Thermal stability evaluation of 0.03μm MO domains in MAMMOS

Abstract

~, ~~ the future of high density MO recording. The problem should be investigated not only for HDD, but also for MO. There are few reports concerning this thermal stability problem for MO. The reason is due to the difficulty in detecting MO signals for the very tiny domains involved, less than 0.1 pm. As one solution for this small domain detection, we proposed a magnetic domain expansion detection named MAMMOS (Magnetic AMplifying MO System). By using this technique, very tiny MO signals reproduced from small domains less than 0.1 vm in length can be enhanced to the saturated signal level”. ’I. In this paper, the thermal stability of these very tiny domains is discussed. Experimental Drocedures The readout layer (GdFeCo:20nm) and recording layer (TbFeCo50nm) were prepared by a magnetron sputtering process on a Poly-Carbonate substrate. The typical TbFeCo for MO recording layer is an amorphous rare earth transition metal alloy. The coercive force at room temperature is quite large (over 20kOe). From this characteristic, it seems that very small domains could be recorded very stable. However, the experimental data were not reported. A non-magnetic intermediate layer separated each magnetic layer. In order to investigate the thermal stability for very small domains, a read-write tester was used with 680nm wavelength, with an objective lens of numerical apeaure, 0.55. The linear disk velocity was 0.8dsec. Exoerimental results and discussions Generally, high readout laser power irradiation causes reformation of the recorded domains because of decreased thermal stability of the domains. This leads to data error. We investigated the change in readout signal before and after irradiation of high laser power to the recorded areas in a MAMMOS disk. In one example, repetition patterns of 0.031.” (30nm) domains and 0.97pm spaces were recorded and the readout waveforms, observed. After this, the high lascr power was irradiated several times onto the recorded areas and the readout waveforms were observed again. In this way, we measured the irradiation number of the high laser power until an error MAMMOS signal was observed. The relation between the laser power and maximum temperature of the recording layer in the laser spot was estimated. According to this, the readout laser powers were transformed to the maximum temperature of the laser spot. The resulting thermal stability is shown in Fig.]. This is a typical graph of Anhenius plot to illustrate the thermal stability. The plots show a linear relation between the inverse temperature and the number of high power laser irradiation corresponding to when error a peared. In the case of a MAMMOS readout temperature region around 1.97e-3 K- , the laser P irradiation number indicates over le10. This is sufficient for actual data reproduction. Also, in the region of 373K (lOOC), the thermal stability of 0.03pm domains was investigated. The laser irradiation number was over le50. This number corresponds to at least over 100 years of archival data storage. In the figure, there are two symbols. The closed squares correspond to the result of the 0.03pm domains and the triangles are those of the 0.10 pm domains. However, there is no apparent difference between them. Conclusion Thermal stability of 0.03pm recorded domain on the MAMMOS disk was investigated. The results indicated that the readout cycle is over le10 times and the archival life at around lO0C is beyond 100 years