1. Field of the Invention
The present invention relates to a magnetic reproduction method of magnetically reading information from a magnetic recording medium, and a magnetism detecting device and a magnetism detecting apparatus for use in the magnetic reproduction method, as well as the magnetic recording medium.
2. Description of Related Art
Magnetically readable cards (hereinafter referred to as magnetic card(s)) have been used in a wide range of applications, for example, telephone cards, prepaid cards such as tickets, commutation tickets and admission tickets. This is because the magnetic cards, which are magnetically recorded, are resistant to dirt and have comparatively low running costs.
The magnetic cards are generally erasable and rewritable, but there are a large number of applications to be met with magnetic cards which are writable only once. If magnetic cards having lower running costs are developed, they can find far wider applications, such as admission cards for events or pass cards for door locks.
The present inventor has noted that toner having magnetism (hereinafter referred to as magnetic toner), for example, iron oxide, is used in copying machines or laser beam printers, and has obtained the idea of printing such magnetic toner on general-purpose plain paper or plastic film to form a magnetic recording medium. Conventionally, the magnetic toner has been used in laser beam printers, copying machines or the like as means for improving the resolution of recorded images, but no consideration has been given to a method of using the magnetic toner for the purpose of magnetic recording.
Although magnetic ink may be available in printing similarly to magnetic toner, magnetic ink easily spreads on paper other than selected kinds of paper, and involves the problem of drying. In addition, since magnetic ink is lower in sensitivity to magnetism than magnetic toner, magnetic ink is only suited to rough detection of magnetism, such as detection of the presence or absence of magnetism.
The present inventor used the aforesaid magnetic toner to print one stripe-shaped continuous recording track on plain paper, and carried out magnetic recording and reading on and from the recording track by means of a magnetic head. As a result, the following problems occurred.
Unlike magnetic powder for magnetic tape, the magnetic toner is not intended for magnetic recording and its coercive force Hc is as small as approximately 100 Oersteds. Accordingly, the magnetic toner is not resistant to self-demagnetization nor an external magnetic field, so that its magnetization is liable to vanish.
In addition to the aforesaid smallness of the coercive force Hc, the particles of the magnetic toner do not have a needle-like shape and are incapable of being oriented so that they can readily be magnetized in a particular direction. Accordingly, the amount of magnetic flux arising from the recorded magnetization is considerably small, so that a conventional inductive type of magnetic head is not able to perform reading with a high SIN ratio. For this reason, it has been extremely difficult to apply the magnetic toner to magnetic cards.