In tape systems, magnetic data is typically written by the fringing fields arising in the vicinity of the poles of a gapped, "ring"-shaped electromagnet. Owing to the fringing, however, the shape of the magnetic transition imprinted in the media—ideally, a straight line segment spanning the exact width of the track—tends to be distorted at the edges, displaying, e.g., track-edge curvature, or a large lateral decay length.
This lateral decay results in an "erase band" of finite width, a region at the edge of the written track in which the media is not uniformly magnetized enough to contribute to the readback signal. As track widths decrease to meet higher areal density requirements, the presence of this erase band implies a less efficient use of the available media area, unless its width can be scaled down at least at the same rate as that of the track.
In collaboration with colleagues at IBM Research - Almaden, we combine modeling and experiments to investigate the mitigation of track-edge distortions by modifying the geometry of conventional writer poles. One such modification, called notching, ensures that the edges of the two write poles are well aligned in the cross-track direction (which is not the case with standard writers, see Figs. 1d and 1e).
Notching decreases the lateral "spillover" of the write field, improving track edge abruptness and suppressing side-writing into the adjacent track. Another geometric modification, called a stepped pole, involves recessing part of the trailing pole (see Fig. 1f) such that the flare out of the write field at the pole's lateral edges is suppressed, thereby reducing transition edge curvature.
Figure 1: Color map of the effective write fields for (a) an unnotched writer, (b) a notched writer, (c) a notched writer with stepped trailing pole. The white line is the calculated transition profile. Dotted lines indicate the writer edges. (d-f) 3D views of the corresponding witer geometries (not to scale). (g-i) MFM images of shingled tracks at 171kfci and 74kfci. (the lower track was written first). (g) Unnotched writer, (h) notched writer, (i) notched writer with stepped trailing pole.