Ferrimagnetic and Metallic Properties of (001) NiCo₂O₄ Films Fabricated at Various Oxygen Pressures

Epitaxial (001) NiCo₂O₄ films with perpendicular magnetic anisotropy were grown on (001) MgAl₂O₄ at various oxygen pressures of 10-200 mTorr using pulsed laser deposition. X-ray diffraction suggested that the lattice constant, crystallinity, and deposition rate displayed distinctive changes around 50 mTorr. The temperaturedependent resistance displayed an insulating behavior in the films grown below 15 mTorr but a metallic one in the films grown above 20 mTorr. Magneto-optical Kerr effect measurement suggested that the NiCo₂O₄ films grown above 15 mTorr are ferrimagnetic at room temperature and possess a distinctive perpendicular magnetic anisotropy. The ferrimagnetic-to-paramagnetic transition temperature reached a maximum of ~385 K at 50 mTorr. During the magnetic reversal, the density of small nucleated domains increased with increasing oxygen pressure from 20 to 200 mTorr, and exhibited metallic ferrimagnetism at room temperature. Consequently, the optimal growth condition for magnetic device applications of NiCo₂O₄ films is believed to be 50-200 mTorr at 320 ℃.