Strategies for Obtaining High-quality Sr₂FeMoO₆ Films Grown via Pulsed Laser Deposition

The basis of this thesis is to discuss the growth and characterization of double perovskite Sr₂FeMoO₆ (SFMO) films grown on SrTiO₃ (100) and (111)-oriented substrates prepared by pulsed laser deposition (PLD). Two different PLD chambers (I and II) were used for depositions in order to verify the universal character of growth conditions between chambers as well as determine the optimal growth parameters. Structural characterization and phase identification were carried out on films using x-ray diffraction (XRD) and Rutherford backscattering (RBS). Magnetic data were obtained from super-conducting quantum interference device (SQUID) measurements and surface characterization was done using atomic force microscopy (AFM).

It was concluded that majority of the films grown in the PLD chamber I were molybdenum rich. Furthermore, most of the films contained a γ-Fe impurity, a metastable phase. The most phase pure films were grown at 850 °C in 50 mTorr of 95% Ar/5% H2, at a substrate to target distance of 89 mm. A magnetization of 2.1 μ_B/f.u. and 1.7 μ_B/f.u. was obtained for films grown on SrTiO₃ (111) and (100) (respectively). The stoichiometry and magnetization of these films correlates well with studies done in the bulk SFMO. Rietveld refinements shows that these films are as ordered as they can be for their stoichiometry, which strongly suggests that the magnetic properties are limited largely by non-stoichiometry.

The impact of ex-situ post-annealing was explored on films containing the γ-Fe phase in PLD system I. It was demonstrated that the γ-Fe phase was not visible in the XRD pattern upon post-annealing. For films grown on SrTiO₃ (111), there was no change in ordering upon post-annealing, but an increase in saturation magnetization was ob-served. Furthermore, there was no significant decrease in either in- or out-of-plane lattice parameters. The lack of structural differences between the pre- and post-annealed samples alluded to the possibility that the increased saturation magnetization was due to the presence of secondary phases not visible by XRD.

Results from growths in a PLD chamber II are discussed. The best growth conditions were determined to be those grown at a temperature of 680 °C, in 225 mTorr of argon with a substrate to target distance of 38 mm. From RBS, a cation ratio of 0.200:0.103:0.108 was obtained for this film, indicating a near-stoichiometric double perovskite phase. An ordering of 83.32% ± 3.12% and saturation magnetization of 2.4 μ_B/f.u. were obtained for the film grown under the above conditions. It was demonstrated that the magnetization for the films with this corresponding ordering is lower than expected as compared to stoichiometric bulk SFMO.