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Microstructure and physical properties of Sr2CrHfO6 ferrimagnetic double-perovskite oxides

Qingkai Tang,Xinhua Zhu

Half-metallic ferromagnets (HMFs) are highly desirable materials for applications in spintronics because of their semiconducting behavior for one spin projection and metallic nature for the other. To operate spintronic devices at room temperature (RT), HMFs should simultaneously have a large saturation magnetization ( M S ), high magnetic Curie temperature ( T C ), and a wide half-metallic gap. However, HMFs that simultaneously fulfill these three criteria are rare. In this work, we report for the first time on the ferrimagnetic double perovskite oxides of Sr 2 CrHfO 6 (SCHO) with a magnetic T C up to 545 K due to the strong Cr 3+ (↑)Cr 4+ (↓) antiferromagnetic spin interactions. The combined experimental results confirmed that the SCHO powders crystallized in an orthogonal crystal structure with Pnma symmetry. The SEM images demonstrate that the powders exhibit a spherical morphology with particle sizes between 150 and 250 nm. XPS spectra verified the presence of Sr 2+ , Cr 3+ /Cr 4+ , and Hf 4+ /Hf x+ (x < 4) ions in the SCHO powder. Oxygen was present as lattice oxygen and adsorbed oxygen species. At 2 K, the SCHO powder exhibited ferrimagnetic behavior with M S of 0.11 μ B /f. u and a magnetic coercive field of 290 Oe. The SCHO ceramics displayed a normal butterfly-like magnetoresistance-magnetic field (MR-H) curve at 2 K due to the intergranular tunneling effect, and the MR (2 K, 7 T) value was −2.05%. The temperature dependence of the resistivity of the SCHO ceramics revealed their semiconducting nature, and the electrical transport data in the temperature range 2−800 K were well fitted by the Mott's variable-range hopping model, thermal activation model, and small polaron hopping model. Optical measurements demonstrate that the SCHO powders have a direct optical bandgap of 2.25 eV in the visible light window. Having both high T C and a direct optical bandgap (2.25 eV) makes SCHO oxides attractive for use in advanced spintronic devices and solar cells.