The magnetic properties required by a potential pigment for magnetic recording tapes are discussed from the point of view of high intrinsic sensitivity. It is pointed out that a high maximum slope of the remanence curve of the pigment is synonymous with high sensitivity, and the respective roles of the coercive force and the saturation remanence in determining this slope are discussed. The geometric characteristics of commercial pigments of (symbol)-Fe2O3, which govern these properties, are demonstrated. These characteristics are summed up by the statement that the individual particles of the pigment must behave like single-domain particles. This is achieved in most American pigments with particles which approximate prolate ellipsoids of revolution measuring about 0.5 x 0.1 micron. Discussed, too, is a German pigment which possesses particles that are roughly spherical and measure about 0.06 micron in diameter. The influence of the manufacturing processes on the sensitivity of the final product is examined, and data correlating tape sensitivity with all of these factors are given. The factors included in this correlation are the maximum slope of the remanence curve of the pigment, the density of the particles in the tape coating, the coating thickness, and the degree of pigment orientation.
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