Recording Density Research Articles

Signal Detection Using Extrinsic Information From Neural Networks for Bit-Patterned Media Recording

To meet the demand for storing large amounts of data, there has been a stronger focus on increasing the recording density of a hard disk drive (HDD). While conventional HDD suffers from thermal effect and superparamagnetic limit, bit-patterned media recording (BPMR) has the potential to resolve these problems and is capable of increasing areal density (AD) beyond 1 terabit per square inch. To increase the AD of BPMR, the distance between islands in both the down- and cross-track directions must be reduced. Consequently, the decreased bit period and track pitch induce more intersymbol interference (ISI) and intertrack interference (ITI), which degrade the bit error rate (BER) performance. In this study, extrinsic information obtained from a multilayer perceptron (MLP) equalizer is used as a priori information in a partial response maximum-likelihood (PRML) detector to improve the detection performance of BPMR. The proposed detection scheme shows better performance compared to both the MLP equalizer and conventional PRML alone. In addition, it was found that the proposed detector provides improved performance when track misregistration (TMR) occurs.

Simulation on near-field light on recording medium generated by semiconductor ring resonator with metal nano-antenna for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is a promising technology for achieving more than 10 Tbit/inch2 recording density. A near-field transducer (NFT), which forms a small light spot on a recording medium, is necessary in HAMR. However, the heat generated by the NFT would melt the NFT itself. To solve this problem, the authors have proposed a novel device, in which a metal nano-antenna is attached to a semiconductor ring resonator. In this paper, the near-field light generated by this device was analyzed through a numerical simulation based on a 3-dimensional model including the recording medium to optimize the structure of the device. It was found that how to excite a desired eigenmode selectively among some eigenmodes is important to make the device effective. A light spot with a diameter of about 25 nm, which corresponds to the recording density of 1 Tb/inch2, was obtained on the surface of the recording medium. It was also found that the design parameters of the device must be optimized considering the recording medium.

Optical disc writing strategy for analog signal recording

Optical discs are being sought as low-cost, earth-friendly storage candidates in response to increasing data volumes. To increase the recording density of optical disc, analog recording technique such as the Orthogonal Frequency Division Multiplexing (OFDM) method has been investigated. Since bitwise information is recorded on optical discs, it is necessary to create a column of marks corresponding to the analog signal for the OFDM method. The Delta-Sigma Modulation (DSM) is known as an analog to digital conversion method in the area of audio. This method transfers efficiently an analog signal to a binary sequence with a simple circuit. In this paper, the application of the method for analog signal recording and the simulation results of the mark recording process on the optical disc are described. The optical disc recording utilizes temperature rise of a multilayer film stack in recording process that is triggered by light focusing. A simplified, 2.5-dimension, thermal analysis is applied to do the large-scale simulation of recording mark sequence.

INFLUENCE OF ABERRATIONS OF OPTICS ON DENSITY OF RECORDING OF HOLOGRAPHIC INFORMATION

The problems of holographic recording of information are analyzed. Traditionally developed memory systems were aimed at storing “cold data” - digitized arrays of long-term storage with rare accesses to them. When recording analog holograms for these purposes, an essential role is played by optical aberrations. The influence of aberrations is analyzed both in binary information recording and image recording schemes. The obtained estimates of the recording density of information even with a decreasing coefficient determined by aberrations indicate the impossibility of transmitting such holograms over the radio channel. To solve the problem of transmitting holographic information over a radio channel in augmented reality and holographic television devices, it is proposed to use a coding method close to the method of transmitting information on one side band, which is known in the radio range, expressed in the transfer of two arrays equivalent in volume to a conventional TV channel and the synthesis of holograms on the receiving end of a channel.

Carbon resistive probe memory designed for ultra-high storage density

Probe-based storage memories are considered one of the most promising solutions to address the mass storage issues in the near future. However, data size arising from conventional probe memories is usually larger than probe size due to the thermal diffusion effect. To eliminate such thermal interference and make data dimension fully dominated by probe dimension, we proposed a concept of carbon-based resistive probe memory and developed a comprehensive computational model to predict its write, rewrite and readout performances governed by electro-thermal and mass concentration processes. The physical reality of such a theoretical model was demonstrated through the good agreement between the calculated and experimental measured threshold voltages for different layered thickness. The data bit of carbon-based resistive probe memory, considered as the sp2 filament inside sp3 background, is formed completely underneath the tip edge due to the localized electric field induced here. This makes the bit size fully determined by the probe tip dimension and allows for the achievement of ultra-high density using an ultra-small probe tip with low energy consumption. Such a conductive filament can be also rewritten back to its pristine sp3 state at relatively high temperature (~250 °C) and detected by sensing the device reading contrast (~1). The designed carbon-based resistive probe memory can retain its bit completeness even if we reduce the bit pitch to 28 nm for a probe size of 25 nm, exhibiting a superior immunity to thermal cross-talk effect. It, however, induces strong readout cross-talk, which is revealed from the resistance image of the multiple bit pattern. This adversely reduces the achievable recording density due to the required large bit pitch, which can be alleviated using either a very sharp tip apex or the optical readout scheme.

Testing a nano-barcodes marking technology for identification and protection of the mechanical products

The article presents a technology for application and identification of nano-barcodes on the surface of industrial products. The essence of this process is to modify the surface of metallic materials by exposure to pulsed laser radiation. As a result of such exposure, the modified surface contains small labels, allowing storing encoded information directly on the machine part, bypassing intermediate carriers. The laser-applied labels are formed using software and encode informative data. The encoding of the input information ensures the uniqueness of the applied nano-barcodes. The proposed technology allows us to apply such unique labels on the materials surface for accounting and identification of industrial products. The proposed marking technology allows us to place a significant amount of information in a small area. Special software has been developed for decoding the information from the applied nano-barcode developed. Studies have been performed that have been conducted to determine the thickness of the marked symbols formed on the materials surfaces and the reliability of the applied symbols. There are also recommendations on the usage of the technology given for direct labeling of materials with the application of information with high recording density for various industries.

Sound Propagation and Quantum-Limited Damping in a Two-Dimensional Fermi Gas.

Strongly interacting two-dimensional Fermi systems are one of the great remaining challenges in many-body physics due to the interplay of strong local correlations and enhanced long-range fluctuations. Here, we probe the thermodynamic and transport properties of a 2D Fermi gas across the BEC-BCS crossover by studying the propagation and damping of sound modes. We excite particle currents by imprinting a phase step onto homogeneous Fermi gases trapped in a box potential and extract the speed of sound from the frequency of the resulting density oscillations. We measure the speed of sound across the BEC-BCS crossover and compare the resulting dynamic measurement of the equation of state both to a static measurement based on recording density profiles and to quantum MonteCarlo calculations and find reasonable agreement between all three. We also measure the damping of the sound mode, which is determined by the shear and bulk viscosities as well as the thermal conductivity of the gas. We find that the damping is minimal in the strongly interacting regime and the diffusivity approaches the universal quantum bound ℏ/m of a perfect fluid.

Development of heat dissipation multilayer media for magnetic hologram memory

AbstractHolographic memory is a strong candidate for next‐generation optical storage with high recording densities and data transfer rates, and magnetic hologram memory using a magnetic garnet, as the recording material, is expected to be used as a rewritable and stable storage technology. We succeeded in recording and reconstruction of two‐dimensional data in magnetic hologram without error. However, the diffraction efficiency is insufficiently high for actual storage devices. To increase the diffraction efficiency, it is important to record deep magnetic fringes, whereas the merging of fringes due to the excess heat near the medium surface generated during the thermomagnetic recording process must be suppressed. To avoid this merge of fringes, we proposed a multilayered structure in which the magnetic layers are divided by the transparent heat dissipation layers (HDL) to control the heat diffusion. In this study, we propose a simple thermal design method for designing the HDL multilayer structure. Using this model, we designed and fabricated an HDL multilayer medium in which the recording magnetic layers are discrete in the film. The HDL multilayer medium exhibited diffraction efficiency higher than that of the single‐layer medium, and error‐free recording and reconstruction were achieved using the magnetic assist technique.

Computer-generated-hologram-based holographic data storage using common-path off-axis digital holography.

A reconstruction method for multilevel complex encoded data-pages is proposed to increase the recording density of computer-generated-hologram-based holographic data storage by using off-axis digital holography. Although the detection process is based on off-axis digital holography, the proposed method keeps the optical setup a simple and common-path configuration owing to the computer-generated holography. Five-level complex encoded data-pages can be experimentally reconstructed.

Development of Heat Dissipation Multilayered Media for Magnetic Hologram Memory

Holographic memory is a strong candidate for next-generation optical storage with high recording densities and data transfer rates, and magnetic hologram memory using a magnetic garnet, as the recording material, is expected to be used as a rewritable and stable storage technology. We succeeded in recording and reconstruction of 2D data in magnetic hologram without error. However, the diffraction efficiency is insufficiently high for actual storage devices. To increase the diffraction efficiency, it is important to record deep magnetic fringes whereas the merging of fringes due to the excess heat near the medium surface generated during the thermomagnetic recording process must be suppressed. To avoid this merge of fringes, we proposed a multi-layered structure in which the magnetic layers are divided by the transparent heat dissipation layers (HDL) to control the heat diffusion. In this study, we propose a simple thermal design method for designing the HDL multilayer structure. Using this model, we designed and fabricated a HDL multilayer medium in which the recording magnetic layers are discrete in the film. The HDL multilayer medium exhibited diffraction efficiency higher than that of the single layer medium, and error-free recording and reconstruction were achieved using the magnetic assist technique.

Numerical Simulation of Thermal Fly-Height Control Slider Touchdown Test for Achieving Ideal Subnanometer Clearance Head Slider Touchdown Behaviors

Understanding the dynamics and instability of a thermal fly-height control (TFC) head slider close to and at touchdown (TD) on a magnetic recording disk is essential for developing a head slider with a clearance of ~ 0.5 nm to enhance the recording density of hard disk drives. This paper presents a theoretical simulation of a practical TD test, performed by continuously increasing the head protrusion according to the TFC power, and demonstrates ideal TD behaviors to be established in design guidelines for an effective head slider–suspension–disk system. The effects of the air-bearing stiffness and magnitude of disk waviness (DW) on the TD behaviors and their repeatability were extensively investigated. Comparison with the experimental TD behaviors of effective head sliders demonstrated that the different TD behaviors at different radial disk positions could be explained by the numerical TD simulation of the DW-excited response of a single-degree-of-freedom slider model. Possible reasons for the discrepancies between the experimental and simulation results in the light contact state were analyzed in detail, and a method of designing the head–disk interface conditions to achieve a head spacing of ~ 0.5 nm was developed. In addition, the deviation from the ideal TD behavior according to the DW magnitude was examined. The TD behavior of a head slider on a disk with a perfectly bonded lubricant was theoretically clarified, and the potential for realizing a head slider with reduced clearance was assessed.

Time Aspects of the 180° Pulsed Magnetization Reversal of Ferrite–Garnet Films with Complex Anisotropy

It is shown that a material having four stable positions of the orientation of the magnetization vector makes it possible to double the density of magnetic recording (for example, in MRAM devices) without substantial modifications of production process. This finding is based on investigations of the time aspects of the 180° pulse magnetization reversal of ferrite–garnet films with planar anisotropy, in the region of external fields where the mechanism of uniform rotation of magnetization operates.

Impact of straightened thermal profiles generated by gapped near field transducers on HAMR SNR

In this work, heat assisted magnetic recording (HAMR) is simulated to assess the areal density advantages of various realistic thermal profiles. Specifically, finite element simulation and micromagnetic modeling are used to assess the areal density consequences of a straighter thermal profile with an associated wider track. This assessment is done by simulating the thermal profiles created by heads with gapped near field optical transducers (NFTs) and split magnetic poles. By varying the gap width of the NFT and the thickness of the medium interlayer, the above variation in transition straightness and associated written transition is achieved. At specific operating points, we find that a wider track with a straighter transition results in a slightly higher recording density (1.2Tb/in2 vs 0.9 Tb/in2). As expected, the former case finds its optimum at higher linear bit densities (1370 kFCI) and lower track densities (850 kTPI) than the latter case (870 kFCI and 1000 kTPI).

Multitrack Detection With Two-Dimensional Hybrid Equalizer for High-Density Bit-Patterned Media Recording

Bit-patterned media recording (BPMR) is one of the most promising candidates to further increase recording density beyond 1 Tb/in2. For high-density BPMR systems, severe intertrack interference (ITI) and media noise caused by decreased track pitch bring severe deterioration to the readback signals. To mitigate the impact of ITI and media noise on bit error rate (BER) performance, we developed a multitrack detection scheme with a two-dimensional (2-D) hybrid equalizer, which utilizes 2-D distinct equalizer coefficients corresponding to the different estimated ITI patterns. By analyzing the probability density distribution of the log likelihood ratio (LLR) output by the decoder, a LLR threshold is implemented to evaluate the reliability of estimated ITI patterns. Then, the 2-D variable equalizer trained with different ITI patterns is implemented for the reliably estimated ITI patterns; otherwise a 2-D fixed equalizer trained with a pseudorandom bit sequence is utilized. Hence, compared to the conventional 2-D fixed equalizer, the 2-D hybrid equalizer decreases the mean squared error of equalization with reduced error propagation risk. The simulation results indicate that the 2-D hybrid equalizer provides increased signal-to-noise ratio gains compared to the 2-D fixed equalizer when the media noise and track misregistration become more severe.

Effects of dot-position dispersion of BPM, thermal distribution, and head field gradient on bit error rate for HAMR

The effects of dot-position dispersion, thermal distribution, and the head field gradients in a cross-track direction on bit error rates for heat-assisted magnetic recording with bit-patterned media were studied using a micromagnetic simulation to achieve an areal recording density of 2.5 Tb in−2. A large dot-position dispersion in a cross-track direction causes on-track bit errors when the full width at half maximum (FWHM) of the thermal distribution was equal to or lower than 25 nm. This is because the centers of the dots with errors were set to be unrecordable in the thermal attenuation area. When the FWHM was larger than 25 nm, there were bit errors for all dot-position dispersion. The bit errors were caused by rewriting, which occurred during thermal fluctuations. The bit error of the down-track direction was more than that of the cross-track direction. Because of these results, the track width should be as narrow as possible.

Recent density decline in wild-collected subarctic crustose coralline algae reveals climate change signature

Abstract Warming surface ocean temperatures combined with the continued diffusion of atmospheric CO2 into seawater have been shown to have detrimental impacts on calcareous marine organisms in tropical and temperate localities. However, greater oceanic CO2 uptake in higher latitudes may present a higher oceanic acidification risk to carbonate organisms residing in Arctic and subarctic habitats. This is especially true for crustose coralline algae that build their skeletons using high-Mg calcite, which is among the least stable and most soluble of the carbonate minerals. Here we present a century-long annually resolved growth, density, and calcification rate record from the crustose coralline alga Clathromorphum nereostratum, a dominant calcifier in Pacific Arctic and subarctic benthic communities. Specimens were collected from the Aleutian Islands, Alaska (USA), a region that has undergone a long-term decline of 0.08 ± 0.01 pH units since the late 19th century. Growth and calcification rates remain relatively stable throughout the record, but skeletal densities have declined substantially since A.D. 1983. Strong correlations to warming sea-surface temperatures indicate that temperature stress may play a significant role in influencing the ability of corallines to calcify. Decreasing algal skeletal density may offset the benefits of continued growth and calcification due to a weakening in structural integrity, which could have detrimental consequences for the diverse reef-like communities associated with algal structures in mid-to-high latitudes.

CoCrPt@(TiO2,CoO) granular thin films grown on Ru/NixPd100-x/NiTa (x = 20, 50, 80)

Structural, morphological and magnetic properties of CoCrPt@(TiO2,CoO) granular thin films deposited on Ru/NixPd100-x/NiTa intermediate layers were investigated as a function of the NixPd100-x composition (x = 20, 50, 80). Using a Ni20Pd80 buffer-layer leads to a significant improvement of the crystallographic texture, grain separation and easy-axis angular distribution with respect to conventional systems using Ru/NiTa as intermediate layers. As a result, a meaningful enhancement of magnetic properties (anisotropy constant, K = 4.1 × 105 J/m3, out-of-plane coercivity, Hc⊥ = 470 kA/m, nucleation field, HN = −115 kA/m) with respect to conventional systems (K = 3.7 × 105 J/m3, Hc⊥ = 374 kA/m, HN = −71.6 kA/m) was achieved, thus indicating that Ni20Pd80 can potentially be used as buffer-layer to enhance the recording density of conventional Hard Disk Drives.

Plasmonic layer-selective all-optical switching of magnetization with nanometer resolution

All-optical magnetization reversal with femtosecond laser pulses facilitates the fastest and least dissipative magnetic recording, but writing magnetic bits with spatial resolution better than the wavelength of light has so far been seen as a major challenge. Here, we demonstrate that a single femtosecond laser pulse of wavelength 800 nm can be used to toggle the magnetization exclusively within one of two 10-nm thick magnetic nanolayers, separated by just 80 nm, without affecting the other one. The choice of the addressed layer is enabled by the excitation of a plasmon-polariton at a targeted interface of the nanostructure, and realized merely by rotating the polarization-axis of the linearly-polarized ultrashort optical pulse by 90°. Our results unveil a robust tool that can be deployed to reliably switch magnetization in targeted nanolayers of heterostructures, and paves the way to increasing the storage density of opto-magnetic recording by a factor of at least 2.

Concatenated Coding for Multilevel Flash Memory with Low Error Correction Capabilities in Outer Stage

One of the approaches to organization of error correcting coding for multilevel flash memory is based on concatenated construction, in particular, on multidimensional lattices for inner coding. A characteristic feature of such structures is the dominance of the complexity of the outer decoder in the total decoder complexity. Therefore the concatenated construction with low-complexity outer decoder may be attractive since in practical applications the decoder complexity is the crucial limitation for the usage of the error correction coding.
 We consider a concatenated coding scheme for multilevel flash memory with the Barnes-Wall lattice based codes as an inner code and the Reed-Solomon code with correction up to 4…5 errors as an outer one.
 Performance analysis is fulfilled for a model characterizing the basic physical features of a flash memory cell with non-uniform target voltage levels and noise variance dependent on the recorded value (input-dependent additive Gaussian noise, ID-AGN). For this model we develop a modification of our approach for evaluation the error probability for the inner code. This modification uses the parallel structure of the inner code trellis which significantly reduces the computational complexity of the performance estimation. We present numerical examples of achievable recording density for the Reed-Solomon codes with correction up to four errors as the outer code for wide range of the retention time and number of write/read cycles.

Recording and reconstruction of volumetric magnetic hologram using multilayer medium with heat dissipation layers.

Hologram memory is a strong candidate for optical storage due to its high recording density and high data transfer rate. We have studied and engineered a magnetic hologram memory medium using a stable magnetic garnet as recording material. To record a deep and clear magnetic hologram, it is important to control the heat diffusion generated during recording. Numerical simulation suggested that a multilayer structure with transparent heat-dissipation layers would be effective to address this. We fabricated a multilayer magnetic medium for a collinear magnetic hologram. This medium exhibited a diffraction efficiency as high as that of the single layered one, and errorless recording and reconstruction was achieved with the magnetic assist technique.