Magnetic Super Resolution Research Articles

All-optical magnetic superresolution with binary pupil filters

A method to produce magnetic superresolution in all-optical magnetic storage is proposed. Two-zone amplitude-only and phase-only filters are designed to improve the quality of the superresolved magnetization pattern, to increase the magnetic spot intensity, to reduce the magnetic spot size, and to control the sidelobe effect. A procedure for designing a rotationally symmetric pupil-plane mask to control the magnetization intensity distribution near focus is presented. As a practical implementation, we have applied our method to obtain superresolving two-zone phase filters that can improve the all-optical magnetic recording density.

Advanced optical and magneto-optical recording techniques: a review

Optical data storage is advancing rapidly as we move forward in the 21st Century. Most people today are familiar with CDR and CD-RW as forms of optical data storage, but DVD-R, with considerably higher capacity, will challenge CD-R as the preferred storage medium soon. This review paper looks at the current state of optical recording and some of the current techniques which offer increased magneto-optical storage density, such as Magnetic Super Resolution (MSR), which is already on the market as the GIGAMO disk, and other methods under development. The latter includes Magnetically AMplified Magneto-Optical System (MAMMOS) and Domain Wall Displacement Detection (DWDD). To complement these technologies are a variety of optical techniques, such as near-field recording, which will enable higher storage densities by overcoming the diffraction limit. Holographic storage has promised much over the years, and has largely remained in the research domain. However, new materials and multiplexing techniques show considerable promise for this volumetric storage medium. The overlapping optical and magneto-optical technologies have yielded the concept of hybrid recording and this will also be addressed in this paper. The paper concludes with a look at alternative storage mechanisms, which utilise electronic, as opposed to thermal transitions, to offer improved data rates and improved storage efficiency, and this may dramatically change the way data is stored.

High Optical Efficiency Integrated Head for Use in Magneto-Optical Disk Drive

We explain the integrated magneto-optical (MO) head module that contains diffractive optical elements (DOE) and a beam-splitter (BS) with cylindrical surfaces. The optical system for servo signal detection is miniaturized by developing a multi-functional DOE. This system yields precise servo signals, and can eliminate the tracking error signal (TES) and focusing error signal (FES) independently. Power detection system is composed by using a multilevel DOE for high-speed automatic power control (APC). We suppress the distribution of the incidence angle on the BS surface, when divergent light is incident on the BS. It enables our integrated head to increase the coupling efficiency of a collimator without the deterioration of optical characteristics. When we test the head in a MO disk drive, the byte error rate (BER) of 10-6 is achieved. We confirm that this drive has the same commercial level performance for a 1.3 GB MO disk with magnetic super-resolution (MSR) technology.

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.

ID PHOTO System –Digital Photo Media for Still Cameras–

We developed a 50-mm-diameter magnetooptical disk system, iD PHOTO as a next-generation system for digital still cameras. The iD PHOTO system is based on the advanced storage magneto optical disk (AS-MO) standard, [rf1,rf2,rf3,rf4] and its media has a maximum capacity of 730 MB on a 50-mm-diameter single-sided disk, which exceeds the compact disc (CD) capacity, while being the world's smallest [rf5] rewritable optical disk. Using this system, it is possible to rewrite data at a high speed of more than 20 Mbps over one million times and moreover, it is suitable for saving data over long periods. Employing the magnetic field modulation (MFM) recording method and the center-aperture-detection (CAD)-type magnetic superresolution (MSR) readout method, the iD PHOTO system realizes high-density recording. The non return to zero inverted plus (NRZI+) modulation and the external clocking system enable high-capacity user data. Finally, we show that the read and write characteristics, which are sufficient for realizing a stable system, are obtained using external clocking and the CAD-type MSR readout method.

ID PHOTO, a New MO Disk for Digital Still Cameras.

A new MO (magneto-optical) disk has been developed as a new memory for digital still cameras, named “iD (image/intelligent disk) PHOTO”. Advantage of the iD PHOTO disk is a larger recording capacity of 730 MB, than those of semiconductor memories on current digital still cameras. The field modulation method for recording and Center Aperture Detection (CAD) type magnetic super resolution for reproducing are employed to achieve this capacity.This disk is designed considering mobile use. For the first point, high field sensitive CAD media is realized so as to reduce electric power consumption. By the addition of the capping layer, the recording field has been reduced to be smaller than 100Oe. For the second point, as well as the mechanical stability, write/read characteristic is enhanced in broad environmental temperature range. More than ±12% read power margin is obtained on each temperature area from -5°C to + 60°C.

Recording and readout using clock marks premastered by groove wobbling

A new recording and readout technique for land and groove recording on a magnetic super-resolution (MSR) disk is described. The technique uses specifically premastered clock marks. To generate a stable clock signal, the clock marks are fabricated with short bursts in grooves by wobbling at a different frequency from address information. The clock marks and the address information can be separated from a track error signal. When the extracted clock signal is applied to precise recording and readout of the magneto-optical data marks, the bit error rate becomes lower than that of the conventional clock recording/readout system. Additionally, the crosstalk of the wobbled address information to the magneto-optical signal can be canceled out electronically. This new format is not only suitable for high-density recording but also convenient in the disk manufacturing process.

Approach to readout aperture control for magnetic super-resolution readout

An approach of readout aperture control for magnetic super-resolution (MSR) readout is described. In order to detect the size of a readout aperture generated by a focused laser beam on an MSR disk, a new mark pattern consisting of short and long marks was recorded. An amplitude ratio of the readout signals between the short and the long marks is proportional to the aperture size. By maintaining the ratio constant by adjusting the laser power, the aperture size can be fixed. This technique makes it possible to reproduce data with a low bit error rate (BER). The sensitivity of the ratio to the readout power deviation and the distribution of the ratio were evaluated in order to estimate the accuracy of the readout power control. This technique is also applicable to readout when the disk is tilted. When the aperture size was maintained by this method, BER was found to be substantially reduced. Consequently, this new technique can realize high density recording on the MSR disk. This is the first approach to readout aperture control based on controlling the laser power.

Magnetic reorientation for high-density magneto-optical recording

We describe a magnetic super-resolution readout process based on a change of the magnetization orientation at high temperature. To realize this process we propose a double layer film composed of a memory layer with perpendicular anisotropy and a readout layer with in-plane magnetization. By heating this film, magnetization in readout layer rotates from in-plane to perpendicular position by the effect of exchange coupling with the memory layer. This transition occurs in a very short temperature range where the readout process is possible. In other temperatures the readout layer magnetization keeps in-plane and prevents any domain detection.

Noise and coupling in magnetic super-resolution media for magneto-optical readout

Interfacial magnetic coupling, transient thermal response, and carrier and noise levels are investigated for two central aperture detection magnetic super-resolution disks. In one of the disks, the two magnetic layers are exchange-coupled, while in the other the coupling is of magneto-static nature. For the exchange-coupled disk, the coupling between the two magnetic layers is fairly strong, and the Kerr loop of the readout layer does not have a square shape. For the magneto-statically coupled disk, the strength of coupling depends on the nonuniformity of the magnetization of the storage layer. The readout layer has a square Kerr loop, but its perpendicular magnetization in the hot region under the focused spot has random orientation if the stray field from the storage layer is weak. This random orientation of magnetization within the readout layer gives rise to a high level of noise during readout.

A MATLAB-Based PRML Model for Magneto-Optic Recording

A computational model describing the bit error rate (BER) performance of a PRML optical recording system has been developed. The model has been implemented in a user friendly MATLABTM environment and is capable of quickly predicting the performance of a PRML channel using either analytical, semi-analytical or full channel simulation techniques. Magneto-optic or phase-change media can be tested and a number of channel conditions (bit size, modulation code, equaliser, PR responses, noise etc.) can be easily adjusted. Advanced formats such as magnetic super-resolution (MSR) can also be incorporated.

READING DYNAMICS IN MAGNETIC SUPER RESOLUTION MEDIA

Reading dynamics in front aperture detection (FAD) and center aperture detection (CAD) magnetic super resolution (MSR) media is studied using a high speed Kerr microscope system with 10-ns resolution. The effects of disk structure and reading conditions on the reading dynamics are investigated. Readout of 0.2 micron domains is successfully demonstrated using 488 nm wavelength light on CAD MSR media.

Control of the Exchange-Coupling Force in Magnetic Super Resolution Magnetooptical Recording Media with In-Plane Magnetization Layers

The exchange-coupling force between GdFeCo readout layers and TbFeCo memory layers has been investigated for magnetically induced super resolution (MSR) magnetooptical recording media with in-plane magnetization layers. The exchange-coupling force can be suppressed in the mask area while mediated in the aperture area by introducing GdFe intermediate layers with large negative perpendicular magnetic anisotropy at room temperature and with relatively low Curie temperatures. As a result, the masking effect is substantially improved, and the recorded information of the memory layer can be copied more completely onto the readout layer at readout temperatures, resulting in a higher performance of MSR media.

Reading dynamics in Front Aperture Detection (FAD) MSR media

Front Aperture Detection (FAD) makes possible the detection of domains smaller than the optical diffraction limit and thus achieves magnetic super resolution (MSR) in magneto-optical (MO) recording. The success of this detection technique depends on good control of the shape and relative position of the thermal mask during the reading process. To have a better understanding of the reading dynamics in FAD, a high speed Kerr microscope system with 10 ns resolution built on an air bearing spin stand was used to directly observe the dynamics of the thermal mask and the effective aperture in FAD disks during readout. A computer simulation of the thermal profiles was compared with the experimental results and provides insight to the data analysis. It is shown that reading power, bias field and linear velocity affect the length of the thermal mask and the effective reading aperture. This effect strongly depends on the temperature gradient. Adding an Al underlayer to sharpen the thermal profile can reduce the dependence of the effective aperture on small changes in power and bias field during reading.

Future Perspective Magnetic Superresolution

The magnetic superresolution (MSR) method improves readout resolution of magnetooptical (MO) signals, with a mask that the readout layer creates. The MSR method is advantageous for high-density recording. However, the recording sensitivity and overwrite function of MO media must be improved in order to render it suitable for general use. To meet these requirements, we investigated the MSR method using static coupling force and its future extension.

Exchange coupling in rare-earth/transition-metal multilayers for magnetic super-resolution

We study exchange coupling in rare-earth/transition-metal multilayers in which one layer has perpendicular magnetization and a second layer has in-plane magnetization at room temperature. The exchange coupling between these layers is mediated by a thinner intermediate layer, and we investigate the dependence of the exchange on the magnetic properties of this intermediate layer from room temperature through the compensation and Curie temperatures of the layers. The structures of interest consist of a 400 Å TbFeCo layer with perpendicular magnetization, a 400 Å GdFeCo layer with in-plane magnetization, and a GdFe intermediate layer of variable composition and thickness. The results indicate that the structures are well-described by either a two-layer or a three-layer coherent rotation model (depending on intermediate-layer thickness) and that the influence of the intermediate layer is a function of its anisotropy energy, its thickness, and the identity of its dominant atomic moment.

READING DYNAMICS IN CENTRAL APERTURE DETECTION(CAD) MAGNETIC SUPER RESOLUTION (MSR) MEDIA

We present, for the first time, the direct observation of the dynamic reading process in central aperture detection (CAD) magnetic super resolution (MSR) media. The stroboscopic images taken by a 10ns exposure time high speed Kerr microscope system on an airbearing spin stand are compared with the corresponding CNR measurements on a MO spin stand. Dynamics of the reading process in samples with and without an intermediate layer and at different reading conditions are studied. Our analysis indicates that the intermediate layer sharpens the threshold for copying the data into the readout layer. We find that optimizing the reading power in CAD MSR readout is critical in order to achieve fast copy speed and provide clear and complete copying. The optimal reading power increases with the increase of the linear velocity of the disk, as expected. Higher disk velocity results in a smaller thermal aperture, which is shifted further from the center ofthe optical aperture, resulting in a decrease in CNR.

ANISOTROPY OF AMORPHOUS Gd-TM FILMS AND ITS APPLICATION FOR DOW AND MSR RECORDING

The recording properties of common single layer magneto-optical (MO) disks can be improved by introducing exchange coupled rare-earth (RE) transition-metal (TM) films, both from the point of view of capacity and data transfer rate. MO multilayer stacks must exhibit exchange coupling in order to be suitable for direct overwrite (DOW) and magnetic superresolution (MSR). Thin DOW stacks require an interface layer to adjust the exchange coupling. Soft magnetic Gd-Fe and Gd-FeCo interface layers exhibit suitable magnetic properties to meet the DOW demands. The best solution would be a RE-TM interface layer with inplane anisotropy at room temperature and perpendicular anisotropy at elevated temperatures. Such a layer can also be used as a read-out layer in MSR stacks. Therefore we studied the anisotropy of evaporated and sputtered Gd-TM films as a function of composition. Gd-FeCo films with a Co/(Fe+Co) ratio less than 0.47 showed a transition from inplane to perpendicular anisotropy with increasing temperature. The transition temperature Ttrans depends on the Co/(Fe+Co) ratio and the Gd content and can be adjusted between 0 K and TC. The temperature range ΔTtrans, where the magnetization direction changes from totally inplane to totally perpendicular, was 70 ± 25 K for all evaporated samples while it depends on Ar pressure for sputtered samples. Triple layer DOW disks showed a maximum CNR of 51.4dB.

High density recording with a super resolution magneto-optical disk using magnetic field modulation method

We have improved the recording magnetic field sensitivity of a magnetic super resolution (MSR) disk consisting of double magnetic layers and examined the magnetic field modulation recording properties. An areal density of 2.2 Gbit/in 2 was obtained with a currently available condition. The experimental optical pickup system had a 680 nm wavelength laser diode and an objective lens of a 0.55 numerical aperture. The MSR disk had 0.7μm-width lands and grooves. The crosstalk values between the land and its adjacent grooves were less than −28 dB. The areal density in the improved MSR disk was accomplished by recording in both lands and grooves and using mark edge recording with a (1,7) run-length-limited coding. Also, if a signaling technique called ‘PRML’ is used, an areal density of 3.1 Gbit/in 2 could be realized.

MAGNETIC SUPER RESOLUTION

A magneto-optical disk memory with a magnetic super resolution (MSR) method can get large capacity beyond the optical limitation. One of the MSR methods is realized by a readout layer that shows in-plane magnetization at room temperature and perpendicular magnetization above a specific temperature. Here, the recording and readout characteristics of the MSR disk are investigated and a magnetic field modulation recording method is examined. The recording density reaches 2.5Gbit/inch2 with a 680nm wavelength laser.