1. Field of the Invention
The present invention relates to a magentic recording medium and a process for the preparation of the same. More particularly, the invention relates to a magnetic recording medium having a magnetic recording layer consisting of at least two layers which is improved in frequency characteristics and a process for the preparation of the same.
2. Description of Prior Art
Magnetic recording media such as an audio tape and a video tape are desired to be excellent in various properties in recent years. For example, an audio cassette tape is required to show low noise, well-balanced frequency characteristics and high output in all frequency bands. Such requirement for the audio cassette tape tends to increase more and more because various developments such as a development of digital system have been recently made in pursuit of ultra hi-fi audio sources or extremely low noise audio sources as shown in a compact disc. A video tape is also required to show higher video output and lower noise.
In order to accomplish the above-mentioned excellent properties of the magnetic recording medium, various methods have been proposed. For example, a method of using iron oxide magnetic particles having a short length in the mean longitudinal length as a magnetic powder of the recording medium is known is lower a noise level. This method is able to lower a noise level of the resulting medium, but unable to give satisfactory frequency characteristics to the medium.
For improving the frequency characteristics, a magnetic tape having a magnetic recording layer which consists of two or more layers has been proposed. In the conventional magnetic tape having two or more recording layers, an upper recording layer is made to have a higher coercive force than that of a lower recording layer so as to give excellent characteristics in the high-frequency band to the upper recording layer, and thereby to obtain a high output level in all frequency bands in the resulting medium. However, such conventional tape does not have a uniform reproduction output in all frequency bands because of the distinct separation between the upper and lower recording layers, and for example, the reproduction output of the tape lowers in the mid frequency band or partially decreases in the extreme in the reproduction procedure. Further, the conventional magnetic tape having plural recording layers is disadvantageous from the viewpoint of practical operation and cost for the preparation (for example, a number of coating procedures are required for the tape), although the frequency characteristics are enhanced in accordance with the number of recording layers.
In other conventional magnetic recording medium having plural recording layers, magnetic particles having a short length in the longitudinal direction are incorporated into the upper recording layer for the purpose of reducing a noise level and magnetic particles having a long length in the longitudinal direction are incorporated into in the lower recording layer for the purpose of increasing frequency characteristics in the high-frequency band, in addition to the above-mentioned consideration on the coercive force. However, when the magnetic particles having a long length in the longitudinal direction is used for the lower recording layer, the surface smoothness of the resulting lower recording layer is decreased more than the case of using magnetic particles having a short length in the longitudinal direction. Thus decreased surface condition of the lower recording layer influences on the upper recording layer, and as a result, satisfactory smoothness is hardly given to the surface of the upper recording layer. Accordingly, although the above-mentioned method (i.e., method of using magnetic particles of the specific length in the longitudinal direction in each of the upper and lower recording layers) can improve the frequency characteristics of the resulting medium to a certain level, the frequency characteristics deteriorate in the high-frequency band in the case of an audio tape, and the S/N ratio decreases because of lowering of sensitivity and increase of noises in the case of a video tape.