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Browsing by Subject "Magnetic recording"

Now showing 1 - 3 of 3
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    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.
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    Heat Assisted Magnetic Recording: Light-Matter Interaction in the Deep Subwavelength Regime
    (2019-01) Ghoreyshi, Ali
    The necessity for low price data storage in emerging Cloud technologies suggests that the Hard Disk Drives (HDDs) will continue to remain the dominant storage technology. To keep up with demands, the hard disk drive industry provided ∼ 40% annual average areal density growth rate over the past 60 years. However, the areal density of current perpendicular magnetic recording technology tends to saturate at ∼ 1.5 Tb/in2. Heat Assisted Magnetic Recording (HAMR) has been considered as the most promising candidate to increase the areal density of HDDs up to 10 Tb/in2. This improvement in the areal density requires employing several aspects of physics - optics, thermal conduction, magnetism, and mechanical control - simultaneously at the smallest regime that a device could operate with the current fabrication technologies. In addition to technological aspects, HAMR provides the opportunity to investigate physical phenomena at the length scale that was rarely investigated before. In this thesis, the light-matter interaction is investigated at the deep subwavelength regime: near the validity limits of macroscopic Maxwell’s equations. First, it is demonstrated that, using plasmonic near-field transducers, light can be focused in an area much smaller than diffraction limit (/100). At this regime, it is demonstrated that the ubiquitous Effective Medium Theory (EMT) breaks down and cannot be used for modeling the interaction between a localized beam and composite structures such as HAMR media. Indeed, the fundamental assumptions of EMT are only valid when the size of the optical beam is much larger than the feature size of composite structures. Instead, by assuming a constant field inside the inhomogeneous phase and neglecting hot spots, an impedance model is proposed for modeling and designing patterned recording media. For the case of the granular recording layer, it is demonstrated that randomness in shape, size, and position of recording grains can lead to localization of depolarization fields. This localization is similar to Anderson localization of electronic wavefunctions in a random potential. In addition, it is demonstrated that for the case of plasmonic particles, random hopping of photons between Localized Surface Plasmonic Polaritons (LSPPs) modes of the system can lead to Anderson localization of the light in the deep subwavelength regime. Finally, the resulting impacts of these localized modes are investigated on the randomness in the absorption of different recording grains. In fact, random absorption can lead to ∼ 3% variation in the temperature of the grains (σT ≈ 3%), which is comparable to the effect of σTc: the common source of transition noise in the recording process.
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    Micromagnetic tests of techniques for reducing pole tip remanence of high density perpendicular write heads.
    (2010-09) Patwari, Mohammed Shariat Ullah
    A multi-scale fast Fourier transform (FFT) based micromagnetic model has been developed to simulate erase after write (EAW) for a 2.4 T FeCo solid pole writer. The simulated remnant state of the writer shows vortices at the pole tip, break and paddle regions that qualitatively matches an experimental MFM image. Dynamic responses show that EAW worsens with a longer breakpoint. Sensitivity of EAW with breakpoint is in good agreement with the experimental data. Modeling suggests that cross track anisotropy reduces EAW risks; however, perpendicular anisotropy is found to be detrimental to EAW. Simulations show that EAW risks are substantially reduced when a uni-polar demagnetization pulse of polarity opposite to that of the last write is applied to the writer. Using the model, the response functions of uni-polar demagnetization pulses have been modeled for reducing EAW events. Simulations show that the value of the initial field created by the demagnetization current is the most effective parameter in reducing pole tip remanence. To avoid driving the head in the opposite direction with the demagnetization pulse, it is important to ramp down quickly with a time constant of about 500 psec. In-plane exchange is found to affect EAW quite significantly; just 25% lower exchange reduces EAW fields by 30%. Modeling shows that introduction of antiferromagnetic coupling in the write pole reduces EAW significantly. Modeling also suggests that non-magnetic holes with in plane dimensions of 35 nm in the middle of the breakpoint region reduce EAW by ~35%. The underlying mechanism to reduce EAW is to make vortex formation in the pole tip energetically inexpensive. Micromagnetics is used to design an unshielded perpendicular writer for an areal density of 1 Tb/in 2 . The head consists of a probe-type tip protruding from a collar. The tip has saturation magnetization ( M S ) of 24 kG while the collar has lower M S . The magnitude and orientation of anisotropy field ( H k ) in the tip is varied to obtain the best recording performance. The combination of high anisotropy write tip with low M S collar is shown to produce effective write fields in excess of 19 kOe and less than 20% track erasure for 10 7 passes. Introduction of in-plane anisotropy within the pole tip reduces head remanence sufficiently that on-track erasure exceeds a benchmark of 10 8 passes. The damping constant in the Landau-Lifshitz-Gilbert equation is varied to improve the frequency response. With damping constant equal to 1.0, simulations show that the head is capable of switching in approximately 0.15 ns.