Magnetic Amplifying Magneto-optical System Research Articles

Mechanical Performance Characteristics of 15000 rpm Ultra-high-speed Rotation of an Ultra-thin Disk with a Magneto-optical Recording Film

We developed a stacked volumetric optical disks (SVOD) system to realize a data capacity of over 1 TB in a cartridge. To increase the read and write speed of the SVOD system, the mechanical performance characteristics of the 92-μm-thick thin optical disk stored in the SVOD cartridge were investigated by combining the magnetic amplifying magneto-optical system (MAMMOS) structure and 15000 rpm ultra-high-speed rotation. The minimum 40 nm mark length has already been reached with MAMMOS recording. This corresponds to 100 GB per disk of CD size. The thin optical disk can be rotated at over 10000 rpm because thin optical disks are flexible, and shatter rarely in comparison with conventional 1.2-mm-thick optical disks. As a result of the study, the mechanical performance characteristics were found to be acceptable for practical use. Therefore, the maximum data transfer rate can be upgraded to more than 2 Gbps by using a combination of thin optical disks, magneto-optical (MO) recording, and 15000 rpm ultrahigh-speed rotation.

Domain Expansion Readout For Magnetic Amplifying Magneto-Optical System

Zero Field Magnetic Amplifying Magneto-optical System (ZF-MAMMOS) is realized by using an exchange-coupled tri-layer. The recorded mark with any length appears as the expanded domain with the size of collapse diameter in the readout. layer without external magnetic field. As the ZF-MAMMOS has a resolution of 1/10 of a laser beam spot, this corresponds to 40 nm using a 405 nm laser diode and an objective lens of NA, 0.9. An AlSiN enhance/cooling layer was arranged for overwriting, and a GdFeCo recording assist layer was developed for high recording field sensitivity. As a result, the durability of overwriting is improved and the recording with ±140 Oe is achieved. This result also indicates that it is possible to realize a heat assisted magnetic recording with a very small recording field.

Materials for Magneto-Optical Recording

AbstractThis article describes the characteristics and development of materials for magnetooptical (MO) recording. Magneto-optical recording is derived from thermomagnetic recording and readout by the magneto-optical effect. Rare-earth/transition-metal alloys, mostly Tb-Fe-Co, have been used for recording materials in MO discs. MO discs were first put on the market in 1988. Magnetic-field modulation for recording was introduced in MiniDisc (MD) systems in 1992. MO discs have an advantage in durability and achievability, because recording by magnetization reversal is not accompanied by any atomic movement. Magnetically induced super-resolution (MSR), reported in 1991, enabled resolving powers that are higher than the optical limit. MO discs based on MSR were commercialized as 3.5–in. discs for data recording and as 2–in. discs for digital cameras.As extensions of MSR, DWDD (domain-wall displacement detection) and MAMMOS (magnetic amplifying magneto-optical system) have been proposed, in which the marks are expanded during readout for a large signal. DWDD technology has been used in Hi-MDs (high-density MiniDiscs), which were commercialized in 2004 with storage capacities of 1 Gbyte per 64-mm-diameter disc.

Mechanism for domain expansion in MAMMOS

A magnetic amplifying magnetooptical system (MAMMOS) model was created which calculates the size of the expanded domains in the readout layer by computing the forces on the domain wall. Using the model, the ratio of nucleation coercivity to domain wall motion coercivity, H/sub c,n//H/sub c,m/, was found to be an important factor governing the final expanded domain size in MAMMOS. Simulation results indicated that H/sub c,n//H/sub c,m/>5 is needed for a good readout signal. Experiments measuring the nucleation coercivity in localized areas on a MAMMOS readout layer agree with the modeling results. In the experiments, a wafer was patterned with islands of magnetic material and the switching behavior was measured. For a MAMMOS film, the ratio of H/sub c,n//H/sub c,m/ was measured to be 13 on average in a 400 /spl mu/m/sup 2/ area.

Three-Dimensional Magneto-Optical Recording

ABSTRACTIn this report, the newly developed three-dimensional magneto-optical (MO) recording scheme and the experimental results are reported. A part of this work has been done as the national project of 3D-MO (3-dimensional MO) project. It started at September 1998 and ended March 2002 as a part of the national project “Nanometer-Scale Optical High Density Disk Storage System” and aimed at achieving 100 Gb/in2 in storage density. Three-dimensional MO recording is one of the prosperous candidates of next generation ultra high density recording. Magnetic amplifying MO system (MAMMOS) is employed for achieving the novel three-dimensional MO recording. Double-MAMMOS scheme consists of 2-recording layers of differing compensation temperature (Tcomp ) and one readout layer was proposed and discussed.With write/read test it is succeeded to show the results corresponding to a 100 Gb/in2 (50 Gb/in2 × 2) recording density. We also proposed and showed results of simulations of a new type of Double-MAMMOS in which the recording layers can hold quadri-valued information by single writing process.

Theoretical Limit to Domain Position Detection Magnetic Amplifying Magnetooptical System

We developed a model that was used to evaluate the performance of a domain position detection magnetic amplifying magnetooptical system (DPD-MAMMOS). Through the use of the model, we explain the operation of DPD-MAMMOS and demonstrate its advantages. Next, we evaluated the performance of DPD-MAMMOS under the influences of position noise from a theoretical point of view. From this analysis, we estimate that a MAMMOS system will need to attain 30 nm or less of position noise to show a density advantage when used with DPD-MAMMOS.

Write Strategies for Improved Readout in RF-Magnetic AMplifying Magneto-Optical System

A promising approach to realize high capacity optical data storage is the use of domain expansion (DomEx) techniques like magnetic amplifying magneto optical system (MAMMOS). However, experiments show that correct readout of long mark domains (multiple bits) is critical and often even impossible, which means that the performance as a (random) data storage technology would be severely limited. Based on a detailed understanding of the MAMMOS readout process, these experimental results are explained and show excellent agreement with numerical simulations: the cause of the readout problems is a decrease in stray field in the centre of long magnetic domains. To solve this problem, write strategies can be applied, where small spaces with opposite magnetization direction are inserted at appropriate locations in each (long) mark domain. From the experimental and simulation results it is clear that this strongly improves the random data readout performance.

Blue Zero-Field Magnetic Amplifying Magneto-Optical System (MAMMOS) Recording

We present results of first-surface blue zero-field magnetic amplifying magneto-optical system (MAMMOS) recording of isolated marks of 60 nm and a single tone carrier of 90 nm. The signals are recorded by laser-pulsed magnetic field modulation using a blue laser, a recording head with numerical aperture (NA) = 0.7 with integrated thin film coil and a free working distance of 12 µm from the disc. The media are covered with a 3 µm thin cover layer to prevent heating induced contamination of the optics.

First-Surface Far Field Magneto-Optical Recording

Magnetic AMplifying Magneto Optical System (MAMMOS) is a promising technology for high storage densities. However, the laser power margin for correct readout is quite small. Based on the understanding of the readout process, recording and readout methods have been developed and tested to improve this margin. First results show that the increased margin allows correct readout of bit lengths as short as 75 nm, which corresponds to an increase in storage density by a factor 1.5 compared to conventional MAMMOS. No changes to the light path or the disk are required. Finally, we measured the rise time of the MAMMOS signal using a fast detector. The result shows that for a numerical aperture (NA)=0.85 and λ=405 nm MAMMOS is capable of data rates around 100 Mb/s.

Zero-Field Magnetic Amplifying Magnetooptical System Applied to Next Generation “iD PHOTO” System

We investigated very high-density recording performance of the zero-field magnetic amplifying magnetooptical system (MAMMOS) with a 635 nm laser diode and a 0.60 N.A. objective lens. It was shown that the linear bit length of 0.13 µm was successfully reproduced with sufficient readout margins. The recording density up to 7.1 Gbit/in2 was demonstrated by combining the zero-field (ZF)-MAMMOS with an in-groove disk structure.

Dynamic Observation of Wall Speed in MAMMOS Reading Layer

In Magnetic Amplifying Magneto Optical System (MAMMOS), a small domain in the recording layer is copied to the readout layer where the domain is expanded to a high level readout signal. After the expansion, the domain should be collapsed for reading out the next information. Therefore, the velocity of expansion and shrinking of the domains is one of the most important parameters which certainly affects the high speed performance of MAMMOS.We have developed a stroboscopic microscope system for observing domain dynamic phenomena. By using this system, domain shrinking processes are observed under a variety of applied fields and the influence of the in-plane magnetic field on the shrinking speed is also measured.

SPATIAL LIGHT DISTRIBUTION IN MAGNETO OPTICAL SYSTEMS FOR HIGH NUMERICAL APERTURE AND SOLID IMMERSION LENS FOCUSING: INFLUENCE ON THERMAL AND MO EFFECTS

In order to achieve higher areal density for optical recording, multilayered magneto optical structures, working at shorter wavelength and higher numerical aperture than the DVD are being developed over the world. MSR (Magnetic Super Resolution) and MAMMOS (Magnetic AMplifying Magneto Optical System) structures use magnetic couplings between magnetic layers and need a very fine temperature control of the recording, switching and reading layers. It has been shown that the power margin for MAMMOS reading is very small. At numerical apertures of NA=0.85 with far field optics and even at higher apertures with the Solid Immersion Lens (SIL), it is essential to determine the spatial distribution of the electric field at the focal point of the objective as well as in the different recording layers. We will present a vector simulation of the electromagnetic field in a MO structure illuminated by a high NA focused field, and discuss absorption in the thin films. The MO properties in the different layers are also calculated at variable incidence angles. We illustrate this with SIL coupling calculations.

Toward Tiny Domains Less Than 1/10 of a Laser Beam Spot

For almost all optical disks, signal decline is mainly due to inter symbol interference (ISI) caused by laser beam spot size or magnetic aperture size. Such 151 does not appear for Zero Field Magnetic Amplifying Magneto-optical System (Zero Field MAMMOS). The limit of the frequency response comes from the frequency range of the MO readout channel. MAMMOS has a resolution for 1/10 of a laser beam corresponding to 0.1μm. Decrease in the carrier level and error of less than 0.1μm is caused by expansion failure. If expansion failure were circumvented, Zero Field MAMMOS would have a possibility of readout with 0.04μm resolution using a laser with wavelength, 650nm and an objective lens of NA, 0.6.

Domain expansion readout system with DC magnetic field

MAMMOS (Magnetic Amplifying Magneto Optical System) is a technique for realizing high-density magneto-optical disk. Conventional MAMMOS readout process needs AC magnetic field for expanding and erasing magnetic domains. As a new readout system, MAMMOS readout that needs only DC magnetic field using RE (Rare-Earth)-rich GdFeCo readout layer was developed. This method exhibited a high readout resolution in spite of an AC field-free method.

Domain Copying and Expansion Phenomena of MAMMOS Media Using A Blue Laser and A High NA Objective Lens.

The combination of Magnetic Amplifying Magneto-Optical System (MAMMOS) media, a blue laser and a high NA objective lens has been investigated using simulations and experiments. This paper describes the experimental and simulated results of domain copying phenomena for the MAMMOS media using a 413nm blue laser and an objective lens with a numerical aperture (NA) of 0.9. Without an external magnetic field, the readout laser was irradiated onto the MAMMOS media. Since the phenomena were related to Rear Aperture Detection phenomena, the signal amplitude simulation assumed temperature dependence of the Kerr rotation, the aperture thermal properties and the Gaussian laser beam profile. This paper also presents the analysis of expansive and shrinking forces per unit wall area concerning the MAMMOS readout using the blue laser and the high NA objective lens.

MAMMOS with DC Magnetic Field by Using RE-rich Readout Magnetic Layer

A new type of MAMMOS (Magnetic amplifying magneto-optical system), which operates under the DC magnetic field in reproducing process, was proposed and evaluated. In this system, the CNR of 43 dB was obtained for 0.25 μm packed domains by applying 10 kA/m of DC magnetic field. Compared with conventional CAD (Center aperture detection)−MSR (Magnetically induced super resolution) disk, it was shown that the CNR for domain size below 0.4 μm was greatly improved.

Spatial resolution of domain copying in a magnetic domain expansion readout disk

In a magnetic amplifying magnetooptical system (MAMMOS) disk, the copying process of a magnetic domain from the recording layer into the readout layer is investigated by using a laser spot diameter of 0.66 /spl mu/m. Interference between neighboring bits occurs when the spatial resolution of the copy process is larger than half the domain size. It is observed that a reduction of the readout power from 2.3 to 2.0 mW corresponds to a decrease of the copy window from 0.5 to 0.06 /spl mu/m. As a result, at 2.0 mW packed magnetic domains with a width of only 0.11 /spl mu/m have been resolved without interference by using domain expansion.

DC磁界により再生領域を制御したMAMMOS再生

Characteristics of new MAMMOS (Magnetic AMplifying Magneto-Optical System) were evaluated. This method needs only a dc magnetic field for expanding the magnetic domains. We obtained 43 dB of CNR for 0.25 μm packed domains by applying 10 kA/m of dc magnetic field. Compared with the conventional CAD (Center Aperture Detection) disk, the CNR for domains smaller than 0.4 μm was greatly improved.

0.04 µm Domain Expansion Readout for the Magnetic Amplifiying Magneto Optical System

Read/write characteristics of the magnetic amplifying magneto optical system (MAMMOS) with alternative readout fields such as carrier to noise ratio (C/N), eye pattern, jitter and bit error rate (BER), are investigated with a 640 nm wavelength laser diode and 0.55 numerical aperture of the objective lens. The C/N value of 0.12 µm is 50 dB and is independent of the mark length. A very clear eye pattern was observed. The results show that the jitter value is less than 3% compared with the window width and it is only dependent on the switching speed of the readout field. The error rate is improved by using a thin recording layer of 50 nm in thickness. The minimum domain length which yields a BER of less than 1×10-4 is 0.045 µm (45 nm); the minimum space length is 0.10 µm (100 nm). These results are confirmed at a track pitch of about 0.40 µm. By using a digital versatile disk (DVD) pickup, the areal density can be estimated to be about 23 Gbit/in2 with the user data capacity of 30 GB/120 mmφ. By using a 400 nm blue laser diode, these values are estimated as 59 Gbit/in2 and 77 GB/120 mmφ.

Effects of Magnetic Properties and Layer Thickness on the Readout Performance of Magnetic Amplifying Magneto-Optical System Disks

The effects of magnetic properties of a GdFeCo readout layer and the thickness of each layer on the readout signal in magnetic amplifying magneto-optical system (MAMMOS) disks were investigated. The readout signals were sensitive to the magnetic properties of the readout layer. However, the readout performance is not strongly dependent on the thickness of the readout layer, memory layer, and Si3N4 dielectric layer. The optimum magnetic properties of the readout layer and the thickness of each layer were obtained.