MAGNETIC RECORDING MATERIALS SINCE 1975

When information is to be recorded and stored on a re-usable medium, magnetic recording, in one form or another, has been and is today the dominant technology. Magnetic particles or thin films having a coercivity of several hundred oersteds, are easily capable of retaining a magnetic pattern of recorded information (at densities of tens of thousands of bits/inch and track densities of thousands of tracks/inch) for hundreds of years and yet when desired, the pattern can be changed by simply writing the new information over the old. Since the recording process requires only a change in direction of the electron spins, the process is infinitely reversible and the new information may be read immediately with no development process being required. This paper deals with developments of the magnetic (and some important non-magnetic) properties of recording media that have occurred since 1975. It describes their current embodiment in particles of cobalt-modified iron oxides, chromium dioxide, barium ferrite and metals and in thin films of metals, alloys and oxides. The curious and currently unexplained aspects of the behavior of these important materials is noted.