| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 92 === Abstract
Ti and MgO underlayer are deposited on corning glass and Si substrate by dc and rf magnetron sputtering, then deposited high magnetocrystalline anisotropy FePt films as magnetic layer on the underlayer. The effects of various underlayers and process parameters on the microstructures, magnetic properties, and easy axis of nanocomposite Ti/FePt, MgO/FePt/MgO, MgO/FePt/Ag, and MgO/(FePt/Ag)5 multilayer films are investigated.
X-ray diffraction and Magnetic properties measurements indicate that the Ti underlayer can promote (111) preferred orientation of FePt magnetic layer. The average grain size, degree of order, and in-plane coercivity (Hc∥) of FePt layer are increased with increasing the Ti underlayer thickness. When the 300 nm thick FePt magnetic layer is deposited on corning glass with substrate temperature of 600℃, the optimum in-plane magnetic properties of the Ti/FePt film are obtained as introducing 100 nm thick Ti underlayer. This Ti/FePt film has Hc∥of 7.3 kOe, saturation magnetization (Ms) of 672 emu/cm3, and its in-plane squareness (S∥) is about 0.8.
Intensity of the (200) peak in the X-ray diffraction pattern of MgO underlayer increases as introduction of N2 gas during deposition of MgO. The maximum intensity of the MgO (200) peak is achieved at N2 flow rate ratio of N2 : Ar = 2 : 5. This MgO underlayer has better crystallinity and its lattice parameter is 4.255 Å. After annealing at substrate temperature of 600℃, the lattice parameter of MgO underlayer will decrease to about 4.198 Å due to stress relief. This is beneficial for reducing interfacial misfit between (200) of MgO underlayer and (001) of FePt magnetic layer.
The (111) preferred orientation of FePt magnetic layer can be changed by introducing a thinner MgO underlayer (≦5nm). Furthermore, Ag atoms can diffuse into FePt grain boundary at higher temperature, which will increase greatly grain boundary energy. Therefore, both the perpendicular anisotropy and coercivity of FePt layer increase by using a Ag capped layer on the MgO/FePt double layer films. A in-plane coercivity of 3923 Oe can be obtained from MgO 5nm/FePt 20nm/Ag 5nm films even heated at a lower substrate temperature of 400℃ for 30 min.
The FePt films with (001) preferred orientation and out-plane coercivity (Hc⊥) of about 2462 Oe can be achieved by stacking structure of (FePt 4nm/Ag 2nm)5 multilayer films deposited on the 5 nm thick MgO underlayer after annealing at substrate temperature of 600℃for 30 min. It is found that both the perpendicular anisotropy and coercivity of (FePt 4nm/Ag 2nm)5 multilayer films are enhanced by introducing a Ag buffer layer (≦20nm) between the (FePt 4nm/Ag 2nm)5 films and MgO underlayer. The high-resolution cross-sectional lattice image shows that the interface between FePt magnetic layer and MgO underlayer is a semi-coherent interface, which has a lower strain energy. However, as introducing Ag buffer layer between FePt magnetic layer and MgO underlayer, a coherent interface associated with higher strain energy is obtained at interface between FePt magnetic layer and Ag buffer layer. Therefore, introduction of Ag buffer layer between FePt magnetic layer and MgO underlayer will promote the formation of hard magnetic fct ��1-FePt phase, and enhances the coercivity of FePt layer. When introducing an Ag buffer layer of 20nm thickness, the Hc⊥value of the MgO 5nm/Ag 20nm/(FePt 4nm/Ag 2nm)5 multilayer films can be increased from 2462 to 4731 Oe, which has significant potential as perpendicular magnetic recording media for high-density recording .
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