<title>Amorphous Thin Film Disk For Magneto-Optical Memory</title> (1982)

AbstractAmorphous rare earth -transition metal films as media of magneto -optical memory are de-scribed. Improvements of amorphous GdCo film in stabilizing recorded bits and in makinguniform film were performed. High speed recording and reading experiment were carried outon GdCo disk of 150 mm in diameter, and excellent stability of recorded bits, considerableuniformity, and high sensitivity for recording were confirmed. Thermal stability andspectra of Kerr rotation angle of GdCo film were also measured. It is shown that GdCo filmis very promising material for magneto -optical memory.IntroductionRequirements for erasable media and system in and with which informations are recorded ofa high density and a high speed are increasing. Magneto -optical recording is a promisingsystem owing to following advantages: This method is (1) erasable, (2) non -contact, (3) random accessible, and (4) non -volatile.In 1973, GdCo amorphous magnetic thin film was presented by Chaudhari et al. as amaterial for a magnetic bubble device, and it was also pointed out that it can be used formagneto -optical memory.' In the past several years, many studies in GdCo film and in newcombinations of rare earth metal (RE) and transition metal (TM) were performed. It becameclear that amorphous magnetic thin films are promising materials for magneto -optical memoryin high density and with high rate.In this paper, improvements in amorphous RE -TM thin films, especially in GdCo films, forthe application to magneto -optical recording, and results of measurements on thermal sta-bility and on the spectra of Kerr rotation angle of GdCo films are presented. Experimentalresults of high speed recording on GdCo disk and of reading from it are also described.Amorphous magnetic thin filmsAmorphous RE -TM films can be prepared easily by sputtering and evaporation technique onvarious substrates. In the case of GdCo film magneto -optical recording is carried out atcompensation temperature. Curie temperature recording is carried out on TbFe2,3, GdTbFekand other materials.Principle of compensation temperature recordingWe show a principle of magneto -optical recording on GdCo film. The compensation tempera-ture varies with the composition of this alloy film. GdCo film having the compensationtemperature near room temperature is suitable for recording.Figure 1 shows a typical temperature dependence of coercive force of GdCo film preparedby rf sputtering method, and Fig. 2 shows schematically the mechanism of magneto -opticalrecording. The film has an axis of easy magnetization perpendicular to the film plane(Fig. 2(a)). A small region of the film is heated by a short pulse of laser beam focusedon the film. The coercive force of the heated region decreases abruptly, as shown in Fig. 1,and the direction of magnetization of the small region turns around with the help of weakexternal bias fild (He) of about 100 Oe (Fig. 2(b)). Thus, information is recorded as asmall cylindrical magnetic domain. For the purpose of erasing the recorded bit selectively,it is enough to heat the bit by the laser beam with negative bias field. Recorded infor-mation