Corrosion protection of magnetic recording media is achieved by providing a thin protective barrier layer comprising ruthenium on the magnetic layer. In the method, a digital-to-analog conversion output signal of a high-speed digital-to-analog converter and a low-frequency sinusoidal carrier wave signal input to a comparator to obtain a low-speed pulse signal. The invention further includes a plurality of port devices coupled to the bus. Attribute information may relate to one or more of: reflectance, fluorescence or height. To solve these problems, the light emitted from a wide stripe semiconductor laser is passed through a mode converter and a wavelength selecting filter, then fed back to the active layer of the semiconductor laser, which fixes the oscillation mode of the semiconductor laser to single mode. A flapper makes an airtight seal with an air duct to prevent air from leaking out of an air chamber.