Multilayer Recording Research Articles

Modeling of multilayer microholographic data storage

We focus on the investigation of multilayer recording in microholographic data storage. We have developed a numerical model for calculating the electromagnetic scattering from thick microholographic gratings using the Born approximation and the direct volume integral. The signal-to-noise ratio and bit error rate were calculated to estimate the noise arising from interlayer and interhologram cross talk. Measurements were done to prove the validity of the model. The results of our calculations and the measurements show good agreement. We present the application of the model to the investigation of confocal filtering at the image plane and to the evaluation of positioning and wavelength tolerances.

Current Status of GaN-Based Laser Diodes Grown on Freestanding GaN Substrate

The development of optical memory devices that use violet laser diodes (LDs) has progressed significantly and these devices have been commercialized. The need for high-speed recording and multilayer recording is increasing, and it is expected that even higher power LDs will be necessary in the future. In this paper, the current state of GaN-based LD research for the next generation optical memory devices is reported, and next targets are discussed. Moreover, we developed high power pure blue laser diodes for full-color laser display. The details of these pure blue LDs characteristics are reported.

State-of-the-Art Magnetic Hard Disk Drives

AbstractMagnetic recording has progressed dramatically over the last 50 years, with an increase of almost eight orders of magnitude in the amount of information stored per unit area of disk space.Two key enablers of this progress have been the recording medium and the read-back head.This article reviews the current state of the art in multilayer thin-film longitudinal recording media and giant magnetoresistive (GMR) read heads, with particular emphasis on the nanostructured magnetic materials that are key to today's high-performance hard disk drives.

Materials for Optical Data Storage

AbstractThe development of recording materials has been a critical component in the advancement of optical data storage. The continual need for improved performance in both consumer and business applications has pushed forward the capabilities of optical storage. In this issue of MRS Bulletin, we review some of the important developments in the materials behind established technologies such as compact discs, digital versatile discs, and magneto-optical recording; the emerging technology of blue recording; and two technologies that seek to extend the performance roadmap for optical storage, multilayer and holographic recording.

Materials for Multilayer Recording

AbstractAn attractive feature of optical discs is their capacity. Recent research has indicated that the capacity of optical discs can be increased by using multiple layers of bitwise data. Layers are spaced along the depth dimension of the discs. Individual layers are recorded and information is retrieved in a manner that is very similar to conventional optical disc systems using a single layer. The data capacity of each layer is nearly equal to the capacity of a single layer, thus increasing the capacity of a disc by a factor equal to the number of layers. In fact, dual-layer optical discs are already commercially available. With some storage materials, it is possible to record hundreds of data layers. However, system engineering trade-offs, like readout speed, are of concern. More conventional – materials can also be used for multilayer recording, but the number of layers is limited by the transmission properties of each layer. This article reviews the materials systems for multilayer recording, the interplay between materials properties and performance, and the optical systems used for multilayer recording.

Electrochemical formation of intermediate layer for Co/Pd multilayered media

Pd nanocluster seeds were formed on a soft magnetic underlayer (SUL) using an electrochemical substitution reaction, and were utilized as an intermediate layer for a Co/Pd multilayered ([Co/Pd] n ) perpendicular magnetic recording medium. A CoNiFeB film prepared with electroless deposition was used as SUL, which was immersed into a PdCl 2 solution for the formation of Pd seeds. The Pd seeds were found to effectively reduce the size of magnetic domains in the [Co/Pd] n film deposited on them. The optimization of the concentration of the PdCl 2 solution and the use of the pretreatment process with a SnCl 2 solution were effective to obtain the smooth SUL surface with fine Pd seeds as small as 5 nm. The 20 nm-thick [Co/Pd] n film deposited on the optimized Pd seeds/CoNiFeB SUL exhibited a high coercivity of 7.8 kOe and a small magnetic domain size of 69 nm. These results indicated that the combination of the Pd seeds and the electroless-deposited SUL was desirable in terms of the improvement not only in the magnetic properties of [Co/Pd] n media but also in the mass productivity of the underlayer.

Liquid Crystal Lens for Compensation of Spherical Aberration in Multilayer Optical Data Storage

In multilayer recording, it is strongly required to compensate the spherical aberration caused by a difference in substrate thickness. A novel liquid crystal (LC) lens is designed and fabricated to compensate the spherical aberration in this study. The new structure of the LC lens includes both concave and convex surfaces, which can compensate the spherical aberration with a relatively long range. Since a previously developed LC panel showed a very low tolerance to the shift of an objective lens, a new component has been proposed with a special LC lens structure to improve both the tolerance and compensation range. By optimized curvature control with a spherical LC lens, the aberration of a Blu-ray optical pickup can be maintained at 0.018λrms even for a thickness variation of ±25 µm. Finally, the LC lens has been fabricated using a standard one drop filling (ODF) process and evaluated by measuring the variation in focal length as a function of applied voltage.

磁気記録媒体用超微小ヘマタイト粒子の焼成状態と非磁性基体の表面平滑性との関係

Non-magnetic particles such as acicular hematite particles with finer size and higher dispersibility are required to produce multi-layered magnetic recording media with better performance. The specific surface area and primary particle size of non-magnetic acicular hematite particles were controlled by adjusting the heating conditions in the dehydration process. A sintering-parameter (SBET/STEM) was defined as the ratio of specific surface area measured by B. E. T. method to the specific surface area calculated from the particle size determined by TEM observation. The lower surface roughness of non-magnetic substrate is used to express the dispersibility of hematite particles. For 1.0≤SBET/STEM≤1.4, the dispersibility of hematite particles was much better in non-magnetic lacquer. When the optimum heating condition was applied to make hematite particles, the lower surface roughness was attained by using finer particles.

Microstructure of a Co/Pd multilayered perpendicular recording medium with Pd seeds prepared by electrochemical process

Effects of the Pd seeds formed by an electrochemical process on the microstructures of Co/Pd multilayered perpendicular recording media were investigated. By immersing a CoZrNb soft magnetic underlayer (SUL) into PdCl2 aqueous solution, island-like Pd clusters were deposited on the SUL, resulting from the electrochemical substitution reaction. Transmission electron microscopic observations revealed that the columnar Co/Pd multilayered grains with clear boundaries grew on the SUL with the Pd clusters. The clear grain boundaries, where the densities of Co and Pd elements were low, were thought to cause the magnetic exchange decoupling between the Co/Pd grains, leading to an increase of perpendicular coercivity and a decrease of magnetic cluster size of Co/Pd multilayered film. Consequently, lower medium noise was obtained for the Co/Pd multilayered medium with the Pd clusters than for that with a sputter-deposited Pd seedlayer. From these investigations, it was clarified that the SUL surface with Pd clusters provided the effective nucleation sites for a well-defined Co/Pd multilayered grain growth.

Co/Pd multilayers for perpendicular magnetic recording media

Co/Pd multilayered media were improved using a three-fold approach. InSn seedlayers were used for higher coercivities, the Pd layers were doped with Si to decrease exchange between grains, and the Pd thickness was decreased to increase magnetization. When InSn seedlayers were used for these Co/Pd multilayered perpendicular magnetic recording media, the coercivity increased to measured values of 12.5kOe and projected values of almost 20kOe. These large values were obtained even though the depositions described here were done at room temperature. In order to lessen intergranular exchange coupling, Si (which has no solid solubility in Pd) was used to dope the Pd layers. Si also slightly decreased the grain size distribution to 23%. Initial recording measurements showed excellent signal to noise ratios, although an excessively wide switching field distribution prevented recording at the highest densities. Finally, the thickness of the Pd bilayers was reduced to increase the magnetization of the samples from 180 to 300emu/cm3.

Femtosecond laser application for high capacity optical data storage

A femtosecond (fs) laser application for multi-layer optical recording is investigated. Information patterns at different layer depths were written inside a transparent glass substrate due to micro-void formation by fs laser ablation, which causes re-distribution in glass materials and a refractive index modification. The information bits recorded in a single layer can be retrieved clearly without interference from the neighboring layers. A fs laser irradiation of a transparent polymer matrix (doped with fluorescent materials for use as low-cost recording media) is also studied. A fs laser induced photo-chemical reaction changes the chemical properties of the fluorescent materials and records information bits inside the matrix. With an ultra-fast laser as a new light source, 3D optical recording can be available for high capacity data storage up to 1 TB per disc.

Cross-Sectional Control of Boron Contents and the Microstructures of Co/Pd Multilayer Perpendicular Magnetic Recording Media

The effects of cross-sectional modulated boron segregation on Co/PdB multilayer perpendicular magnetic recording media have been studied. Gradually reduced boron contents from thin PdB seed layer (33 at.%) to the top of the Co(0.2 nm)/PdB (0.8-nm) multilayer (12.5 at.% at the center of the multilayer) were successively achieved and confirmed by the depth-profile of X-ray photoelectron spectroscopy (XPS). Observed magnetic properties and magnetic force microscopy images suggested that this gradient diffusion supports reduction of intergrain exchange coupling and improvement of the magnetic properties. The signal-to-noise ratio over 22 dB was obtained at 450 kFCI in playback tests, in combination with amorphous cobalt boron soft-magnetic underlayer (SUL). It is suggested that these properties are brought by a combination of the dispersed nucleation sites with rich boron content in the initial layer and decreasing boron content during the growth of the multilayer. In addition, this simple medium, which consists of only three elements, Co, Pd, B, for the SUL, the seed layer, and the magnetic layer, has a very unique feature of the dull interfaces, observed by a transmission electron microscope.

Long-Range Erasure of Nearby Recorded Tracks

A thin-film write element at 0/spl deg/ skew angle can produce partial erasure, affecting previously recorded tracks as far as 2 /spl mu/m away. This symmetric erasure is due to fringe fields from the write gap or adjacent to the gap side of the leading pole. The amount of erasure varied with the design of the write yoke, and a yoke made with a higher saturation moment material also produced more erasure. Disks with lower coercivities and multilayered recording media were more easily erased. The erasure was also aggravated by increasing the flat-to-flat write current, the amount of write current overshoot, and the frequencies of both the aggressor and victim data tracks. There was no correlation between the amount of this off-track erasure and the on-track overwrite. The amplitude reduction increased logarithmically with the number of aggressor write operations.

Microexplosion Recording in Spin-Coated Polymer Films Including ZnO Nanoparticles for Three-Dimensional Optical Memory

As a microexplosion recording material, we propose polymer films including ZnO nanoparticles (ZnO polyester composite) for write-once multilayered recording media. These media with the ZnO composite material can be fabricated by a spin-coating method and can be read at the violet wavelength of 0.405 µm. By the electromagnetic analysis of diffraction loss, we clarified the pit design and the optical performance for void formation recording. From the results of experiments performed using three kinds of mode-locked pulsed lasers (pulse widths of 150 fs, 16 ps and 6 ns), with a clear reflection microscope image of submicrometer pits, the microexplosion sensitivity was confirmed to be greatly improved by 14, 38 and 50 times, respectively.

Three-Dimensional Multi-Layered Bit-Oriented Optical Storage

Three-dimensional multi-layered optical storage is one of the most promising methods that break the limitation of recording density in optical storage. It stores the bit information onto the multilayered recording planes inside the thick recording medium and realizes the tremendous large recording capacity. This paper describes the recording/reading principles, the systems, and the materials suitable for three-dimensional multi-layered memory. We also present the experimental results that using photochromic organic materials and rhodamine-B-doped and Au(III)-doped PMMA for recording medium.

Direct observation of magnetic bit transition and physical grain boundary in perpendicular recording media

The correlation between magnetic bit transition line and physical grain boundary in a multilayer perpendicular recording medium is examined by combined magnetic force and electrostatic force microscopy. Reducing the scan height results in stronger electrostatic interaction between the tip and the sample disk. This electrostatic interaction causes the topographical information to be superimposed onto the magnetic force image and allows simultaneous imaging of the columnar grains and the magnetic bits. It is found that the magnetic transition follows the physical grain boundaries with high accuracy. The correlation of both is quantified by image processing and a correlation coefficient of 0.92 has been determined.

Reduction of medium noise of co-pd multilayered perpendicular magnetic recording media by a thin carbon interlayer

We report the effect of introducing a thin carbon interlayer to the recording layer on the read-write characteristics of Co-Pd multilayered perpendicular magnetic recording media. A 1-nm-thick carbon interlayer markedly reduced the medium noise and improved the output resolution, D/sub 50/. As a result, the signal-to-noise ratio at the recording density of 200 kFRPI for the medium was higher by 5 dB than that for the medium without the interlayer. We attribute these improvements in read-write characteristics to a decrease in the crystal grain diameter of the recording layer, resulting in suppression of the transition fluctuation of recorded bits. The Co-Pd multilayered perpendicular magnetic recording medium with carbon interlayer has a high thermal stability despite a slight degradation of perpendicular magnetic anisotropy.

Perpendicular media: alloy versus multilayer

Properties and performance for alloy and multilayer perpendicular recording media designs utilizing a soft magnetic underlayer are compared. Among samples considered here, grain size and grain size dispersion are more highly refined for alloy media deposited at high substrate temperature, and are beginning to approach those now available in longitudinal recording. Multilayer media made at ambient temperature typically sacrifice film density and surface smoothness for interface quality. Although microstructural development and the manufacturing process for multilayer media are less mature versus alloy, multilayer media remain attractive due to their high anisotropy potential and the ease with which H/sub n/ and H/sub c/ can be controlled. For thermally stable alloy media made on a pilot production sputtering machine, a spin-stand areal density of 61 Gb/in/sup 2/ has been demonstrated at 350 Mb/s data rate with an on-track bit-error-rate reference level of 1e-6. Using the same media, a working perpendicular drive has been demonstrated at 32 Gb/in/sup 2/ and 500-800 Mb/s data rate.

Dual Layer Phase Change Optical Disk Using Limit Equalizer

A simple manufacturing process of multilayer recording disks has been developed to fabricate a large-capacity optical disk of more than 40 GB, using a short-wavelength laser diode and high numerical aperture (NA) objective lens. Some existing production facilities for the digital versatile discs (DVD) are applicable to this process. These disks can be utilized for long-time video recording of high-definition TV broadcasting. We have developed a dual-layer rewritable optical disk with capacity of more than 43 GB per one side of a 120 mm disk by this process. We obtained good performance of playback jitter for disks of 43 GB and 46.6 GB capacities with on-groove recording and limit equalizer at a user data transfer rate of 36 Mbps as well as good direct overwriting characteristics over 1,000 cycles.

Effect of the Preparation Conditions of a Pd/Si Dual Seedlayer on the Magnetic Properties of Co/Pd Multilayered Perpendicular Magnetic Recording Media

A Pd/Si dual seedlayer consisting of a 5-nm-thick Pd layer on a 5-nm-thick Si layer was deposited on a substrate prior to the formation of a [Co/Pd]n multilayered perpendicular magnetic recording medium, and the effect of the deposition conditions of the dual seedlayer on the magnetic properties of the [Co/Pd]n, multilayered film was investigated. The addition of N2 to the sputtering Ar gas during the Pd upper seedlayer deposition was effective for decreasing the value of α, which is the slope of a perpendicular M-H loop, defined as 4π(dM/dH)H=Hc, and reflects the magnitude of intergranular exchange coupling The value of α was further decreased by post-annealing of the Pd/Si dual seedlayer. By combining the effects of N2 addition and the post-annealing in preparing the Pd/Si dual seedlayer, both an increase in the reproducing resolution (D50) and a reduction in the medium noise were achieved. Furthermore, the preparation conditions were found to influence the surface morphology and the state of chemical bonding of the Pd/Si dual seedlayer.