Browsing by Subject "Hard Disk Drive"

Now showing 1 - 4 of 4
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    Computational Studies on Magnetic Information Storage Devices and Systems
    (2020-01) Ahmed, Rizvi
    Micromagnetics and information technology, combined, has emerged as a flourishing field in the area of digital data management. In order to explore new ideas in a flourishing field, computational studies are essential alongside experimental implementations. In this thesis, some of the physics-based computational techniques have been utilized to investigate underlying principles, propose new design, and compare different technologies, in the area of applied magnetism. Most of the computations have been performed on the micromagnetic simulation platform. Readily available commercial CAD tools in the area of magnetism/spintronics are not customizable enough to study specific physical phenomena, with insightful details. Magnetism related phenomena, being originated from quantum mechanics, cannot be analyzed accurately with classical/SPICE models. Hence, most of the studies in this thesis were performed using self-developed/customized physics-based simulation frameworks. Over the course of time, the thesis has branched into three different projects. In the first project, the effectiveness of voltage assisted switching in a large-area magnetic tunnel junction (MTJ) has been explained. More specifically, micromagnetic simulation has been performed to investigate the possible reason behind the observed large change in the switching field with applied voltage in experimental MTJs. It has been found that the film roughness plays a significant role in boosting the intrinsic effect of an applied electric field in experimental devices, with dimensions in the sub-micron range. The second project of the thesis involved voltage-controlled exchange bias (VCEB) in hetero-structures with magnetoelectric (ME) Cr2O3, and an exchange-coupled ferromagnet (FM). Monte Carlo simulation has been employed to study temperature-dependent magnetic properties of the ME Cr2O3. Using the calculated temperature-dependent properties, exchange bias (EB) mechanism has been explained in a ME Cr2O3/FM system. It has been pointed out that domain formation inside the ME Cr2O3 most probably explains the experimentally observed temperature dependence of the EB. As an extension to the VCEB project, a fully electric-controlled magnetoelectric switching device has been proposed. The proposed device addresses the obvious challenges faced by the traditional ME Cr2O3-based EB switching device. Advantages in terms of scaling, and low electric field operation, have been demonstrated through a developed micromagnetic simulation. The simulation framework is capable of effectively handling a linear ME device in useful dimensions. The thesis ends with a signal-to-noise (SNR)-based comparative study between two most prominent magnetic recording technologies, used in the hard drive. One of them is the perpendicular magnetic recording (PMR), which is the current technology for the shipped hard drives. The other one is the upcoming heat-assisted magnetic recording (HAMR), a soon expected replacement of the current PMR. It has been pointed out that even an unoptimized, bare-minimum heat-assisted magnetic recording (HAMR) design, yields higher SNR than an optimized PMR. SNR comparisons have been calculated for various changes in the head design parameters. The comparisons are expected to increase the motivational drive towards migrating to HAMR from PMR.
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    Effects of Skew Angle and Transition Curvature in HAMR Hard Disk Drives
    (2017-05) Cordle, Michael
    Continued areal density growth in hard disk drives (HDD) is becoming increasingly difficult to achieve as Perpendicular Magnetic Recording (PMR) approaches the super paramagnetic limit of ~1Tb/in2. Heat-Assisted Magnetic Recording (HAMR) is on the verge of becoming the next generation of high-density recording technology. Understanding the physical mechanisms behind the unique recording characteristics will be a critical step in the maturity of HAMR technology as it continues to make progress towards production. A notable difference between HAMR and PMR that has drawn a lot of recent attention is the curved shape of a recorded transition. Minimizing transition curvature is understood to be crucial for improving ADC, and current studies have shown that it could be imposing a significant limitation for HAMR. Here, we provide a comparison of HAMR and PMR ADC profiles in an HDD. We explore a new technique proposed for capturing magnetization footprint images through HDD testing, and take full advantage of a significantly improved cycle time to apply a statistical treatment to experimental curvature data to provide a quantitative analysis of factors that impact transition curvature in HAMR and PMR HDDs. We identify geometric effects resulting from skew angle that correlate well to changes in transition curvature. We also show the impact of laser power on transition curvature, and discuss how an understanding of this information can be used to quickly identify uncontrolled variables in an experiment.
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    System and Media Optimizations for improved HAMR Performance
    (2020-08) Natekar, Niranjan
    It is said that data never sleeps. It has a ubiquitous presence and is being generated at an exceptional pace from different sources. The insurmountable demand for data must be met with an equally fast paced data supply which has led to the development of data servers by companies like Microsoft and Google. In spite of facing an existential crisis due to the development of SSD’s, the Compound Annual Growth Rate (CAGR) of ~ 40% maintained by the HDD industry has ensured that these devices are a necessity when it comes to large scale data storage. The growth of the HDD industry is helped by the fact that a huge amount of research is dedicated to developing new data recording technologies to improve the storage capacity of HDD’s. Significant investment has gone into developing a new data recording technology called Heat Assisted Magnetic Recording (HAMR) that is expected to improve the storage density up to at least 5Tb/in2. In conjunction with other improvements (like the development of Bit Patterned Media BPM), the expected density output for HAMR can be even higher. The optimization of the HAMR technology has focused on different aspects of the HAMR system, optical, magnetic, mechanical and electrical. In this thesis, different system and media optimizations that may help improve the HAMR performance are explored. The effect of doped L10 FePt media, which is an extremely popular HAMR media, is considered to understand how a change in its Curie temperature (Tc) can actually influence its intrinsic magnetic properties. This is followed by the implementation of micromagnetic simulations with the use of the stochastic Landau Lifshitz Gilbert (LLG) equation that is used to mimic the magnetization dynamics of grains. These simulations are used to vary the media properties and HAMR process parameters to optimize a thin 3nm (write layer)/6nm (storage layer) Thermal Exchange Coupled Composite (ECC) HAMR media. Introducing finite exchange coupling between the grains of the write layer and scaling the damping in the write layer are techniques that can help reduce the DC noise and improve the Signal to Noise Ratio (SNR). The Ensemble Waveform Analysis technique identifies Transition SNR as the main cause of SNR variation. This optimization process lends credence to the idea that a thinner composite media may be used to realize significant enhancements of SNR. Micromagnetic simulations are also used to address an important issue related to HAMR; the high temperature for writing data can cause heating issues with long term HAMR use. A low temperature Thermal ECC media is proposed that can significantly reduce the writing temperature (by about 34%) and that can reduce the peak temperature of the heat spot used to heat the media in the HAMR process by 200K. This is followed by an analytical formulation that is derived to calculate the transition jitter in the HAMR process. The jitter is known to depend on the grain size as well as the heat spot thermal gradient. It also depends on the Voronoi Grain Size Distribution, as well as exhibiting a surprising nonlinear dependence on the reader width. By combining the noise due to these dependencies the analytical formulation can be derived. This simple formulation provides both physical insight and conserves computational time relative to lengthy (and complex) recording simulations. A detailed analysis of the Adjacent Track Erasure (ATE) in the HAMR process is also explored. The numerical extent of ATE in different HAMR media is established and techniques are implemented using micromagnetic simulations in an attempt to reduce the ATE effect. A hypothesis is established to explain the presence of extent of ATE in different HAMR media. Research in the area of HAMR process and system optimization is of huge importance especially since the data storage industry has invested a lot in terms of research and manpower in this technology. Potential directions of research include techniques to reduce the ATE, improving the designs of different HAMR system components and developing better data post processing techniques like Neural Networks and 2D detectors.
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    Temperature Dependent Robust Control Of Hard Disk Drives Using Parameter Varying Techniques
    (2016-10) Honda, Masanori
    A hard disk drive (HDD) is a device that stores digital data by writing and reading magnetic signals onto a disk using a magnetic transducer. The data is organized into circular tracks that are less than 100 nm wide. Modern HDDs use a dual-state actuator to position the transducer onto these tracks. Since the tracks are extremely narrow, a high performance controller is required to reject disturbances from various sources and maintain the position of the transducer on a single track. This dissertation focuses on designing a robust controller for the HDD system. The controller must be robust to its external environment, such as changes in temperature, and provide good performance to thousands of drives. An adequate uncertainty model designed using first principles is not available for robust controller design. Thus a set of frequency response data (FRD) measured from a number of HDDs and at different temperature points is used to design the uncertainty model of the system. A basic method of averaging the set of FRD to create an uncertainty model is used to design the baseline controller through a standard D-K synthesis method. A numerical algorithm is then developed to create an optimal uncertainty model for the system using the experimental FRD. Using this algorithm, a temperature dependent model is designed for the purpose of designing a temperature dependent robust controller. Finally a temperature dependent controller is designed to increase the performance of the HDDs compared to the baseline controller, and the theoretical validation for the method is given.