Magneto-optical Media Research Articles

Magnetic properties and microcolumnar structure of a TbFeCo memory layer for high-density magneto-optical recording

The MsHc value of the TbFeCo magneto-optical medium was reported as a key factor in high-density recording. Microstructure control of the magnetic underlayer was identified as an effective method for increasing the value of MsHc. This article describes the relationship between the magnetic properties and microcolumnar structure, which depends on the growth process and sputtering conditions of the TbFeCo recording film and the layer structure. It has been found that the columnar structure of the film self grows under the low migration energy process without any magnetic underlayer. The process uses a target in-face type dc magnetron sputtering system with a low temperature substrate. The amount of sputtering gas was observed to play a dominant role in the self growth of the film. The coercivity Hc value increases by creating a microcolumnar structure, but the squareness ratio of the Kerr hysteresis loop of the film is reduced. And the perpendicular anisotropy constant Ku is decreased to less than 1×106 erg/cm3 when Hc is more than 8×103 A/cm. As a result, it is difficult to record a tiny mark because the magnetic transition width of the magnetic domain wall would be extended. This is considered that the self-grown columnar structure is intrinsically different from the film grown over a magnetic underlayer because the TbFeCo film grown over a magnetic underlayer with seeding effect has large coercivity Hc and a high squareness ratio of Kerr hysteresis. These results indicate that it is suitable for tiny mark recording of the recording film with the seeding effect.

A spin rotator for spin-polarized scanning electron microscopy

A Wien filter, which is a common energy analyzer, was modified as a spin rotator for use in a spin-polarized scanning electron microscope. By switching the spin rotator on and off, magnetic domain images of all three magnetization vectors can be produced in one scan. The electrodes and the magnetic pole pieces were specially designed by using a three-dimensional computer simulation for electric and magnetic fields, electron trajectories, and spin rotation; the broad beam of the secondary electrons passes through to the spin detector with a 90° rotation. The structure is simple with only two electrodes that have hyperbolically curved surfaces to create a stigmatic focusing effect, while the surfaces of the magnetic pole pieces are flat to enable a uniform rotation of all electron spins. The performance was tested and confirmed to be effective by observing the magnetic domain structures of Fe(001) with in-surface-plane magnetization and a TbFeCo magneto-optical medium with surface normal magnetization.

Nonlinear magneto-optical effects in cold atoms of 87 Rb

With laser-cooled cold 87Rb atoms as a magneto-optical medium, a weak right circularly polarized probe field and frequency modulation technique are used to detect the magnetic distribution of the quadrupole field. A two-peak dispersion-like signal other than that of the usual nonlinear magneto-optical effect mentioned in other papers is obtained.

Optimization of surface-plasmon-enhanced magneto-optical effects

Kretschmann prism couplers, including magneto-optical media such as Co, Fe, and Ni, allow enhancement of the magneto-optical response, such as conversion of light polarization, thanks to the resonant excitation of surface plasmons. Since these media have high losses, they support overdamped surface plasmons. A way to get high-quality surface-plasmon resonance is to use noble-metal/ferromagnetic-metal multilayer thin films. The question that arises is, which surface-plasmon resonance leads to the largest enhancement: is it the overdamped or the high-quality one? We show that the best optimization not only depends on the orientation of the magnetic field but also on the magneto-optical coefficients. The most effective enhancer of the magneto-optical effects is the high-quality surface plasmons for the transverse orientation and may be the overdamped surface plasmons for the polar and longitudinal ones. This applies when the noble metal is gold and the magneto-optical medium is cobalt.

Development of a universal interface for the exchange of medical image data using magneto-optical media

Digital images generated by medical imaging form the basis for radiological diagnosis and surgical planning. Despite the advent of the DICOM 3.0 standard for medical image communication, widespread application of the existing information is often limited by incompatibility of the data formatting used by different equipment generations, and the manufacturer-specific standards employed. An exchange interface based on magneto optical discs has been developed to retrieve and present medical image data regardless of the technological hardware and the specific formats used. Specially adapted routines to retrieve the data first had to be developed. A modular program structure was used to allow flexibility in the implementation of further routines and other exchange media. Over 20,000 CT and MRI images including header information obtained from different General Electric and Siemens scanners were extracted successfully from MO discs. The image data were used for follow up and surgical planning and were transferred to a PAC-server. The interface proved reliable and easy to use. Support for further proprietary formats is currently being developed. The present exchange interface permits reliable retrieval of digital images for diagnostic and surgical planning purposes, regardless of the hardware generation and manufacturer-specific formats.

Effect of RE-TM underlayer on the microstructure of TbFeCo memory layer for high-density magneto-optical recording

A magneto-optical (MO) medium has been investigated as high-density data storage. This MO medium sputtered from an intermetallic compound alloy target has advantages for mass production. However, it may have some difficulties for advanced high-density recording due to the controllability of the magnetic properties. In order to use the mass-production advantages of this MO medium, the intrinsic magnetic properties of its film must be clarified. This paper investigates the relationship between MsHc and the magnetic underlayers, which are controlled by the film's microstructure. It is found that the MsHc of the memory layer can be increased by controlling the first magnetic underlayer; furthermore, the arranged microcolumnar structure in the memory layer is also found to be closely related to the underlayer. As a result, as much as 1.8/spl times/10/sup 6/ erg/cm/sup 3/ MsHc can be obtained even by sputtering from an alloy target. Adopting this film for the memory layer in a domain wall displacement detection system is promising approach for high-density recording that features reliably written tiny marks.

Ultrahigh-density magneto-optical recording based on the effect of plasmon excitations

The analysis of ultrahigh-density recording in magneto-optical media formed with the use of granular CoAg(Au) films is performed. Due to excitation of plasma oscillations of electrons in granular CoAg(Au) films, the latter make it possible to perform additional focusing of recording radiation and to significantly decrease the size of memory cells. It is suggested that two-layer film CoAg(Au)/TbFeCo/substrate systems be used as recording media for magneto-optical recording. It is shown theoretically that the angle of the Kerr rotation of the plane of polarization θK upon magnetization of such a two-layer structure increases significantly in the range of incident-photon energy from 2 to 4 eV in comparison with the case of TbFeCo magnetic film. Some specific features were detected experimentally in the spectra of θK in the region of excitation of plasma oscillations in nanogranular CoAu and CoAg films at ℏ ω P ≈ 2.5 and 3.5 eV, respectively, as well as in two-layer CoAg(Au)/TbFeCo film systems.

Mapping spatial variations of the coercivity on patterned magneto-optical materials

The spatial variation of coercivity on patterned magneto-optical (MO) materials has been studied with a Kerr microscope. Our results show that the coercivity values on arrays made by holes are larger than those observed on the land regions. In contrast with nonpatterned MO media, which present a sharp Gaussian-like distribution, the coercivity distribution in the patterned material is much wider and shows an exponential-type distribution. This article maps the spatial variation of the coercivity on patterned magnetic materials with a spatial resolution of 0.1×0.1 μm2.

Magnetic properties of multilayers of nano thin Co,Ce-doped and undoped yttrium iron garnet films for magneto-optic applications

Improved magnetic properties of as-deposited multilayers consisting of alternate nano-thin films of Co,Ce-doped and undoped yttrium iron garnet (YIG) formed by low pressure chemical vapor deposition using metalo-organic precursors are described. Multilayer garnet films consistently show high M s and H c values, which depend on the period number and period thickness. Multilayer films also show increase in anisotropy in the direction perpendicular to film plane. Multilayers having doped and undoped YIG films in each period in the thickness ratio 25 nm/30 nm with 6 periods show high saturation magnetization, M s value of 137 kA m −1 and high perpendicular to planer coercivity ratio ≈2. Maximum obtained coercivity, H c for a 6 period, 14 nm/17 nm multilayer film was 197 kA m −1. Multilayer films do not a require high temperature 900 °C, 5 h oxygen annealing for realizing improved magnetic properties which is inevitably needed for single layer Co,Ce-YIG films. Thus, as-deposited multilayer garnet films offer considerable technological advantage for application in magneto-optic media.

Analysis of the picosecond magneto-optical phenomena in scattering media of biological interest

The behaviour of a magneto-optically active biological-like medium under picosecond optical excitation is analysed. The new technique is based on the fact that photons trapped in multiple scattering events inside the magneto-optical medium leave the medium with larger induced rotation angles, as they travel longer distances. Two- and three-dimensional displacements of the photons in the medium are separately analysed. The dependence of this effect on the applied magnetic field strength, the value of the magneto-optical constant of the medium and the standard deviation of the statistical distribution of the photons scattered inside the turbid medium are studied. The best values for the magnetic field and optical parameters of the biological medium are proposed for the experimental observation of the picosecond magneto-optical phenomena in scattering media of biological origin. We also make some prospective studies to evaluate the potential application of the magneto-optical effect as a tool for optical tissue biopsy. Values for the optimum magnetic field intensities and for the expected experimental sensitivity in diverse conditions are reported.

The Recording of Tiny Marks Less Than 100 nm on Magneto-optical Medium Sputtered from RE-TM Alloy Target

Use of magneto-optical (MO) medium sputtered from an intermetallic compound alloy target has advantages for mass production but may not be suitable for high-density recording due to its poor perpendicular magnetic anisotropy. The relationship between Ms Hc, the product of saturation magnetization Ms and coercivity Hc, and film microstructure is discussed in this paper; we obtained an Ms Hc of 1.3 × 106 erg/cm3 using microcolumnar structure even when the medium was sputtered from an alloy target. By adopting this film for the memory layer in a DWDD system, tiny marks less than 100 nm in size can be reliably written. As a result, high-density recording at 15 Gbit/inch2 is achieved.

Developing the Potential of Plasmon Enhancement in Perpendicular (PtCo Multilayer) Magneto-Optic Recording Media

A PtCo multilayer magneto-optic recording medium is deposited on a nano-structured 2D sinusoidal grating substrate to study the concept of plasmon enhancement of magneto-optic readout and other phenomena associated with this type of surface. Comparison is made with the magneto-optic behaviour of a simple cobalt film deposited on the same structured substrate and PtCo multilayers in the Kretschmann-Raether configuration.

Suppression of crack formation in garnet film by using a compound glass underlayer

Garnet film made as a magneto-optical medium suffers from the problem of crack formation, caused by the high temperature of the garnet film fabrication process. We solved this problem by using a compound glass with a yield point lower than the garnet heat-treatment temperature as an underlayer. This successfully suppressed the formation of cracks in the heat-treated garnet film. As a result, we obtained a garnet film recording medium with excellent crystalline structure and high coercive force.

The evolution of filmless radiology teaching

The transition from hard-copy film to the picture archiving and communication systems (PACS) arena is often a long and difficult path, but the complete transformation to the digital environment does not end with the successful clinical practice utilizing PACS. The next hurdle for academic institutions is similar changes in the teaching field. The loss of hardcopy films can greatly hinder the radiology teaching file and teaching conference experiences. The next step in our growth is the conversion to digital teaching files and conferences. The original work 10 years ago with utilizing computers linked with laser disks and magneto-optical media, have now evolved into complex networks with expanding and relatively cheap storage media, such as CD-ROM and easily to navigate graphic-user interfaces such as hypertext markup language (HTML) and extensible markup language (XML), for use with multimedia teaching tools. The transition into the digital arena for radiology education and interdepartmental conferences can be accomplished through several different paths. These include direct transfer of images into a presentation program, as well as exportation of images into suitable image file formats for later use. There is also the ability to expand the PACS network to include conference rooms. Similar training and teaching can allow radiologists to transition into the digital environment for future digital teaching file creation as well as correlative radiology interdepartmental conferences.

Polarization dependence of readout signals from periodic one-dimensional arrays of magnetic domains in magneto-optical media and crystalline-amorphous line pairs in phase-change media of optical recording

Polarization dependence of signals from periodic one-dimensional arrays of magnetic domains in magneto-optical (MO) media and crystalline domains in amorphous phase-change (PC) media has been studied by theoretical calculation and experiment. The MO signal in the small-period regime depends on the direction of incident polarization. The relative strength of the E(?) and E(?) signals changes depending on the period of the pattern, the wavelength of the light, and the numerical aperture of the objective lens. For PC media, the reflected signal has similar polarization dependence, but this dependence is weak.

Magneto-optical Faraday and Kerr effect of orthoferrite thin films at high temperatures

Orthoferrites present, as bulk materials, reorientation transitions of their magnetic moment alignment at temperatures depending on the rare-earth (RE) ion. In particular, orthoferrites (REFeO3) with RE = Sm, Dy, present this transition at T SRT = 443 K and 36 K, respectively. The spectra of the complex Kerr and Faraday angle have been measured on orthoferrite thin films (RE = Sm, Dy, Y), which were prepared by pulsed laser deposition on amorphous quartz substrates. The obtained spectra exhibit contributions of both surfaces and interfaces. Propagation effects of the polarized light in the magneto-optical medium which is interpreted in terms of a simplified theoretical formalism, is also observed. For selected photon energies, temperature dependent Faraday rotation measurements, , on orthoferrite thin films (RE = Sm, Dy, Y) have been performed. A quite different thermal variation compared to the bulk magnetization has been observed. Curie temperatures are found to be close to the bulk values or slightly larger by 10 K to 20 K as in the case of DyFeO3 and YFeO3. For RE = Sm and Dy, increases with increasing temperature contrary to the saturation magnetization, passes through a maximum at about 460 K and vanishes with a T C of 647±18 K, 695 K for RE = Sm and Dy respectively.

A new Faraday rotator for high average power lasers

The new design of a Faraday rotator is proposedwhich allows one to compensate partially the radiation depolarisation in magneto-optical elements induced by heating dueto the laser radiation absorption. The new design is comparedanalytically and numerically with a conventional design forthe cases of glass and crystal magneto-optical media. It isshown that a rotator, which provides the compensation forbirefringence in active elements with the accuracy up to 1 %at the average laser radiation power of 1 kW in the rotator,can be created.

Influence of Groove Shape on Rear Expansion Detection Readout

We have investigated the influence of groove shape and recording scheme on readout of rear expansion detection (RED) magneto optical media. Marks recorded by laser pulse magnetic field modulation (LP-MFM) were found to expand without a field while those recorded by light intensity modulation (LIM) required a field for expansion. Media with steep wall grooves showed a high carrier-to-noise ratio (CNR) of 42.7 dB at a mark length of 0.2 µm when recorded by LP-MFM and required a low readout field of less than 100 Oe even when recorded by LIM.

Modulational instability and four-wave mixing in anisotropic χ(3)magneto-optic media

Nearly degenerate four-wave mixing (FWM) in anisotropic magneto-optic media is theoretically investigated. It is shown that the anisotropic nature of the nonlinear χ(3) tensor is responsible for the appearance of a novel FWM instability branch for which phase matching can be conveniently controlled by the application of an axial static magnetic field.

後方拡大型光磁気記録媒体における磁区拡大再生の機構

Rear Expansion Detection (RED) magneto-optical media with different intermediate layers were studied to investigate the mechanism of domain expansion readout. A TM-rich GdFe-M intermediate layer was found to improve the readout margin. A domain written with light intensity modulation (LIM) expands with the wall energy gradient as a driving force under a readout field, compensating for the shrinkage force of an expanding domain. After the expansion, the expanded domain shrinks through wall motion toward the high temperature region with the wall energy gradient as a shrinking force. The above intermediate layer reduces friction during wall motion by reducing both wall coercivity and the demagnetizing field. Domains written with laser pulse magnetic field modulation (LP-MFM) were found to expand with a zero field if the groove wall angle of the substrate is sufficiently high. At mark length of less than 0.3 μm, the shape of marks written with LP-MFM are more suitable for RED.