Ferroelectric Probe Data Storage

Fig.1 Artificial nano-domain dot written on a LiTaO3 single
crystal having a diameter of 3 nm.

Fig.2 Digital data written on a ferroelectric recording medium
with areal recording density of 4 Tbit/inch2.

There is growing interest in probe data storage technology, which would realize ultrahigh recording density over 1 Tbit/inch2. Especially, probe data storage using ferroelectric materials as recording media is one of the most promising methods, because artificial nano-sized domains with high stability can be formed on ferroelectric materials. Figure 1 shows an artificial nano-sized domain dot formed on a ferroelectric LiTaO3 single crystal. The diameter of the dot is as small as 3 nm, and is far smaller comparing with the bit sizes of conventional data storage devices. (HDD, optical disk, etc.) This means that ferroelectric materials have large potential for data storage device application from the view point of improving the recording density.

SNDM technique is used in the read/write head of the proposed data storage method, because SNDM can detect the ferroelectric polarization with high spatial resolution. Up to now, we have demonstrated that real information data can be recorded with the areal recording density of 4 Tbit/inch2 on a ferroelectric recording media. (Fig.2) We are now developing a prototype storage system based on the above mentioned techniques aiming for the practical realization. (Fig.3, Fig.4) More specifically, we are working on improving read/write speed for this method because this is necessary for mass data storage application as with high recording density. At the present time, the read speed is confined to Mbps order, and thus its significant improvement is required. (Fig. 5) As one approach for this purpose, we are developing novel ferroelectric material which is tuned for our application with enabling the read back signal intensity to be enhanced.

Fig.3 Prototype test system of the ferroelectric probe data storage.

Fig.4 Data bit array written using prototype test system.

Fig.5 Readback signal observen with a PZT recording mrdium. (Bit rate: 3.72 Mbps)

Recording Media for High-Speed Readback

As mentioned in the previous section, one of the major issues for the practical application of ferroelectric probe data storage is the improvement of readback speed. For this purpose, a recording medium that can obtain a large readback signal that is not buried in noise even during high-speed reading, that is, a recording medium having a large nonlinear dielectric constant is required. Controlling the material properties of ferroelectrics by doping is a commonly used method. And it has been found that this approach is also effective for increasing the nonlinear permittivity. Therefore, we are currently working on increasing the nonlinear permittivity by doping various elements into the existing ferroelectric materials, and aiming at improving the readback speed.

On the other hand, there is a significant tradeoff in the development of recording media. Increasing the nonlinear permittivity often causes deterioration of recording retention characteristics, which is one of the most important requirements for recording media. Therefore, as a means to overcome this tradeoff, we are also working on developing double-layer recording media, in which a material with a large nonlinear permittivity is used as the upper layer and a material with excellent retention characteristics as the lower layer.