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. Figure 1 illustrates the geometry of typical tape write transducers. 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 the Almaden Research Laboratory, 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. 2d and 2e). 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 Figs. 2f and 3) such that the flare out of the write field at the pole's lateral edges is suppressed, thereby reducing transition edge curvature.
- Figure 1. (a) Optical microscope image of early generation tape write transducer. (b) Cross sectional view through the center of a 3D model of a tape write transducer.
Figure 2: 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.
The in-plane shape of the imprinted magnetic transition can be estimated using maps of the effective write field Heff( x,y) obtained with finite element modeling (FEM), as illustrated in Figs. 2a-c for the three main geometric types. The model predicts that using stepped poles with well-chosen geometric dimensions is effective in suppressing transition-edge curvature, and results in a larger proportion of the written track width being usable.
To validate the predictions [1,2], we used a focused ion beam (FIB) to modify the pole geometry of conventional tape writers to compare the three pole types described above. The unnotched tape write heads had a 10-µm-wide trailing pole, a much wider leading pole, and a write gap of 0.3 µm. Lateral notches 0.6 µm long and 1 µm deep were added in the leading pole of the notched and stepped-pole writers. For the stepped writer, the trailing pole was additionally recessed by 100 nm from the head surface, except for a 150 nm × 10 µm area adjacent to the write gap (see Figs. 2d-f).
Figure 3. Scanning electron microscope (SEM) image of the stepped-pole writer of Figure 2f produced by FIB trimming of a conventional tape writer.
Write performance was tested by recording shingled tracks of periodic waveforms with alternating frequencies (171 kfci and 74 kfci) on BaFe tape. Figures 2g – i show magnetic force microcopy (MFM) images of four shingled tracks written on perpendicularly oriented BaFe medium with the three writer types. The images demonstrate that notching reduces overwriting with the leading pole but does not eliminate transition curvature at the track edge. With the stepped writer, the transition curvature is practically eliminated and the erase band is further reduced. Cross-track profiling reveals that using the stepped writer widens the usable track by approximately 0.4 µm compared to unnotched writers.
References
[1] A stepped-pole writer geometry to minimize side erasure on barium ferrite tape,
P.-O. Jubert, E. Delenia, J. Frommer, H. Rothuizen, and M. A. Lantz,
Digest of Intermag 2012, Vancouver, Canada (May 2012), EC-06.
[2] A Stepped-Pole Writer to Minimize Side Erasure on Barium Ferrite Tape,
P.-O. Jubert, H. E. Rothuizen, E. Delenia, J. Frommer, and M. A. Lantz,
IEEE Trans. Magnetics 48 (11) 2012.