Between Dec 19, 2024 and Jan 2, 2025, datasets can be submitted to DRUM but will not be processed until after the break. Staff will not be available to answer email during this period, and will not be able to provide DOIs until after Jan 2. If you are in need of a DOI during this period, consider Dryad or OpenICPSR. Submission responses to the UDC may also be delayed during this time.

Browsing by Subject "FePt"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    FePt based advanced magnetic recording media
    (2013-02) Wang, Hao
    In future extremely high density magnetic recording, FePt is considered as a promising candidate for future recording media materials. In this thesis work, FePt media with composite structure have been systematically studied in the forms of both granular media and bit patterned media (BPM). Continuous FePt films with surface roughness of less than 0.3 nm are achieved in FePt hard magnetic films, exchanged coupled composite (ECC) films and graded films. Nanoimpriting and block-copolymer lithography are employed to fabricate BPM. The switching field distribution (SFD) broadening and degradation of FePt BPM are studied. The reduction of SFD has been achieved using a post-annealing process. Both ECC and graded FePt BPM with sub-30 nm dot size have been experimentally demonstrated on large substrates for the first time. It is confirmed that the patterned graded BPM sample has smaller switching field and larger thermal energy barrier than the ECC sample does. Ultra-thin FePt granular media with graded composition was directly fabricated using a spontaneous layer diffusion process between the FePt and Pt layers during film deposition. A large gain factor of 3.74 was found in this spontaneously formed FePt graded granular media. A nanopatterning process, named as the Embedded Mask Patterning (EMP), is proposed and experimentally demonstrated based on the FePt magnetic recording media. In this process the granular structure is defined by a sputtering-deposited mask layer, while the magnetic properties are determined by the FePt continuous film. Grain size can be decreased by optimizing the mask layer only. A non-ideal surface anisotropy effect has been observed on the magnetization reversal process of both L10 phase FePt nanoparticles, and (001) textured L10 FePt thin film with island structure. The broken symmetry of the surface creates surface anisotropy and also weakens the exchange coupling. The elimination of the surface effect has been experimentally demonstrated by epitaxially capping a Pt layer on FePt. After being embedded in a Pt matrix, the exchange coupling between the surface portion and internal portion of FePt islands was enhanced.
  • Thumbnail Image
    Item
    Magnetic Thin Films With High Perpendicular Anisotropy For Magnetic Recording Media Applications
    (2014-08) ZHAO, HAIBAO
    In order to meet the basic thermal stability requirement in future extremely high density magnetic recording, magnetic materials with high magnetocrystalline anisotropy constant (Ku) are needed. In this thesis work, the two most-promising candidates of high Ku materials, i.e. CaCu5-type Sm(Co, Cu)5 and L10-type FePt thin films, have been systematically studied. SmCo5 has the highest Ku among practical magnetic materials. Prior studies on SmCo5-based thin films for magnetic recording media applications is reviewed. SmCo5 thin films with good perpendicular magnetic anisotropy were only grown on Cu underlayer. However, the uncontrollable Cu diffusion from the Cu underlayer and the relatively large Cu grain size make it not suitable for magnetic recording applications. In this study, polycrystalline Sm-Co-Cu films consisting mainly of highly (0001) textured Sm(Co, Cu)5 grains have been successfully fabricated on non-Cu containing underlayers (Ru or Ru(Cr)) on glass substrates. Strong perpendicular magnetic anisotropy of Sm(Co, Cu)5 thin films was achieved. It was found that increasing Cu is like increasing deposition temperature - both could improve the SmCo5 phase formation and the crystallinity of Sm(Co, Cu)5 (0001) films. The median composition of nanocrystalline Sm(Co, Cu)5 grains estimated from structural unit volume and Curie temperature matches reasonably well with each other. In-plane compressive strain in Sm(Co, Cu)5 films is inferred from the differences of lattice constants between the thin film and bulk material. The use of Ru(Cr) underlayer with a proper Cr doping could improve the perpendicular anisotropy of Sm(Co, Cu)5 films. Microstructure of Sm(Co, Cu)5 thin films was studied using SEM, AFM, and TEM analysis. The grain size of Ru underlayer is about 10-30 nm, much smaller than that of Cu underlayer (~200 nm) reported in the literature. Magnetization reversal in Sm(Co, Cu)5 thin films is dominated by the domain wall pinning mechanism. Many types of pinning sites are found: voids, grain/matrix boundaries (or crystalline/amorphous boundaries), grain boundaries between crystalline grains, and the composition inhomogeneity in grains. The key finding of TEM elemental mapping analysis is that Cu atoms were found to be rich in the inner part of Sm(Co, Cu)5 grains or particles, instead of the outer part, such as grain boundaries or edges of voids. Cu served as an alloying element in Sm(Co, Cu)5 grains, not as a doping element to form Cu-rich grain boundaries. A model of Sm(Co, Cu)5 films with in-plane graded anisotropy due to composition/crystallization variation can explain the huge difference between the Hc and HK as well as the angular dependences of coercivity and remanence coercivity. A simple analytical expression of the angular dependence of switching field for graded media has been derived and shown to match well with experimental results. Chemical stability of Sm(Co, Cu)5 thin films has been studied. Ta-capped Sm(Co, Cu)5 thin films are stable in terms of structural and magnetic properties in a normal laboratory environment (25 °C) over 3 years, but they did not pass the accelerated corrosion test (130°C, 95%RH, 6 hours). A capping layer consisting of a hcp-phased CoPt-alloy layer and carbon overcoat should help Sm(Co, Cu)5 thin films meet the requirements for future high-density magnetic recording applications. L10-type FePt thin films were studied as perpendicular ECC media with potentially high gain factor due to domain-wall assisted magnetic switching. Ultra-thin exchange-coupled-composite (ECC) FePt granular recording media with different soft layer anisotropy were fabricated by controlling the soft layer deposition temperature. The structural and magnetic properties of soft layers (FePt-SiO2) confirmed the feasibility of controlling the soft layer anisotropy by changing its deposition temperature. The effect of soft layer anisotropy field on the coercivity (Hc) and the remanent coercivity (Hcr) of ECC FePt thin films showed a "V" shape relationship, with the minimums at TSoft of 200 °C. It is consistent with the theoretical prediction based on domain wall assisted magnetization reversal mechanism. The ECC FePt thin film with TSoft of 200 °C may achieve a gain factor larger than 2.
  • Thumbnail Image
    Item
    Micromagnetic study of heat assisted magnetic recording using renormalized media cells
    (2014-05) Huang, Pin-Wei
    Perpendicular magnetic recording is currently very near to its physical limit, making it difficult for researchers to keep the pace of the growth of areal density of hard disk drives. Heat-assisted magnetic recording (HAMR) is considered to be the next generation technology for magnetic recording beyond 1 Tb/in2. Complete understanding of HAMR processes is necessary to optimize the design parameters. In this thesis, current state-of-the-art modelling methods are developed aiming at HAMR recording. First a simple torque-based method for calculating the transient behavior of temperature-dependent magnetic anisotropy is introduced. By using this method several physical quantities at finite temperature including effective anisotropy, anisotropy field, and their fluctuations are obtained. A composite grain that includes a high Curie temperature soft layer can reduce the anisotropy fluctuations. Then a new scheme for the simulation of HAMR that systematically includes fluctuating material properties above a predefined length scale, while retaining magnetostatic interactions is introduced. Renormalized media parameters, Ms, Ku, Aex and &alpha, suitable for useful length scales, are found numerically. These renormalized parameters are then used to model the Voronoi-cell-composed medium in the HAMR simulation. Transition jitters are obtained under various conditions. The results show that moderate maximum temperature of the heat spot, intergranular exchange coupling, media thickness of at least 10 nm, nonzero canting angle of the head field, relatively low head velocity, and large head-field strength are helpful for a successful recording. This scheme of HAMR simulation is used to find the dependencies of recording performance on the grain size and damping. The simulated results are used to compare with an experimental demonstration. Finally, composite FeRh/FePt for HAMR media is investigated with micromagnetic simulation. It is found to potentially lower recording temperature, while retaining high anisotropy field gradient. The transition width is predicted to depend on the media cooling rate. The thickness of the FeRh layer and the applied field can significantly affect the switching time of FePt layer, and therefore alter recording performance. Applied field magnitudes and angles are identified that allow successful switching within 100 pS. It is shown that by using up to 15 nm of FeRh with 6 nm of FePt, the jitter for 5.6 nm grains can be nearly equal to the grain-size limited value, for head velocities as high as 20 m/s.