On the design of magnets for the orientation of particles in tapes

In designing an efficient magnet for aligning particles in a magnetic recording medium it is not sufficient to consider only the peak value of the orienting field. The sequence of fields must also be controlled since it is possible for particles oriented in a field of 1000 Oe to be disoriented by the application of a field of only 100 Oe in the opposite direction. Such a reversed field is likely to be found in the common magnet design in which the source of magnetomotive force is symmetrically placed with respect to the air gap. The paper addresses the design of an orienting magnet for tapes. The undesirable reversed field is reduced in two steps. First the source of mmf is moved to a position upstream of the gap. Second, antennae are added to the exit port and further reduce the reversed field by extending its spatial distribution. An orienting magnet embodying these changes had the following characteristics: orienting field = 1800 Oe, maximum reversed field at the exit port: 10 Oe, and an orienting performance indistinguishable from that of a solenoid having the same peak field.