Magnetic Dispersions

Abstract

DURING the manufacture of magnetic recording tape a dispersion of acicular γFe₂O₃ particles in a suitable lacquer is spread on to plastic film and is passed through a strong orientating magnetic field before appreciable drying has occurred. It has been found that if the orientated coating is passed while still fluid through a second magnetic field acting in the opposite sense to the orientating field, both the orientation and original remanence are destroyed at a certain critical field-strength (Fig. la, b). This phenomenon, which has been named “Dorf effect”, is related to the rheological breakdown and consequent disturbance in the dispersing medium, induced by the rotation of elongated elements of the magnetic particle structure. It can thus be used as a highly sensitive method for the study of such structures, and in the field of general rheology. A machine which automatically measures the Dorf effect and which is suitable for the routine checking of dispersions during manufacture is currently in use.