A magneto-optical storage medium that protects against the effects of cross-talk, making it possible to further reduce the level of track density, and which also makes it possible to conduct high-density recordingplayback with a reduced magnetic field for erasure and a favorable level of playback signal quality. In an example, the apparatus includes a cantilever-based sensor that includes an optical sensor and a cantilever structure. The server appends billing information and a digital signature to the first message based on account information of one of the users stored in the database. A magnet is utilized to attract a leading end portion of the guide rod. A hydraulic classifier for separating particles having a mean particle size of from about 2-7 \u03bcm in accordance with the present method is provided also. A fourth circuit selectively couples the second input bus pair to the reference voltage.