 1800Å is the same for both samples. Hence, twice as many interfaces are encountered for the A = 90Å than for the A = 180Å sample. SQUID magnetometer measurements performed on these multiferroic samples show that the magnetization is significantly larger for the smaller wavelength sample thus pointing to a multilayer effect between the magnetic susceptibility and the number of interfaces per unit volume.

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We report a temperature-dependent high-resolution X-ray diffraction investigation of 200 nm epitaxial BiFeO 3 thin films grown on (001) SrTiO 3 . We were successful in making epitaxial thin films thermally stable, a key requirement for this study. Our results provide evidence that BiFeO 3 undergoes three hightemperature transitions: an antiferromagnetic transition at 425 C, a first-order -phase transition between 745 C and 780 C and a smoother transition toward the -phase at 860 C. Both the -and -transitions take place 60 C lower than the bulk values whereas the antiferromagnetic transition occurs 55 C higher. This underscores the part played by strain and also emphasizes that BiFeO 3 is not a conventional ferroelectric perovskite. Reciprocal space maps reveal the unusual result that the thin films remain monoclinic on crossing the -phase transition. Linear extrapolation of the in-plane lattice parameters to higher temperatures rules out cubic symmetry for the -phase.