Business, science and entertainment applications depend upon computers to process and record data, often with large volumes of the data being stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, optical disk cartridges, floppy diskettes, or floptical diskettes. Typically, magnetic tape is the most economical means of storing or archiving the data. Storage technology is continually pushed to increase storage capacity and storage reliability. Improvement in data storage densities in magnetic storage media, for example, has resulted from improved medium materials, improved error correction techniques and decreased areal bit sizes. The data capacity of half-inch magnetic tape, for example, is now measured in hundreds of gigabytes on 512 or more data tracks.
The improvement in magnetic medium data storage capacity arises in large part from improvements in the magnetic head assembly used for reading and writing data on the magnetic storage medium. A major improvement in transducer technology arrived with the magnetoresistive (MR) sensor originally developed by the IBM® Corporation. The MR sensor transduces magnetic field changes in an MR stripe to resistance changes, which are processed to provide digital signals. Data storage density can be increased because an MR sensor offers signal levels higher than those available from conventional inductive read heads for a given bit area. Moreover, the MR sensor output signal depends only on the instantaneous magnetic field intensity in the storage medium and is independent of the magnetic field time-rate-of-change arising from relative sensor/medium velocity. In operation the magnetic storage medium, such as tape or a magnetic disk surface, is passed over the magnetic read/write (R/W) head assembly for reading data therefrom and writing data thereto.
The quantity of data stored on a magnetic tape may be increased by increasing the number of data tracks across the tape. More tracks are made possible by reducing feature sizes of the read and write elements, such as by using thin-film fabrication techniques and MR sensors.
The interleaved R/W magnetic tape head with MR sensors allows increased track density on the tape medium while providing bi-directional read-while-write operation of the tape medium to give immediate read back verification of data just written onto the tape medium. Tape recording systems may alternatively implement arrays of “piggyback” R/W pairs, where the writer and reader in each pair are aligned in the direction of tape travel.
Tape and other magnetic heads in particular suffer from adverse tribological interactions, which include electrical discharge, tape changes, head wear, head erosion, debris buildup, chemical conversion, head MR shorting, etc.
Tape and other magnetic heads in particular suffer from head-tape interactions caused by motion of the magnetic recording tape. Repeated passes of the tape medium over the wear-resistant tape head surface may eventually produce head-tape interface changes, which can impair head performance. This can be a particular problem for thin-film magnetic heads where the thin-film layer structure sees intense exposure to large regions of tape with brief operation, giving a higher risk of such effects as accumulation, corrosion, wear, shorting, etc. which in turn reduce the effective lifetime of the magnetic head assembly. Practitioners in the art may provide wear-resistant layers on the air bearing surfaces of magnetic heads to inhibit these interactions, for example, a sputtered layer of diamond-like carbon or hard aluminum oxide, but such layers are also very thin, being perhaps 20 nanometers thick to minimize tape-to-head spacing loss, and must generally be deposited onto pre-recessed heads.
While tribological mechanisms are not perfectly understood in the art, one problem is believed to arise from accelerated tribological interactions in line with the write gap, which is disadvantageous for head-assembly life-expectancy. The interactions are typically media-dependent and can be severe enough to make certain media incompatible with such head assemblies.
In piggyback heads, because of the close proximity of the reader and the writer in each R/W pair, tribological effects are believed to be caused by voltage swings on electrically floating writer poles. The relative motion between the head and recording medium may produce huge voltage swings on the writer poles, which are electrically isolated in current piggyback heads. Large voltages are strongly implicated in unfavorable tribological processes such as wear, accumulations and corrosion. For example, the high potentials generated on the writer poles can aggravate electrochemical reactions, poletip corrosion, and electrostatic accumulation of debris.
Additionally, in piggyback heads, one write pole is in very close proximity to the nearest reader shield, separated therefrom by only a thin insulator that may be less than 1 micron thick. The voltage differences between the adjacent reader shield and writer pole tip is problematic due to their close proximity. Electric potentials generated on the writer pole tips create huge potential gradients, i.e., large electric fields. This can potentially lead to electrostatic discharge from the writer pole tip to the reader shield, which in turn causes a noise spike in a readback signal. The noise spike in turn results in a readback error. Worse, the electric fields are implicated in aggravated accumulation of conductive materials that can actually short the MR sensor to its shields.
Consider the following example. Suppose reader shield S2 and writer pole P1 in a piggyback R/W pair are only separated by a thin insulator. Suppose S2 is at 1.5 V, and P1 is at 6.5V. The difference is 5 V. If the space between them is 0.5 microns, the electric field (gradient) is 10V per micron, which is a huge gradient at those dimensions. For comparison, a spark from a sweater occurs from a gradient of 1V per micron. In the example presented, the gradient is 10× that. Other unusual effects have been observed, including formation of solid water (essentially ice) at room temperature in the effect of a very high electric field. This problem is less of an issue in interleaved heads, where the writer poles and reader shields are hundreds of microns apart.
Several solutions have been contemplated in the prior art, but are not favorable. These solutions include connecting each writer pole directly to ground, possibly via a resistor or resistors, or to a separate power supply, or to a bus bar connected to ground, possibly via a resistor, or to a power supply or voltage controller. Connecting each writer pole to ground clamps voltages of all the writer poles in a multi-track head to one value. But the voltages of the companion reader shields generally vary from track to track and change depending on drive operation. This produces writer pole-reader shield voltage differences which are smaller but still unfavorable from a triobological as well as electrostatic discharge standpoint. In addition, such writer pole grounding methods require additional processing steps and wafer real estate, which may not be available. Finally, heads with write poles tied to ground may malfunction when there is a single point conductive defect between the write coils and poles.
There is accordingly a clearly-felt need in the art for a wear-resistant piggyback read/write head assembly with improved tribological characteristics. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.