Powered by Indico
v3.2.8
Title: 준강자성 스피넬 산화물 NiCo$_2$O$_4$ 박막의 격자변형과 자기 이방성 연구 Lattice strain and magnetic anisotropy of ferrimagnetic spinel oxide NiCo$_2$O$_4$ epitaxy thin film
The ferrimagnetic NiCo2O4 films were grown on (0001)Al2O3, (110)MgAl2O4, and
(001)MgAl2O4 at various oxygen pressures PO2 and deposition temperatures by using pulsed laser deposition. Their structural, magnetic and electrical properties have been studied. First, hetero-epitaxy growth conditions with the change of substrate temperature and PO2 on the (0001)Al2O3 substrate were confirmed by X-ray diffraction measurements. Below 20 mTorr, not only (222) peak but also (311) peak were also observed, implying a growth of a polycrystalline film. As the oxygen pressure increased to 100 mTorr and the substrate temperature decreased to 350 °C, the (311) peak remarkably disappeared, indicating that an optimal growth condition is around 350°C and 100 mTorr. In addition, it was confirmed that the ferrimagnetic property could be improved by a post-annealing at 500 °C in oxygen environment to reduce the oxygen defect.
Second, the characteristics of the (110)NiCo2O4 films on (110)MgAl2O4 substrate were investigated with the changes of deposition temperature and PO2. When the (110)NiCo2O4 film was deposited at 350°C, it exhibited the most distinctive uniaxial magnetic anisotropy within the film plane. As the oxygen pressure was increased, the out-of-plane lattice constant,
the coercivity, and the ferrimagnetic-to-paramagnetic transition temperature of the (110)NiCo2O4 film displayed increasing tendencies. This means that the PO2 in the deposition process crucially affects on the ferrimagnetic property of the NiCo2O4 film as well as on the lattice strain of the film.
Third, the (001)NiCo2O4 film was grown on (001) MgAl2O4 substrate at various PO2 and substrate temperature. In the case of the films at 320°C, the (001) peak position in the XRD data was gradually shifted to a high angle position without a change of peak intensity as the oxygen pressure increased from 10 mTorr to 20 mTorr. Above 50 mTorr, the peak position and intensity were almost constant. This is presumably due to a decrease of oxygen vacancies with increasing the PO2 during the deposition process. On the other hand, the (001)NiCo2O4 films deposited at various temperatures displayed a similar thickness of about 30 nm, but their out-of-plane lattice constants increased as the deposition temperature increased. In addition, all the films deposited at 50 - 200 mTorr and 250 - 350°C displayed ferrimagnetic and metallic properties. Finally, a heterojunction structure of Co/Pt multilayer and (001) NiCo2O4 film which have perpendicular magnetic anisotropy were prepared to observe the exchange coupling at the interface. A further study is required to make an optimal perpendicular exchange coupling heterostructure.
Supervisor: Prof. Joonghoe Dho