Browsing by Subject "Micromagnetic Simulation"

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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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    Micromagnetic Modeling of Magnetic Storage Devices
    (2021-03) Hsu, Wei-Heng
    Hard disk drives (HDDs) are the dominant mass storage devices for personal and cloud storage due to their low cost and high capacity. Heat-assisted magnetic recording (HAMR) is considered to be next-generation recording technology for HDDs. While HAMR shows the potential for areal density to go beyond one terabit per square inch, this new recording mechanism requires further understanding and optimization before commercialization. First, I examine the relationship between media noise power and linear density in HAMR. I observe that there is a noise plateau at intermediate recording density and show that the plateau can be shifted to different recording density regions depending on the temperature profile. This effect is argued to be a consequence of the competition between transition noise and remanence noise in HAMR. To extend the recording density limit, heat-assisted shingled magnetic recording is studied. The transitions are no longer symmetric about the track center after shingled writing, especially when the transitions are highly curved as a result of the temperature profile generated by the near-field transducer. I propose a new reading scheme by rotating the read head to match the curved transitions. For a single rotated head, more than 10% improvement in user density over that of a single non-rotated head is achieved. I found that the optimal rotation angle generally follows the transition shape. With an array of two rotated heads, a track pitch of 15 nm, and a minimum bit length of 6.0 nm, the user areal density reaches 6.2 terabits per square inch, more than 30% above previous projections for recording on granular media. Magnetoresistive random-access memory (MRAM) is another type of magnetic storage device that is mainly used as computer memory. As semiconductor-based memory begins to hit physical limits, spin-transfer torque (STT) MRAM and spin-orbit torque (SOT) MRAM appear to be strong candidates for future memory applications. I start first by studying SOT switching in magnetic insulators. Magnetic insulators (MIs), in particular rare-earth iron garnets, have low damping compared to metallic ferromagnetic materials due to lack of conduction electrons. Analogous to STT devices, their low-damping nature is presumed to be an advantage for SOT applications. I report that perpendicular magnetic anisotropy (PMA) material with low damping does not favor reliable SOT switching, but increased damping, interfacial Dzyaloshinskii–Moriya interactions, or field-like torques may help SOT switching in some cases. Notches in a nanometer-scale element, which is a more realistic size for practical applications, can also improve switching stability. To fully utilize low damping MIs with SOT, an in-plane exchange-coupled composite free layer SOT-MRAM is proposed. The free layer consists a low-damping soft MI and a high anisotropy material. The adoption of high anisotropy materials, such as L10 alloy, not only facilitates the achievement of ultra-high-density memory but also allows for the reduction of heavy metal layer volume and thus a reduction in write energy not seen in previous CoFeB-based SOT-MRAM. A write energy of 18 attojoules per bit for 1 ns switching is achieved which is only 72 times more than the theoretical limit of 60kBT. It also represents a factor of more than five hundred times improvement relative to state-of-the-art dynamic RAM.
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    Micromagnetic study of perpendicular magnetic recording media.
    (2011-01) Dong, Yan
    With increasing areal density in magnetic recording systems, perpendicular recording has successfully replaced longitudinal recording to mitigate the superparamagnetic limit. The extensive theoretical and experimental research associated with perpendicular magnetic recording media has contributed significantly to improving magnetic recording performance. Micromagnetic studies on perpendicular recording media, including aspects of the design of hybrid soft underlayers, media noise properties, inter-grain exchange characterization and ultra-high density bit patterned media recording, are presented in this dissertation. To improve the writability of recording media, one needs to reduce the head-tokeeper spacing while maintaining a good texture growth for the recording layer. A hybrid soft underlayer, consisting of a thin crystalline soft underlayer stacked above a non-magnetic seed layer and a conventional amorphous soft underlayer, provides an alternative approach for reducing the effective head-to-keeper spacing in perpendicular recording. Micromagnetic simulations indicate that the media using a hybrid soft underlayer helps enhance the effective field and the field gradient in comparison with conventional media that uses only an amorphous soft underlayer. The hybrid soft underlayer can support a thicker non-magnetic seed layer yet achieve an equivalent or better effective field and field gradient. A noise plateau for intermediate recording densities is observed for a recording layer of typical magnetization. Medium noise characteristics and transition jitter in perpendicular magnetic recording are explored using micromagnetic simulation. The plateau is replaced by a normal linear dependence of noise on recording density for a low magnetization recording layer. We show analytically that a source of the plateau is similar to that producing the Non-Linear-Transition-Shift of signal. In particular, magnetostatic effects are predicted to produce positive correlation of jitter and thus negative correlation of noise at the densities associated with the plateau. One focus for developing perpendicular recording media is on how to extract intergranular exchange coupling and intrinsic anisotropy field dispersion. A micromagnetic numerical technique is developed to effectively separate the effects of intergranular exchange coupling and anisotropy dispersion by finding their correlation to differentiated M-H curves with different initial magnetization states, even in the presence of thermal fluctuation. The validity of this method is investigated with a series of intergranular exchange couplings and anisotropy dispersions for different media thickness. This characterization method allows for an experimental measurement employing a vibrating sample magnetometer (VSM). Bit patterned media have been suggested to extend areal density beyond 1 Tbit/in2. The feasibility of 4 Tit/in2 bit patterned recording is determined by aspects of write head design and media fabrication, and is estimated by the bit error rate. Micromagnetic specifications including 2.3:1 BAR bit patterned exchange coupled composite media, trailing shield, and side shields are proposed to meet the requirement of 3×10-4 bit error rate, 4 nm fly height, 5% switching field distribution, 5% timing and 5% jitter errors for 4 Tbit/in2 bit patterned recording. Demagnetizing field distribution is examined by studying the shielding effect of the side shields on the stray field from the neighboring dots. For recording self-assembled bit-patterned media, the head design writes two staggered tracks in a single pass and has maximum perpendicular field gradients of 580 Oe/nm along the down-track direction and 476 Oe/nm along the crosstrack direction. The geometry demanded by self-assembly reduces recording density to 2.9 Tbits/in2.