on the spatial extent of the write field for the sake of a simple argument we assume that the effective write field extends just to the outer edge of the pole pieces and that the shoulders of a written magnetization profile have the same spatial extent then at t5 the leading edge of the second write pulse the head needs to be separated from the recorded bit at least by a distance single polarity s2 2p g alternating polarity s2 p g where g and p are the of the gap and the pole pieces respectively this minimum separation defines t2 s2 v the min spacing between write pulses by t6 the second transition freezes in the medium the final illustration at t7 shows both transitions recorded and moving with the medium in the rz mode the attainable recording density is limited by head geometry medium velocity and duration of the write pulse as shown in fig1 this figure shows reciprocal bit spacing as a function of medium velocity for three different head geometries and three different pulse lengths from fig1 it can be seen that the attainable density is predominantly limited at lower media velocities by head geometry and at higher media velocities by the length of the write.