Takashi Tokunaga

Novel direct overwriting technology for magneto‐optical disks by exchange‐coupled RE‐TM quadrilayered films

A light‐intensity‐modulation method for direct overwrite using a magneto‐optical disk with exchange‐coupled RE‐TM quadrilayered film has been developed. This film consists of a ‘‘memory layer,’’ a ‘‘writing layer,’’ a ‘‘switching layer,’’ and an ‘‘initializing layer.’’ The initializing layer is used to initialize the writing layer, and the switching layer is used to switch off the coupling between the writing layer and the initializing layer above the Curie temperature of the switching layer. These two layers have made direct overwrite with only a conventional bias magnet possible.

Study of interface wall energy in exchange-coupled double-layer film

Recently, exchange‐coupled multilayer films have been investigated intensively for the magneto‐optical recording medium. We studied the interface wall energy density (σw) in the exchange‐coupled double‐layer TbFeCo films which were prepared by the dc‐sputtering method. The composition of the first layer was Tbx(Fe85Co15)100−x (x=14.0–35.0), and that of the second layer was fixed at the compensation composition of Tb23.8(Fe85Co15)76.2. The magnitude of σw stored in the boundary was determined by the shift field of the minor hysteresis loop of the first layer measured with VSM. It was found that σw depended on the Tb content of the first layer. As a result, σw was correlated with the perpendicular anisotropy (Ku), and well explained by the formula, σw∝(Ku)1/2. We can treat the interface wall energy practically as Bloch wall energy; however, σw was determined by the magnetic properties of both layers.

A New Direct Overwrite Method by Light Intensity Modulation with Multi Layered Magneto-Optical Discs

We have found possibility of performing direct overwriting by light intensity modulation with only one conventional bias magnet using a new type of exchange-coupled double-layered film. The biggest characteristic of this film is existence of hysteresis of temperature distribution in the film for the light intensity and that makes overwriting possible.

Edge Recording on Direct Overwrite Exchange-Coupled Multilayer Media

The possibility of edge recording on direct overwrite exchange-coupled multilayer media was evaluated. It has been established that overwrite media are not inferior to conventional magnetooptical (MO) media in edge recording because overwrite methods which use light power modulation have an advantage of reduced standard deviation of edge jitter even if the residual signal level is high. In this paper, direct overwrite exchange-coupled quadrilayer media were examined to prove this advantage.

A new z-axis capacitive accelerometer with high impact durability

This paper reports a newly developed z-axis capacitive accelerometer with high durability to impacts. Our accelerometer has a unique sensing mechanism by which the displacement of an inertial mass caused by an acceleration in the z-axis direction is converted into the rotational displacement of a pair of detection plates. Our accelerometer has a high degree of design freedom in terms of selecting the resonant frequency and sensitivity. While maintaining a constant sensitivity, the resonance frequency and etching hole within the detection plate were designed so as to have an appropriate damping effect. As a result, it achieved an output saturation level higher than 5000 G under the impact acceleration of 5 kHz.

Recording Characteristics of Double-Layered TbFeCo Disk

Exchange-coupled double-layered TbFeCo films, consisting of a transition metal-dominant first layer and a rare earth-dominant second layer, were found to serve as high-performance media for erasable optical disks. In this paper, we describe studies of the dependence of the recording characteristics on the magnetic properties of each layer. The temperature dependence of the switching field for each layer as measured using the Hall effect is described, and the change in the magnetization process with temperature is discussed. It is suggested that the recording noise in the large-H c1 (coercive force of the first layer) film at lower write powers results from differences in the magnetization process with the recording temperature.

Direct Overwrite Magneto-Optical Disks with Magnetically Induced Super Resolution Readout Function

Threshold temperature design for a light intensity modulation direct overwrite magneto-optical disk with magnetically induced super resolution readout layers was investigated. It was confirmed that the magnetically induced super resolution read power margin can be enlarged by selecting adequate threshold temperatures for writing and erasing. Also it allowed a sufficient erase power margin for direct overwrite operation and suppressed a cross write. High recording density with a track pitch of 0.7 µm and a mark length of 0.35 µm with a sufficient read and erase power margin was demonstrated.

High density overwriting in exchange-coupled magneto-optical multilayer films

The possibility of high-density overwrite recording using the light modulation method in magnetooptical disks has been investigated. It was found that the area around a mark written by irradiation of high power was erased spontaneously, regardless of the prerecorded states. It is effective for high-linear-density recording. On a disk with a track pitch of 1.4 mu m, signal deterioration by thermal interference was not observed after recording at an adjacent track with an off-track of 0.2 mu m. >

Multilevel Recording with Multilayer Magneto Optical Media by Light Intensity Modulation

Multilevel recording is one of the promising methods for increasing recording density. In this paper we describe multilevel recording using multilayer Magneto optical (MO) media by light intensity modulation. The MO disk has four layers, they consists of the recording layer, the record backup layer, the switching layer and the initialization layer from the substrate side. The data are recorded on and played back from the recording layer. A narrow stripe whose width is less than that of a regular mark is written on the initialization layer when the disk is manufactured. A gray mark is formed by controlling the transcription of the thin stripe written on the initialization layer to the recording layer. Therefore, the write margin is wider than in conventional multilevel recording. This method has additional advantages in that a conventional optical head can be used, and cyclability is unlimited. The possibility of multilevel recording is thus proved.

MAGNETIC DOMAIN EXPANSION PHENOMENA IN A DOUBLE MASK TYPE MAGNETICALLY INDUCED SUPER RESOLUTION MAGNETO-OPTICAL DISK

The readout mechanism in a trilayer double mask type rear aperture detection (DRAD) magnetically induced super resolution (MSR) disk was studied by readout waveform analysis. A signal enhancement phenomenon was observed, which was ascribed to magnetic domain expansion. Two kinds of magnetic domain expansion mechanisms for the erase direction readout field (Hr) and for the write direction Hr were found. In the case of write direction Hr, a magnetic domain can expand toward the rear mask before the mark contacts with the rear mask from the estimation of mark size and mask position. Edge jitter was evaluated in each readout mode and characterized according to the readout mechanism. Both high-resolution power of MSR and signal amplitude enhancement due to magnetic domain expansion contributed to improvement of DRAD disk readout signal qualities.