MnBi films for magnetooptic recording

Progress made during the past years in the area of magnetooptic data storage by the computer industry has been most impressive. Many material media and physical phenomena have been developed for this particular application. It now appears that a large capacity (gsim 10^{10}bits) magnetooptic data store possessing major advantages over the conventional recording techniques could be developed. Of the many materials and techniques advanced to date, the use of thin films of MnBi for thermomagnetic writing, erasing, and magnetooptic reading has received particularly intensive study because of the many unique properties of this film medium. In order to provide an assessment of the potential of this medium for optical memory application, we have included in this review the pertinent material physical properties of MnBi; the memory characteristics in regard to read, write, and erase operation; the physical process involved in the writing and erasure by thermomagnetic technique; the technique for detection of written information; and the utilization of this medium for magnetic holographic storage. Emphasis is given to the material properties and physical phenomena, rather than the systems considerations in using MnBi films for optical memory.     

Measurement of the thermal conductivity of erasable phase-change optical recording media

We describe a method to estimate the thermal conductivity of the substrate, the dielectric layer, the phase-change (PC) layer, and the reflective layer of PC optical recording media. The method relies on the amorphous-to-crystalline phase transition that occurs in the PC layer and takes advantage of the difference in the thermal diffusion behavior under different-sized focused spots. All the results obtained here are reliable with better than ?5% accuracy, which is within the margin of our experimental error.     

Media for high density magnetic recording

Longitudinal recording is limited at high bit densities by recording demagnetization, self-demagnetization, and adjacent-bit demagnetization, which occur during the writing-demagnetization processes. To minimize these adverse effects it becomes necessary to resort to extreme scaling of the media parameters and their thickness, with the consequence of greatly increasing the difficulty of fabrication and the cost of such optimized media. Pure perpendicular recording circumvents these writing and demagnetization problems because of the strong head coupling of a single pole head with a double layer medium, positive interaction between adjacent bits, and low self-demagnetization at high bit densities. Therefore, it does not require any extreme scaling of the media magnetic parameters and their thickness. Of great interest, at least for the next several years, are the quasi-perpendicular particulate media which can support perpendicular magnetization. These include the isotropic, high-squareness media, and oriented perpendicular media employing particles with uniaxial crystalline or shape anisotropies. The attractiveness of these media derives from their excellent recording performance and from the fact that they preserve the existing head/media interface and they utilize existing coating facilities which should reflect favorably on their cost. In this paper the advantages and disadvantages of the various media under development for high density magnetic recording are compared, and predictions are made for their potential application in future systems.     

Media requirements and recording physics for high density magneticrecording

Relevant aspects concerning the ultimate achievable recording densities for particulate as well as for thin-film media are discussed. This review covers the entire range starting from micromagnetics of individual single domain particles, moving on to their magnetic behavior in a particle assembly under particular consideration of the structure being actually obtained in the process of manufacturing recording media, and finally embarking on an outline of recording physics. These considerations are not only carried out for longitudinally and perpendicularly oriented recording media but also for media having an arbitrary orientation of the easy axis of magnetization. All aspects are discussed and illustrated for the first commercially available thin-film medium on a flexible substrate, which is the metal evaporated tape, i.e., the obliquely deposited Co-Ni-O layer for the Hi8 video system     

Hot pressed GdIG for magnetooptic memory application

This paper describes the preparation of polycrystalline GdIG by hot pressing and reports the most relevant properties of the material. Hot pressed samples were prepared using various conditions of time (1 to 20 hours), temperature (900 to 1300°C), and pressure (300 to 1000 kg/cm²). Dense specimens with grain diameter ranging from 1 to 10μm were prepared. Thin wafers made by sectioning and polishing samples have high coercive field (higher than 200 Oe), good hysteresis loop squareness (Br/Bm = 0.85), and high figure of merit (2beta/alpha sim 2). These properties make them well suited for optical mass memory application. This was confirmed experimentally, and a bit density of5 times 10^{5}bit/cm2was obtained by laser beam thermomagnetic writing.     

Media and tip trajectory optimization for high-density MFM-based perpendicular recording

In this paper, we investigate the feasibility of using a magnetic force microscopy scheme for recording and retrieving magnetic marks for ultrahigh-density, ultralow power applications. We will address the main design considerations while designing such a system. Then, using the impulse-response and inverse-convolution technique, we deduce a novel tip trajectory for the optimum recording process. We will also apply extensive optimization for a Co/sub x/Cr/sub y/Pt/sub 1-x-y/ perpendicular media structure to maximize the signal-to-noise ratio (SNR). An areal density of up to 0.3 Tb/in/sup 2/ is shown to be achievable with thermally stable magnetic marks and a SNR of 20-25 dB in the existence of additional electronic noise.     

A survey of magnetooptic effects

The interaction of light with matter involves the electronic structure of the matter. In particular, the interaction is affected by the magnetic state of the medium, and this gives rise to the magnetooptical effects. The Faraday, Kerr, and Voigt effects are described, and their derivation from Maxwell's equations and the constitutive relations are given. The microscopic origins of the effects are discussed heuristically. A few representative applications of the magnetooptic effects to magnetic recording and to the control of light are explained.     

Studies of magnetooptic effects for thin-film optical-waveguide applications

A computer study of mode conversion in thin-film optical waveguides using magnetooptic GdIG as a substrate is reported. The efficiency of the mode-converting structure is analyzed and compared to other structures in order to gain design insight. It is shown that 100-percent mode conversion (TErightleftarrowTM) is possible if both modes satisfy the phase condition required of mode propagation.     

Ferrimagnetic garnet films for magnetooptic information storage

Reversible magnetooptic memories are believed to compete favorably with magnetic bubble devices and CCD's if economic low-power light sources can be used. For optical information storage at data rates of >10⁶bit/s and blocks of 10³bit, memory materials are required exhibiting an optical recording sensitivity on the order of the photographic plate. The presented contribution outlines the recently proposed concept of a magnetooptic/photoconductive sandwich, called MOPS, and shows that certain ferrimagnetic garnet compositions can especially be adapted to such applications.     

The magnetooptic bubble display

The various factors that affect the properties of magneto-optic (M/O) bubble displays which use bismuth substituted iron garnets are discussed. A composite bias magnet-ferrite drive coil assembly is described which allows optical access to the display chip. The drive coils required 300 mW to produce an in-plane field of 30 Oe when operated at 10 kHz, and the bias field was uniform to ± 1.5% over an area of radius equal to 1/10 that of the bias magnet. Also discussed is the brightness and heating of the display. A screen brightness of 160 candelas/m²is possible using a X20 projection lens, and the temperature rise of the display chip could be less than 1°C if adequate heat sinking were used. Finally a prototype hand-held display is described which contains a 1.3 k bit capacity display chip. Examples of characters written into that chip are given.     

Pulse-read on erasable thermal phase-change superresolution disks

New erasable thermal phase-change superresolution (EPSR) disks composed of mask and recording layers can increase recording density by the detection of the below-diffraction-limited marks within the readout spot. The formation of the aperture and the readout signal on the EPSR disk were analyzed. The feasibility of optically designed EPSR disks was evaluated by thermal simulation. A carrier-to-noise ratio of 32 dB at a mark size of 0.4 mum, 8 dB higher than that of a conventional disk, was obtained by application of a pulse-read method to the EPSR disks at a wavelength of 780 nm and a numerical aperture of 0.55.     

Investigation of polarization magnetooptic responses of a low-concentration ferrofluid

A highly sensitive laser polarization-optical technique was developed and applied to investigations of a ferrofluid, which was placed in a magnetic field. A colloid solution of magnetite in kerosene was used as a sample. It was shown that the magnetooptic response of this substance is reliably registered at a volume solid-phase content of down to ~10^–5. It was revealed that within a very wide range of changes in the solution concentration, a quadratic dependence of the observed birefringence value on the low-intensity field (up to several tens of oersted) is retained.

     

Reversibly erasable nanoporous anti-reflection coatings from polyelectrolyte multilayers

For nearly two centuries, researchers have sought novel methods to increase light transmission in optical systems, as well as to eliminate unwanted reflections and glare. Anti-reflection coatings and surfaces have enabled the increasing performance demands of optical components fabricated from glass-based optical materials. With the current trend of technology moving towards optically transparent polymeric media and coatings, the need for anti-reflection technology and environmentally benign processing methods for polymeric materials independent of shape or size has become quite apparent. We describe an economical, aqueous-based process controlled at the molecular level that simultaneously coats all surfaces of almost any material. Systematically designed nanoporous polymer films are used, which are suitable for optical applications operating at both visible and near-infrared wavelengths. These high-efficiency anti-reflection coatings are created from phase-separated polyelectrolyte multilayer films that undergo a reversible pH-induced swelling transition. Furthermore, such films, easily patterned by an inkjet printing technique, possess potential for pH-responsive biomaterial and membrane applications.