Toshio Niihara

Magnetic and magneto‐optical properties of Tb‐Fe‐Co amorphous films

Characteristics of Tb‐Fe‐Co amorphous films have been systematically investigated in order to optimize compositions suitable for magneto‐optical recording medium with high readout signal. In order to optimize alloy composition, Curie temperature TC and compensation temperature Tcomp are summarized as functions of both Tb and Co contents. Crystallization temperature Tx of Tb‐Fe‐Co amorphous films has been also measured to be about 400 °C and tends to increase with replacing Fe by either Co or Tb. To evaluate Tb‐Fe‐Co films as a magneto‐optical recording medium, carrier C and modulation noise Nmod (defined by the difference in noise before and after writing) are measured as a function of external field Hex. It is found that written bits of large Nmod show an irregular shape, while those of small Nmod are of regular one. To reduce Nmod and obtain high readout C/N, films with lower saturation magnetization Ms (that is, low demagnetization field) at just below writing temperature are preferred.

Study on recorded domain characteristics of magneto‐optical TbFeCo disks

Domain characteristics (size and shape) of TbFeCo disks of several compositions are studied systematically through direct optical observation under various recording conditions. The relationships among recorded domains, magnetic properties, and recording conditions are also investigated. Magnetic films suitable for very high‐density recording are found to have a compensation temperature between room temperature and 100 °C and a Curie temperature near 200 °C. In those films, the formation of domains is stable against fluctuations in laser power and external field. These results can be explained by a simple domain formation model in which coercive force, demagnetizing field, and wall energy at the domain wall are taken into account. Teardroplike domains are observed in the case of relatively long laser pulse recording. Such domains cause detection errors during the domain edge detection. It is also found that recording noise is high under certain recording conditions. The origin of this noise is supposed to...

Domain and write–read characteristics for magnetic field modulated magneto‐optical disk with high data transfer rate

Magneto‐optical domain shape and write–read characteristics were investigated in an overwrite scheme using a flying magnetic head. Through domain observation and domain shape numerical simulation on magnetic film, it was found that ‘‘tails’’ of domains must be shortened to increase the carrier‐to‐noise ratio (C/N) at high‐density conditions. To reduce the length of domain tails, isotherms on the magnetic film must be circular. For quick heat flow, we constructed a quadrilayered disk which has a thin metal film. C/N of the disk was about 50 dB at 7.4 MHz (0.75‐μm domain length) with linear velocity 11.3 m/s. Compared with the light power modulation method, the magnetic field modulation method is a more suitable overwrite technique, since the thermal profile is the same for any recording mark length.

Magneto-optical and magnetic properties of RE-TM/Pt multilayer films

Abstract The magneto-optical Kerr effect and magnetic properties of RE-TM/Pt multilayers are investigated (RE-TM = TbCo, TbFeCo, GdTbFeCo). The Kerr rotation of the RE-TM/Pt multilayers with amorphous Pt layers is larger than that of the Pt/Co multilayer in the region of 550–800 nm wavelength for next generation high-density magneto-optical memory.

High corrosion-resistant magneto-optical film on a new plastic substrate

To realize high-data-rate and high-reliability magnetooptical (MO) disks, a corrosion-resistant MO film and a novel plastic substrate were developed. Corrosion was reduced by adding a metallic element (M=Ti,Nb,Ta,Cr) to TbFeCo film. The magnetic characteristics of TbFeCoM film were designed to be the same as those of the TbFeCo film. The plastic substrate (photocurable casting resin, PHC) shows low retardation and high heat resistance. MO disks with a trilayer structure on PHC showed high reliability even in severe conditions. The performance of a prototype MO recording system using the PHC disk is presented. >

Perpendicular magnetic anisotropy of Tb‐Co amorphous films sputtered in H2‐added Ar gas

Very large perpendicular magnetic anisotropy has been observed in Tb‐Co amorphous films deposited by nonbiased rf sputtering using H2‐added Ar gas. As the hydrogen partial pressure of the gas increases, the perpendicular anisotropy constant Ku increases remarkably and rectangular Kerr hysteresis loop appears. For 10% H2, Ku reaches near 105 J/m3, which is sufficient to direct the magnetization perpendicular to the film plane. These films can be used as magneto‐optical recording media because of the relatively large Kerr rotation angle and coercive force. The mechanism of increasing Ku remains to be clarified.

2 GB/130 mm Capacity Direct-Overwrite Magneto-Optical Disk

Precise mark-edge recording on a high signal-to-noise ratio (S/N) exchange-coupled direct overwrite magneto-optical (MO) disk provides 2 GB capacity on a 130 mm disk with a sufficient edge jitter margin. Write compensation technology was proven to be effective for both non- and direct-overwrite MO disks.

Perpendicular anisotropy in Tb-Fe and Tb-Co amorphous films sputtered in H 2 -added Ar gas

It has been found that the perpendicular anisotropy Ku in amorphous Tb-Fe and Tb-Co films prepared by rf sputtering changes remarkably when H 2 gas is added to Ar sputtering gas. Ku in Tb-Fe films decreases with increasing H 2 partial pressure, while Ku in Tb-Co films increases and reaches almost 106erg/cm3. The internal stress in the films changes slightly with increasing H 2 partial pressure. Consequently, the change in Ku can not be explained in terms of magnetostriction. Mass spectroscopy shows that those films sputtered in H 2 -added Ar gas contain H 2 about three times more than those sputtered in pure Ar gas. It is likely that H 2 induces change in microstructure in films causing large change in Ku. However, the mechanism and the reason for different behaviors between Tb-Fe and Tb-Co are not clear yet.

High Linear Density Recording of 0.3 µm Domain Length on Magneto-Optical Disk

High linear density magneto-optical recording of 0.3 µm domain length is studied for both laser power modulation and magnetic field modulation methods, through optical observation and numerical simulation of recorded domains on a rotating 5.25 inch TbFeCo disk. In the laser power modulation method, domain length of around 0.3 µm changes largely with fluctuations in recording power. By decreasing the wavelength of the incident laser beam, the amount of change can be reduced. On the other hand, in the magnetic field modulation method, a 0.3 µm domain length can be recorded stably against power deviation even in the case of an 830 nm wavelength at linear velocities of 5~15 m/sec.

Magnetic and Magneto-Optical Properties of TbFeCo Amorphous Films

The relationship between the magneto-optical characteristics and the medium noise of TbFeCo films is studied. The Ms changes with temperature in Fe-rich and Tb-rich films differ markedly. Magnetic inversions occurred at high temperatures in films with a large Ms, causing a demagnetizing field which left blank sections in the recorded bits and increased the playback noise. At normal temperatures, Tb-rich films have a small Ms and, as a result, less modulation noise.