Magnetic Structure and Phase Separation in Epitaxial SrCoO$_{\mathrm{x}}$ Thin Films*

Bulk SrCoO$_{\mathrm{x}}$ separates into three distinct ferromagnetic phases as the oxygen content is increased from x $=$ 2.75 to 3.0, corresponding to T$_{\mathrm{C}} =$ 165 K (SrCoO$_{2.75})$, T$_{\mathrm{C}} =$ 220 K (SrCoO$_{2.88})$, and T$_{\mathrm{C}}$ $=$ 280 K (SrCoO$_{3.0})$. Over this composition, the lattice evolves smoothly and remains a single crystallographic phase. Using pulsed laser deposition and electrochemical oxidation, we have prepared epitaxial films of SrCoO$_{\mathrm{x}}$ of varying thickness and orientation on SiTiO$_{3}$ substrates. While in polycrystalline samples intermediate oxygen concentrations show a two-phase magnetic behavior, 100nm thick (0 0 1) films remain single phase but still favor the same ferromagnetic transitions. Thicker, 150 nm (1 1 1) films also order at comparable T$_{\mathrm{C}}$'s, but again show two-phase behavior during deoxidation. Resonant x-ray diffraction on these samples reveals both commensurate and incommensurate ordering dependent on the oxidation state. This implies a charge or orbital ordering which may be influenced by finite size effects.