An experimental approach for obtaining perpendicular FePt-SiOx thin films with a large height to diameter ratio FePt(L1 0 ) columnar grains is presented in this work. The microstructure for FePt-SiOx composite thin films as a function of oxide volume fraction, substrate temperature, and film thickness is studied by plan view and cross section TEM. The relations between processing, microstructure, epitaxial texture, and magnetic properties are discussed. By tuning the thickness of the magnetic layer and the volume fraction of oxide in the film at a sputtering temperature of 410 C, a 16 nm thick perpendicular FePt film with $8 nm diameter of FePt grains was obtained. The height to diameter ratio of the FePt grains was as large as 2. Ordering at lower temperature can be achieved by introducing a Ag sacrificial layer. FePt(L1 0 ) is considered to be one of the most promising materials for ultrahigh density recording applications. [1][2][3][4] For achieving high density, the film media needs to be granular in nature, of small and uniform sized grains with a nonmagnetic phase at the grain boundaries. [5][6][7] Well defined carbon regions at the grain boundaries with small magnetic grain sizes have been achieved in FePt(L1 0 )-C films. However, non-columnar film microstructures in these films leads to a layer with new secondary grains, leading to undesired magnetic properties. 8 The difficulty in obtaining columnar growth of L1 0 FePt grains with large height to diameter ratios lies in perhaps the high surface energy associated with the material choice.In this work, an experimental method for obtaining perpendicular FePt-SiOx thin films with large height to diameter ratio FePt(L1 0 ) columnar grains is presented. It was found that the microstructure of composite films can be influenced by not only the deposition pressure, temperature, and oxide volume fractions, but also the choice of matrix material and thickness of the thin film.FePt/oxide multilayers and MgO/Ta underlayers were deposited on Si substrates by RF sputtering. The base pressure was approximately 5 Â 10 À7 Torr and the argon pressure during deposition was maintained at 10 mTorr. The MgO/Ta underlayers were deposited at room temperature; the FePt/ oxide multilayers were fabricated by the alternating sputtering of the Fe 55 Pt 45 alloy and the SiOx targets on a heated substrate. The substrate temperature was varied between 370 C to 475 C during the multilayer deposition. X-ray diffraction and transmission electron microscopy were used to study the texture and microstructure of the films. An alternating gradient force magnetometer (AGFM) and a polar magneto-optical Kerr effect looper (MOKE) were used to investigate the magnetic properties.The cross section TEM images in Fig. 1 show the microstructure of FePt-SiOx composite films as a function of oxide volume fraction (OVF). For this set of samples, the FePt/ SiOx multilayers deposition temperature was maintained at 410 C and the FePt film thickness was fixed at 16 nm. As shown in the figure, in the low OVF f...
Cross-plane thermal conductivity kth measurements of vertical stacks of FePt/C were used to estimate the in-plane thermal conductivity of Heat Assisted Magnetic Recording (HAMR) media that consist of columnar FePt grains segregated by thin C grain boundaries. FePt/C multilayers with varied repeat units and FePt layer thicknesses (chosen to represent HAMR media grain sizes) were measured using Frequency-Domain Thermoreflectance to determine kth in the direction normal to the layers. The data suggest that when FePt grains are less than 8 nm in diameter, the in-plane kth for HAMR media is below 1 W/m-K and the anisotropy of kth (cross-plane/in-plane) will exceed 10.
An approach to enhance the height-to-diameter ratio of FePt grains in heat-assisted magnetic recording media is proposed. The FePt-SiO x thin films are deposited with a decrease of the SiO x percentage along the film growth direction. When bi-layer and tri-layer media are sputtered at 410 C, we observe discontinuities in the FePt grains at interfaces between layers, which lead to poor epitaxial growth. Due to increased atomic diffusion, the bi-layer media sputtered at 450 C is shown to (1) grow into continuous columnar grains with similar size as single-layer media but much higher aspect ratio, (2) have better L1 0 ordering and larger coercivity. Considerable attention has been given to L1 0 FePt as the most probable material for the next generation magnetic recording media due to its high magnetocrystalline anisotropy of $5-7 Â 10 7 erg/cm 3 . Achieving good grain morphology, strong perpendicular (001) texture, and high chemical ordering still remain challenging for its commercial applications to be realized. Control of grain morphology including small grain size with a narrow size distribution, good boundary isolation, and exchange decoupling has been attempted by adding segregants into the FePt magnetic thin films. Various segregants such as C
We present a study on atomic ordering within individual grains in granular L10-FePt thin films using transmission electron microscopy techniques. The film, used as a medium for heat assisted magnetic recording, consists of a single layer of FePt grains separated by non-magnetic grain boundaries and is grown on an MgO underlayer. Using convergent-beam techniques, diffraction patterns of individual grains are obtained for a large number of crystallites. The study found that although the majority of grains are ordered in the perpendicular direction, more than 15% of them are multi-variant, or of in-plane c-axis orientation, or disordered fcc. It was also found that these multi-variant and in-plane grains have always grown across MgO grain boundaries separating two or more MgO grains of the underlayer. The in-plane ordered portion within a multi-variant L10-FePt grain always lacks atomic coherence with the MgO directly underneath it, whereas, the perpendicularly ordered portion is always coherent with the underlying MgO grain. Since the existence of multi-variant and in-plane ordered grains are severely detrimental to high density data storage capability, the understanding of their formation mechanism obtained here should make a significant impact on the future development of hard disk drive technology.
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