We investigated the influence of oxygen vacancies on the magnetic and transport properties of ferrimagnetic NiCo2O4 (NCO) epitaxial films. Oxygen vacancies were introduced by annealing under reducing atmospheres NCO films whose cation composition was close to the stoichiometric one. We find that annealing NCO films under the vacuum reduces their magnetizations and increases their electrical resistivities. The perpendicular magnetic anisotropy, on the other hand, is almost unaffected by annealing treatments. X-ray absorption spectroscopy shows that oxygen vacancies introduced in NCO films preferentially lower the Ni valence state while leaving the Co valence state unchanged. The lowering in the Ni valence state explains the reduced magnetizations and the increased resistivities for the films annealed under the vacuum. On the other hand, the Co valence state, which dominantly determines the orbital magnetic moments responsible for magnetic anisotropy, is insensitive to oxygen vacancies. Therefore, the perpendicular magnetic anisotropy is maintained even when the oxygen vacancies are introduced.

中文翻译：

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氧空位对垂直磁化NiCo2O4外延薄膜磁性能的影响

我们研究了氧空位对亚铁磁性 NiCo2O4 (NCO) 外延膜的磁性和传输特性的影响。氧空位是通过在还原性气氛下退火 NCO 膜引入的，其阳离子组成接近化学计量的。我们发现在真空下退火 NCO 薄膜会降低它们的磁化强度并增加它们的电阻率。另一方面，垂直磁各向异性几乎不受退火处理的影响。X 射线吸收光谱表明 NCO 薄膜中引入的氧空位优先降低 Ni 价态，同时保持 Co 价态不变。Ni价态的降低解释了在真空下退火的薄膜的磁化强度降低和电阻率增加。另一方面，Co价态主要决定负责磁各向异性的轨道磁矩，对氧空位不敏感。因此，即使引入氧空位，垂直磁各向异性也得以保持。