1. Field of the Invention
The present invention relates to a heat-mode recording medium comprising an organic recording layer capable of recording information by the application of heat thereto or irradiation of light, and more particularly to a heat-mode recording medium capable of recording information by utilizing the change in the crystallization states of the above-mentioned organic recording layer.
2. Discussion of Background
A heat-mode recording system, which enables information to be recorded therein by applying thermal energy thereto and changing the configuration or physical properties of a recording layer of the recording medium, has lately been put to practical use.
The above-mentioned recording media can be classified into an inorganic-type recording medium and an organic one. The inorganic-type recording medium comprises a metallic material consisting essentially of Te, Bi, Se, Tb or In. On the other hand, the organic-type recording medium comprises an organic dyestuff, for example, polymethine-type dyes such cyanine dye; or macrocyclic azanulene-type dyes such as phthalocyanine dye, naphthalocyanine dye and porphyrin dye; naphthoquinone dyes anthraquinone dyes; and dithiol metal complex type dyes.
When the thermal energy is applied to the above-mentioned recording medium by projecting a converged laser beam onto the recording medium, a portion of a recording layer where the laser beam is applied is fused or evaporated to form a pit, which enables information to be recorded therein. However, those recording media has no reversibility of erasing the recorded information and recording another information.
As the development of the aforementioned reproduction-only or write-once heat-mode optical recording media, a need for a reversible recording medium capable of recording, reproducing and erasing information becomes pressing.
As the above reversible recording medium, there is known a magneto-optical recording medium comprising a thin layer made of an alloy of a rare earth metal such as Gd, Tb or Dy and a transition metal such as Fe, Ni or Co. The magneto-optical recording medium, however, has the shortcoming that the sensitivity is not so high that the S/N ratio in reproduction is still poor. In addition to the above, the recording sensitivity is gradually deteriorated because the recording medium is oxidized with time, so that high recording stability cannot be obtained.
Furthermore, there is known a reversible recording medium which comprises an inorganic recording layer essentially consisting of an inorganic material such as Ge, Te, Se, Sb, In and Sn. In this reversible recording medium, information can be recorded by taking advantage of the phase transition from a crystalline phase to an amorphous phase of the recording layer. This reversible recording medium can perform the recording and erasing operation in a heat-mode only by applying a laser beam thereto. However, in the recording layer of the above reversible recording medium, a recorded area cannot offer a sufficient contrast to a non-recorded area, which cannot stabilize the recording operation. In addition to this, the above-mentioned reversible recording medium has the shortcoming that the materials for the recording layer themselves are not stable.
Furthermore, a method of preparing a recording medium which comprises an organic recording layer comprising a fatty acid o fatty acid derivative is disclosed in Japanese Laid-Open Patent Application 63-289354. By this method, a predetermined amount of a fatty acid or derivative thereof is fused on a substrate by increasing the temperature of the substrate higher than the melting point of the employed fatty acid or fatty acid derivative. Another substrate is placed on the fused fatty acid or derivative thereof, with a pressure applied to the upper substrate side, to spread the fused solution of the fatty acid or derivative thereof between a pair of substrates. By cooling the substrates and the fused solution of the fatty acid or derivative thereof sandwiched therebetween, as a whole, a recording medium can be prepared in such a fashion that a recording layer comprising a fatty acid or derivative thereof is interposed between a pair of substrates.
The thus prepared recording medium is allowed to stand in a constant temperature bath maintained at a temperature higher than the melting point of the employed fatty acid or derivative thereof for several hours, while a pressure is uniformly applied to the recording medium by placing a weight thereon. After several hours, the temperature of the constant temperature bath is lowered at a fixed rate. As a result, the fused fatty acid or derivative thereof interposed between a pair of substrates is crystallized, and a recording medium comprising a pair of substrates and an organic layer interposed therebetween, as shown in FIG. 1, can be initialized. The thus formed organic layer is excellent with respect to the uniformity of its thickness and refractive index.
However, in the process of crystallizing the above-mentioned organic recording layer, the phase transition of the organic recording layer from a fused state to a crystalline state proceeds as the crystalline nucleuses are spontaneously formed in the fused organic recording layer and then grow in the course of cooling. In the above-mentioned method of preparing the recording medium, numerous crystalline domains tend to be oriented in different directions, as shown in FIG. 2, because the organic recording layer is crystallized by polynuclear growth of crystals. This is not always suitable for precise recording of information.