Oxygen Pressure Dependence of Lattice Strain and Magnetic Anisotropy in Ferrimagnetic NiCo₂O₄ (110) film

Epitaxial nickel cobaltite (NiCo2O4) (110) films were grown on magnesium aluminate (MgAl2O4) (110) at oxygen pressures ranging from 5 to 200 mTorr using pulsed laser deposition. Using X-ray diffraction, a shift in the (110) peak to a lower angle was suggested to have occurred as the oxygen pressure increased with a relatively large change in interplanar spacing at approximately 20 mTorr. The rocking curve supported the view that NiCo2O4 films had high crystallinity, and atomic force microscopy images suggested that films had few irregularities below 1 nm. The measurement of the magneto-optic Kerr effect suggested that the NiCo2O4 films were ferrimagnetic at room temperature and possessed a distinctive uniaxial in-plane magnetic anisotropy. As the oxygen pressure increased, the ferrimagnetic-to-paramagnetic transition temperature increased from ∼305 to ∼365 K. All of the films displayed metallic and ferrimagnetic properties. Consequently, an increase in oxygen pressure is likely to induce a decrease in oxygen defects and an increase in nickel(III) ion occupation of the octahedral sites, resulting in strengthening of the ferrimagnetic properties. In addition, this will presumably enhance lattice strain, resulting in a reinforcement of the magnetic anisotropy due to spin-lattice coupling.