Pump-pulse induced magnetization reversal of amorphous Gd 23.1 Fe 71.9 Co 5.0 showed a subpicosecond magnetization collapse followed by a slower reversal. The reversal dynamics is well described by the Bloch equation via a reversal time that does not depend on temperature, but strongly decreases with increasing pump fluence. A comparison to data obtained in external saturation field and in remanence opened the way to separate the contributions due to temperature induced effects within single domains from those related to field induced domain formation.

An amplified magneto-optical signal readout by using a dynamic domain expansion effect has been proposed and experimentally confirmed. Small domains of around 0.3 μm in diameter were expanded by a pulsed external field on the readout layer copied from the recording layer. An amplified readout signal up to three times larger than that of the original one has been achieved.

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Single-beam overwrite in an optical disk is reported using In-Se-Tl amorphous-crystalline phase-change recording films having very short erasing (crystallization) time of 0.2 μs. The lifetime of the amorphous state in an In-Se-Tl film is estimated to be longer than 10 years at 60 °C. This is due to its high crystallization temperature (135 °C) and high activation energy (2.6 eV). The phase-change cycles can continue over 106 cycles in stationary state experiments using test samples. The possibility of single beam overwrite (rewriting without prior erasing as with a magnetic disk) is verified by using a 1.6-μm-diam round laser beam spot irradiated on a 5-in.-diam disk rotating at 2400 rpm.

To broaden magneto-optical disk application, the most important issue is a remarkable increase of recording density. This paper firstly opens magnetic multi-valued (MMV) MO disk. Especially. here, quadri-valued MO recording was studied by a field modulation overwriting method using a new layer structured disk. Such attempt gives very attractive approach to make double or more higher capacity disk not depending on laser wavelength.1. Dual Write State MO Media It was found that rare-earth rich TbFeCo coupled with Pt84C016 shows dual writing state against external field as shown in Fig. l(a). This TbFeCo film has a Curie temperature of 200°C and has no compensation point. Even at a erase side field direction (minus sign), there occurs write state as noted by B. Also, at a larger positive field there occurs again write state as C. Such behavior is explained by the similar phenomena with the capping layer effect of PtCo [1]. Magnetic Multi-Valued MO Media (MMV)The quadri-valued MO media was invented combining the above mentioned bi-Iayer and a conventional TbFeCo. The external field dependence of a conventional TbFeCo is an ordinarily type as shown in Fig. 1 (b). The important thing is a thickness which make the film see-through behavior. Typical thickness of conventional TbFeCo is Ilnm. The role of PtCo layer is both a magnetic capping layer and a optical reflective layer. Very thin SiN layer was inserted between two ThFeCo layers.The layer structure is shown in Fig. l(c). Four level switching field is applied corresponding to the above mentioned 4-area ; A, B, C, D of the dual writing state. As a result, four coupled magnetization states are available on such a combined media.With this MMV media, reproduced signal is shown in Fig. 2(a). Each state is recorded by DC field, -900, -500, +500, +900 Oe respectively with the DC laser beam. Clearly separated 4 signal levels are observed.It is necessary for certifying a multi-valued media that every new recorded state should be written regardless the previous recorded state. We confirmed this performance by the following experiment. First, the 4 signal DC states like Fig. 2(a) are written on a track. After that, 400 ns pulsed light was irradiated at a fixed DC field on different those 4 signal states. In the Fig. 2(b) each photo shows the re-recorded results for each external fields respectively. You can see that re-recorded state is always transposed to the essential state defined by the external field. This also guarantees the direct overwrite by the field modulation method for multi-valued MO media. Fundamental Experiment of Field Modulation OverwritingWith this MMV media, we actually confirmed the multi-valued recording performance on field modulation method. Four external field levels using a magnetic head was applied irradiating fixed DC laser beam. The fI;:,:produced signal is shown in Fig. 3(b), which shows a quadri-valued overwrite recording corresponding to the external field.429