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  4. Fabrication of low ordering temperature L10 FePt thin films and study of its application in perpendicular magnetic recording media
 
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Fabrication of low ordering temperature L10 FePt thin films and study of its application in perpendicular magnetic recording media

Date Issued
2005
Date
2005
Author(s)
Sun, An-Cheng
DOI
zh-TW
URI
http://ntur.lib.ntu.edu.tw//handle/246246/55370
Abstract
There are three subjects in this dissertation, we illustrate them below: 1. Fabrication of low ordering temperature L10 FePt thin films Polycrystalline Fe52Pt48 alloy thin films were prepared by dc magnetron sputtering on preheated natural-oxidized silicon wafer substrates. The film thickness was varied from 10 to 200 nm. The as-deposited film was encapsulated in a quartz tube and post-annealed in vacuum at various temperatures for 1 hour followed by furnace cooling. Before annealing treatment, the structure of the as-deposited FePt thin films is in fcc FePt phase with ~4 nm grain size. L10 FePt phase with hard magnetic properties began to occur after annealing at 350 oC. FCC FePt phase transform completely into L10 FePt phase after annealing at 400 oC. Therefore, the ordering temperature from as-deposited soft magnetic fcc FePt phase to hard magnetic fct L10 FePt phase could be reduced down to about 350 oC by preheating substrate and furnace cooling treatment. The ordering rate of L10 FePt phase increases with increasing the annealing temperature. The ordering rate is non-existent at 300 oC. However, it is enhanced dramatically when temperature is raised above 300 oC and becomes nearly fully completed at 400 oC. From TEM bright images, it is clear that by lowering the ordering temperature it helps reduce the grain size of L10 FePt thin films. Grain grows from ~6 nm to ~9 nm when the annealing temperature increases from 350 oC to 400 oC. The magnetic properties measurements indicated that the in-plane coercivity of the films increase rapidly as annealing temperature is increased from 300 oC to 400 oC, but decrease when the annealing temperature is higher than 400 oC. The grain growth, in-plane coercivity, ordering rate, and formation of L10 FePt phase are impeded in thinner films. The critical order-disorder transformation in grain size of FePt is about 4.5 nm. L10 FePt phase and hard magnetic properties only occurred when the grain size is larger than 4.5 nm. After annealing at 400 oC, the in-plane coercivity of Fe52Pt48 thin film with film thickness of ~100 nm is 10 kOe, Ms is 580 emu/cm3, and grain size is about 9 nm. Factors responsible for reducing the order-disordering temperature of L10 FePt phase include higher sputtering rates, lower annealing temperature accompanied with a longer annealing treatment, and substrate preheating. Among these three factors, the higher sputtering rate of FePt plays the dominant role to help reduce the ordering temperature. 2. Study of perpendicular anisotropy of L10 FePt thin films FePt/Pt/Cr trilayer thin films with perpendicular magnetic properties were deposited on amorphous Corning 7059 glass substrate and natural-oxidized silicon (100) wafer substrates. Before sputtering, the base pressure of the sputtering chamber is better than 5×10-9 Torr. The perpendicular anisotropy of FePt(001) texture was not discernible right after depositing the FePt magnetic layer on top of the Cr(200) underlayer. The Cr-rich epitaxial barrier will be formed at FePt/Cr interface distorting the epitaxial growth of FePt(001) magnetic layer above the Cr(200) underlayer. After inserting a Pt buffer layer at the FePt/Cr interface, perpendicular magnetic properties with FePt(001) preferred orientation was observed. Squareness of the L10 FePt film was close to 1 when a magnetic field was applied perpendicular to the film plane. Pt buffer layer serves as a good barrier to impede diffusion of Cr into the FePt layer and modulate the lattice misfit between Cr underlayer and FePt magnetic layer. Semi-coherent epitaxial growth was initiated from the Cr (002) underlayer, continued through the Pt buffer layer and extended into the L10 FePt (001) magnetic layer. In this investigation, the ordered FePt phase in FePt/Pt/Cr film was found to show up at 250 oC substrate temperature. As the substrate temperature is increased to 300 oC, perpendicular texture and magnetic properties of L10FePt(001) become firmly established. Thus, the formation temperature of the ordered FePt(001) preferred orientation can be identified as low as ~300 oC. This is an important fact may be proven to be very useful for practical industrial perpendicular recording media application. 3. The effects on inserting a soft stability layer in perpendicular FePt/Pt/Cr trilayer thin films When a FeTaC soft layer was inserted in FePt/Pt/Cr trilayer thin film, the FePt(001) preferred orientation of the FePt/Pt/Cr trilayer thin films become distorted. Both Cr(200) and FePt(001) orientations were enhanced by adding a Si3N4 layer between FeTaC layer and Cr underlayer. FePt(001) texture will be preserved if Fe is used as the soft layer. But, the dominant magnetic properties of the films so prepared were longitudinal. This is due fact that there exists strong exchange coupling between soft and hard magnetic phase and very large shape anisotropy in the film. If a CoCr layer was used instead as the soft layer, the intensity of FePt(001) orientation will be reduced while the that of FePt(111) orientation enhanced. The perpendicular magnetic properties of FePt/Pt/Cr were distorted when CoCr is used as the soft layer. When a Cr intermediate layer was inserted at the interface of Pt/CoCr, the Cr intermediate layer may grow along the CoCr(11-20) texture and obtain the Cr(200) preferred orientation. Thus, depositing FePt/Pt bilayer films on this new Cr intermediate layer will exhibit FePt(001) texture and obtain perpendicular magnetic anisotropy properties again.
Subjects
FePt
磁性
磁記錄
垂直磁性質
magnetic
magnetic recording
perpendicular magnetic properties
Type
thesis

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