The crystal structures and magnetic properties of the 40 nm brownmillerite SrMnO2.5 film, perovskite SrMnO3-δ film, and mixed-phase film have been systematically investigated. The features of the oxygen vacancy ordering superstructure in the brownmillerite SrMnO2.5 film are observed from HRSTEM as follows: the dark stripes with a periodicity of four (110) planes of the cubic perovskite appearing at an angle of 45° with the substrate-film interface and extra reflection spots in fast Fourier transformation patterns along the (001) plane. When annealing the brownmillerite SrMnO2.5 film under higher oxygen pressure, the top portion undergoes structure transition into perovskite SrMnO3-δ as seen in the mixed-phase film consisting of the perovskite SrMnO3-δ phase dominating at the top part and the brownmillerite SrMnO2.5 phase dominating at the bottom part. The magnetic properties and Mn valences of the brownmillerite SrMnO2.5 film indicate that this film, similar to the bulk, is antiferromagnetic with TN at 375 K. However, the strained tetragonal perovskite SrMnO3-δ film exhibits ferromagnetic behavior with Curie temperature at 75 K and a saturation magnetization of 2.5μB/Mn at 2 K. Moreover, the top perovskite SrMnO3-δ phase of the mixed-phase film also exhibits ferromagnetic behavior evidenced by the existence of the exchange bias. We propose that the ferromagnetic properties in both the perovskite SrMnO3-δ film and the top perovskite SrMnO3-δ phase in the mixed-phase film originate from Mn3+–Mn4+ double exchange coupling. However, the formation of Mn3+ differs for the two samples in that it is caused by oxygen vacancies in the former and the distribution of oxygen content across the film during annealing in the latter.
The magnetic and optical properties of multiferroic GdMnO 3 nanoparticles synthesized by a polymerized complex method have been investigated. The GdMnO 3 nanoparticles crystallized in orthorhombic perovskite-type structure. The zero-field-cooled and field-cooled magnetizations show that complicated magnetic transitions occur in a temperature range from 2 to 60 K, which were confirmed by magnetic hysteresis loops. Three shoulder absorption peaks centered at about 2.0, 2.3, and 2.7 eV are attributed to the Mn ͑3d͒-electronic transitions, while an absorption peak at around 4.1 eV corresponds to the charge-transfer transitions between O ͑2p͒ and Mn ͑3d͒ states. UV emission at about 396 and 406 nm and blue emission at around 466 nm have been found, which may be attributed to the spin-allowed charge-transfer transitions.
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