Browsing by Subject "Composite media"

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    Micromagnetic analysis of co-based magnetic nanostructures.
    (2010-12) Hernandez, Stephanie
    Micromagnetic analysis was employed in order predict the dynamical behavior of a variety of magnetic structures utilized in information storage devices. First, the surprising behavior of homogeneous perpendicular recording media was micromagnetically investigated. It is common to model recording media as interacting coherently rotating magnetic moments, but real materials frequently exhibit perpendicular switching fields less than the anisotropy field and a different angular dependence than theoretically expected. Micromagnetic simulations were performed, which included multiple elements per grain and magnetostatic interactions between elements. Two likely explanations have emerged from this analysis: the existence of low anisotropy regions within the first few atomic layers of the sputtered film or anisotropy gradation throughout the grain thickness. Both explanations offer appropriate coercivity reductions; however, grains including anisotropy gradation display this effect at more realistic values of intragranular exchange. Secondly, the lack of inclusion of spin-dependent scattering effects in most micromagnetic studies was addressed in this work. An analytic expression that includes the effect of multiple reflections within the interface of a tri-layer spin-valve composed of materials with partial spin polarization was obtained. Inclusion of this term in a micromagnetic calculation demonstrates the effect of the spin polarization of the magnetic material on the current induced behavior of the structure. We show that neglecting to include interfacial scattering events results in an underestimation of the switching current compared to the method detailed in this thesis. Multiple reflections also produce a strong dependence of the switching current on the magnetocrystalline anisotropy of the fixed layer. This approach was then extended to structures consisting of more than two ferromagnetic layers. Micromagnetic calculations employing this method achieved good agreement with electrical measurements performed on Co/Cu multilayer nanowire arrays.
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    Micromagnetic Study of Composite Media for High Density Heat Assisted Magnetic Recording
    (2017-06) Liu, Zengyuan
    The hard disk drive industry is a market with around 25 billion revenue each year. The annual average areal density growth rate is about 40% before 2012. With cloud computing and storage technology emerge, hard disk drives with high area density and low price are required. However, the areal density of current perpendicular magnetic recording technology tends to saturate at 1 Tb per square inch. Therefore, new technologies like Heat Assisted Magnetic Recording (HAMR) are needed. On the other hand, the Solid State Drive (SSD) has developed quickly as another candidate for high density and high speed information media which makes this situation urgent. In this thesis, micromagnetic simulations of HAMR media are conducted based on the renormalized Landau-Lifshitz-Gilbert (LLG) method. L10 FePt is one promising recording media candidate for HAMR. The transition noise and transition jitter are calculated through magnetic recording simulation accelerated by GPU parallel computing. Thermal fluctuations and Curie temperature variance are verified to be two import noise sources for FePt recording media besides the grain size distribution and anisotropy variance. A more easily implemented method called thermal switching probability distribution (SPD) is proposed. It can provide two important factors for evaluating the recording performance: 𝜎𝑆𝑃𝐷 and write temperature. Under a certain fabrication technology (certain average grain size), the transition jitter can be estimated by 𝜎𝑆𝑃𝐷 . Furthermore, the grain volume dependence of 𝜎𝑆𝑃𝐷 and write temperature are investigated. The dependence follows 1 ⁄ 𝑉 and 1 ⁄ √𝑉 power law respectively. This knowledge greatly helps the noise analysis and new media design. To mitigate the noise from thermal fluctuations and Curie temperature variance, a new composite media design based on a bilayer structure with two different Curie temperatures is proposed. The substantial anisotropy of the write layer differentiates this design with respect to previous work. This ensures that the composite structure has small transition noise and high areal density. The user density can reach 3.4 Tb per square inch under traditional recording and 4.7 Tb per square inch with shingled magnetic recording (SMR) technology. The interlayer exchange coupling effects are found to affect the energy barrier during the dynamic recording process. Both the thermal effects and write temperature can be tuned by optimizing the interlayer exchange coupling effects. Further research verified that this is due to the linear temperature dependence of energy barrier at temperatures close to Curie temperature. More research need to be done to offer a good explanation of the high areal density achieved by PMR-ECC-like structure for HAMR. Possible directions include the effective field temperature gradient and switching speed during the recording process as the temperature changes.