Hitachi Maxell, Ltd.
Whereas conventional magneto-optical recording on a disk uses only one recording layer, MAMMOS technology is made possible by using a newly developed disk with two recording layers. Conventional recording methods involve changing the direction of the external magnetic field while shining a laser beam on the disk's surface. This process is used to determine the magnetic moment of the recorded spot (about 0.6$B%*(Bm). Playback of the data depends on direction of the recorded spot's magnetic orientation. Accordingly, to increase capacity, a smaller recording spot is required, and although smaller recording spots can be technically produced, accurate playback is not possible using the above conventional method. MAMMOS technology solves this problem by employing a double layer recording film which assures accurate playback of a much smaller magnetic moment (0.1 - 0.3$B%*(Bm). The double layer consists of a bottom magnetic layer (the "recording layer") and a top magnetic layer (the "magnetic amplifying layer") which performs a special function explained as follows:
During playback, the small magnetic domain (recorded mark) of the recording layer is heated using a laser beam causing a new magnetic domain with the same magnetic orientation to form on the magnetic amplifying layer (due to magnetic transformation). Then by applying an external magnetic field in the same direction as the magnetic orientation, the magnetic domain of the amplifying layer grows, assuring accurate signal recognition during playback. The actual amplitude of the signal created during playback is more than three times as large as the signal created using the conventional recording method. And, to assure the amplified magnetic domains are precisely read one by one, an external magnetic field is applied in the opposite direction of the magnetic orientation of the amplified domain causing it to vanish before the next domain is amplified. To verify these processes, we have developed a high quality optical signal playback technology that precisely changes the direction of the external magnetic field to synchronize amplification and de-amplification of the magnetic domains during high speed operation.
Hitachi Maxell, Ltd. and Sanyo Electronics Co., Ltd. applied the superior technical know-how gained as leading recording media and drive manufacturers, respectively. The principle of amplifying electricity has been widely developed in the United States for use in the semi-conductor industry. The amplifying magnetism principle for use in the memory media industry has now been confirmed, leading to high expectations in various fields. For example, previously, optical disk capacity increases mostly depended on the development of short wavelength lasers to make even smaller recording spots. Now, MAMMOS technology increases optical disk capacity by changing the disk itself.
(1) Principle of MAMMOS (Magnetic Amplifying Magneto-Optical System)
Previously, data was written onto magnetic optical disks on the recording layer TbFeCo (terbium, iron, cobalt alloy) in the size of the laser beam (figure 1). The lasers wavelength is 0.68$B%*(Bm, which is almost the size of the magnetic domain (0.6$B%*(Bm). However, an extremely small magnetic domain between 0.1$B%*(Bm to 0.3$B%*(Bm (figure 2) is employed by MAMMOS. The top layer (Magnetic Amplifying Magneto-Optical) is made of GbFeCo (gadolinium, iron, cobalt alloy) formed by sputtering. This material has the characteristics to facilitate copying the magnetic domain onto the top layer by warming the top layer to about 100$B!<(BC with the laser. At the same time, applying a positive external magnetic field, as in fig. 2, helps the magnetic domain quickly grow larger to just the size of the light spot. Also, the magnetic orientation of the recording layer will not be affected within this temperature range so high density is kept as it is. This phenomenon is called magnetic amplifying. The amplified magnetic domain is the size of the light spot, allowing clear, large signals to be easily picked up. For continuous reading of data, the magnetic moments must be read precisely one by one. So, to make sure the amplified magnetic domain will not interfere with the reading of adjacent domains, the direction of the external magnetic field must be reversed, causing the amplified magnetic domain to shrink and disappear instantly. The magnetic amplifying layer will return to its original magnetic orientation and be prepared for the next amplification.
(2) The MAMMOS Experiment
First, we made a CD-size magneto-optical disk and recorded data onto the recording layer with a 0.3$B%*(Bm magnetic domain. The conventional method of playback displayed only extremely small signals (fig. 3). Next, magnetic amplification using MAMMOS technology was attempted and resulted in a signal size three times larger when applying a magnetic field in the positive direction (fig. 4). Also, by applying a magnetic field in the minus direction, the amplified magnetic domain signal disappeared. Additionally, an amplifying experiment for lining up randomly recorded magnetic domains was reproduced precisely. High speed switching of the magnetic field from plus to minus is possible before passing to the next magnetic domain, creating amplifying and de-amplifying processes. When there was no magnetic domain to be amplified, no signal was reproduced, achieving precise data playback.
(3) Developing a CD-ROM sized disk with 20 times the storage capacity.
MAMMOS disk technology and a 635nm infrared laser, will allow a rewritable optical disk that can store 14GB, 20 times more data than CD-ROM and 3 times more than DVD-ROM. So a MAMMOS disk can store up to 5 hours of digital video data at a data transfer rate of 6mbps. Additional applications include massive data storage required by network systems. The technology for this media is similar to that of MiniDisc so experience in the MD field is advantageous in developing MAMMOS related products, such as media and drives.