This paper is a review article for magneto-optical (MO) recording. Principles of writing (erasing) and reading and an outline of a MO recording apparatus are described firstly. Secondly, recording media (including material requirements and properties of rare earth-transition metal amorphous films), methods of film preparation, and the construction and properties of the MO disk are illustrated shortly. Lastly, future problems to be solved for practical use are discussed.
Phase-change type optical disks record optical information through thermal phase transition of thin amorphous films from the disordered to the ordered state upon laser beam irradiation. A recorded signal is read out as a change in the optical reflectivity. It is shown that phase transition of Te-TeO2 thin film is related to the growth of Te grains and that it is sufficiently stable and commercially available as write-once type media. For erasable type, thin films of Te-TeO2-Ge-Sn and Te-Ge-Sn-Au systems have been developed. In these fllms, an increase in the crystallization rate is required to simplify the optical system.
The mechanism of memory of liquid crystals is mainly the boundary aligning method by which liquid crystals are obligated to orient one direction in origin. Liquid Crystal Devices are used in this character. Some new application or arrangement might be possible if a new mode is found and new liquid crystals have been obtained.
Recent developments in the new spectroscopic technique concerning photochemical hole burning (PHB) and its possible application to the wavelength-multiplexed high density optical memory systems are reviewed. The basic concept of PHB is the site-selective optical bleaching usually observed in the solid systems doped with organic dye molecules at liquid helium temperature region. The molecular conditions and the fundamental mechanisms for the appearance of the holes are discussed in relation to the ordinary photochromic molecules. The review also includes discussions on the present research activities on PHB materials for future optical storage systems.
A TV camera tube equipped with a photoconductive thin-film target is, in a sense, a two-dimensional rewritable optical memory. To apply the target of a camera tube to memory devices, however, its storage time, that is, time constant, must be sufficiently long to store optical information. This is, in general, possible by optimizing the composition and structure of photoconductive targets. In this article, various kinds of photoconductive materials are reviewed in detail. To realize optical recording media, it is advantageous to combine a photoconductive film with another material. Various combinations of photoconductor and insulator, elastomer, thermoplastics, electro-optical single crystal, and ferroelectric ceramics are also discussed.
The studies on hole formation type optical recording films, which are mainly studies on three Te-based recording films are reviewed. Pb-Te-Se recording films, are deposited by vacuum evaporation or by sputtering. The role of Se in the film is to inhibit the oxidation. By more than 14% of Se addition, film oxidation is completely inhibited even at 60°C relative humidity 95%. A selenium condensed layer is found at the inner part of an oxidized surface layer by Auger electron spectroscopy. The surface Te oxide layer and the Se rich layer inhibit the film inside oxidation. The role of the metallic elements In, Pb, Sn, Bi, and Sb in the film is to inhibit cracking and to decrease noise in reproduced signals by decreasing the size of crystal grains. Lead is found to be the best among these metallic elements, because its recorded hole shape is clean even when recorded after 15 days accelerated oxidation at 60°C relative humidity 95%. A storage life longer than 50 years at room temperature is expected for the Pb-Te-Se optical recording film. Te-C films are deposited by sputtering a Te target in mixed a gas of Ar and CH4. The film structure shows small particles of Te surrounded by organic compounds made from the CH4 gas. CS2-Te films are deposited by plasma polymerizing CS2 gas while vacuum-evaporating Te. The film has a structure in which small particles of Te are surrounded by polymerized CS2. In these films, the organic compounds protect the Te particles from oxidation and a long storage life is expected.
By using a perpendicular magnetization mode to the surface of a recording medium, a new recording system that has extremely high volume density will be realized. Its minimum bit-length corresponds to the crystalline particle size which is about one-tenth the thickness of a perpendicular magnetic anisotropy film used for recording media. For writing and reading, a single pole type magnetic head made of a soft magnetic thin film has been also proposed. In this paper, the principles, methods and feasibilities of perpendicular magnetic recording are described.
The recent trend of high density magnetic disks was reviewed. The magnetic hard disk and head were introduced from the standpoint of the manufacturing process and material. Recent future in the field was briefly discussed.
Since its invention in the early 1930's, the powder-coating recording medium has been playing a principal role in magnetic recording by making coninuous improvements possible so that the ever present demand for higher quality and greater recording density can be met. The major improvements are as follows : a. Increase of saturation magnetization, squareness ratio and coercive field. b. Size reduction of magnetic powder pigments. c. Improvement of the homogeneity of powder dispersion and the surface smoothness. d. Reduction of coating thickness. In 1., the structures and manufacturing methods of both flexible and rigid media are described with an emphasis on the importance of binder selection to achieve homogeneous particle dispersion and higher wear resistance. In 2., brief explanations are given about the characteristics and preparation methods of the magnetic powders used for coating media, such as γ-Fe2O3, CrO2, Co-modified γ-Fe2O3, Fe particles and Ba-ferrite for perpendicular recording.
The magnetic bubbles are cylindrical magnetic domains where the direction of magnetization is antiparallel to that of the surrounding area. The magnetic bubble memory devices use the “bubbles” for information storage. This memory has several advantages such as being solid-state, non-volatile, and having very high storage density. Here, we focus on materials for magnetic bubbles. The single crystal-thin films of garnets are used for bubble materials. In order to increase the storage density the bubble diameter should be reduced. The bubble diameter is closely related to the magnetic properties of the garnet film such as saturation induction, anisotropy energy, and exchange constant. The relationship between the film composition and the magnetic characteristics has been described along with the fundamental properties of magnetic bubbles and garnets. The liquid phase epitaxial growth method for preparation of garnet films has also been described.