Structure and magnetism of epitaxial SrTi0.78Cu0.22O3−δ films with mixed-valence Cu ions

The structure and magnetic behavior of Cu-substituted Sr2+Ti4+O3-δ (STCu) thin films with ∼22% Cu on the Ti sites, grown on CeO2/yttria stabilized zirconia buffered (001) Si substrates were investigated. STCu films grew with (110) orientation or with a mixed (001)+(110) orientation and were under in-plane compressive strain. They showed weak room temperature ferromagnetism with a saturation magnetization of 0.3 ∼ 1.7 emu/cm3. Cu ions in STCu grown in an oxygen atmosphere existed mainly as Cu3+ with a small proportion of Cu2+, while in STCu grown in vacuum, Cu1+ ions were dominant with small amounts of Cu2+ and Cu3+. The saturation magnetization was attributed to the Cu2+ (3d9, S = 1/2) concentration, and the majority of Cu3+ (3d8, S = 0) ions were stabilized in the low-spin diamagnetic electron configuration similar to that found in mixed-valence layered perovskite superconductor compounds.