Magnetic Multi-Valjued Magneto-Optical Disk

Abstract

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

Multilevel magnetic media in continuous and patterned films with out-of-plane magnetization

“…The multilevel storage deals with a discrete number of levels and belongs to the numerical storage family which is different from the well-known analog storage [5] which deals with a continuum of levels. Such a multilevel approach is already used in other technologies as, for example, magneto-optical storage [6] or flash memories [7]. Multicoercitive systems have also been reported for GMR head application or MR enhancement [8].…”

Codes for digital recorders

“…In all such studies, however, the potential increase in recording density that might accrue from the application or adaptation of coded-modulation techniques developed for digital communications has been outweighed by the increase in detector complexity and, more fundamentally, the cost in signal-to-noise ratio that accompanies the linearization process. However, several novel storage technologies can support multilevel alphabets, such as electron-trapping optical memories ETOM [148], [31] and optical recording with multivalued magnetooptic media [176].…”

Multi-Wavelength Read-Out for Double Layered Mo Disk

“…3 (6). However, the amplitude of read-out signal of MMV disk is decreasing with increasing of number of signal levels by using multi-value read-out.…”

“…However, for the case of a small amplitude of the external field (HI and H 2 ), the directions of the magnetization of written domains for only 2nd layer are the opposite direction of the external field. This phenomenon is caused by magnetically exchange coupling between TbFeCo and PtCo layers (6). Therefore, data are individually written in each memory layer by using magnetic effect between magnetic layers without changing a focal point.…”

“…Nnns ""\/Sl1+S12 (6) where (7) SNR XI and SNR X2 are the ratios for the signal amplitude (o-p) and the r.m.s. values of noise Nrms.…”