Thin film head read recovery.

Background

of

the

Invention

This invention relates to thin film magnetic read/write heads.

When a write current is applied to a typical thin-film read/write head, the resulting write field causes the full width of the tip of each pole of the head to be spanned by a single magnetic domain. When the write current is removed, so-called closure domains and central domains are reformed within the head in a configuration that minimizes the sum of the magnetostatic, anisotropic, and exchange energy of the magnetic particles in the head. The precise configuration of the reformed domains will depend on, for example, stress from the alumina layers in the vicinity of the permalloy material that forms the head, the iron/nickel ratio in the permalloy composition, defects in the permalloy material, and the profile of the write current as it is reduced to zero. Thus, the domain configuration after a write operation may differ slightly from the domain configuration prior to the write operation

When the head is used to read data on a track of a magnetic disk, the field generated by the data element being read from the disk is passed along the pole of the head by slight rotation of the magnetizations in the central domains and small movements of the domain walls, but without altering the general domain configuration. The effectiveness of a read operation will depend on the particular domain configuration that resulted from the most recent write operation.

Viewed at a less microscopic level, the domain configuration determines the head permeability and hence the efficiency with which the head can read data; and the most recent write operation determines the domain configuration. In some cases, the reading efficiency of a given domain configuration may be so low as to produce multiple symbol and uncorrectable errors and an inability to read header information.

In IBM Technical Disclosure Bulletin, Vol. 13, No. 1, June 1970, p.202 there is described a method for the reduction of read error rates in magnetic recording systems. The method describes the application of a DC bias current to a read/write head, after a read operation ("playback") in which an error has been detected.

In EP-0137676 there is described a method of obtaining an invariable output characteristic of a magnetic head by, prior to any playback operation, applying a decaying alternating current superimposed on the bias current which provides the bias magnetic field to the head's magneto-resistive element.

Summary

of

the

Invention

A method and an apparatus, according to the invention, are defined in claims 1 and 9, respectively.

Preferred embodiments of the invention include the following features. The write operation is performed in a separate region of the medium from the region where data is stored. The existence of errors in data read from the medium is detected as an indication of whether the read operation was performed acceptably. After the write operation, the head performs the first (read) operation again. The error detection, writing, and reading steps may be repeated more than once, if necessary, until the head is able to perform the read operation acceptably.

The invention takes advantage of the fact that a write operation is capable of reconfiguring the domains and thus returninc the head to a condition in which it can read from the disk. The procedure reduces the number of uncorrectable read errors, and may be included as one of several techniques used in a disk drive to recover from read errors.

Other advantages and features will become apparent from the following description of the preferred embodiment and from the claims.

Description

of

the

Preferred

Embodiment

We first briefly describe the drawings.

Fig. 1 is a block diagram of a host computer and a disk storage system.

Fig. 2 is a diagrammatic top view of a portion of a magnetic disk.

Fig. 3 is a flow chart of a procedure for conditioning a read/write head.

Structure

Referring to Fig. 1, in a magnetic disk storage system 10, a disk drive unit 12 includes a rotating stack 14 of magnetic disks. A microprocessor 16 (controlled by a stored program 18) regulates the operation of a read/write head assembly 20 to cause it to read and write bits on the magnetic disks in accordance with instructions provided via a link 22 from a disk controller 24. Link 22 also carries the bits back and forth between disk controller 24 and disk drive 12, and carries status information from the disk drive back to the disk controller

Disk controller 24 includes error correction circuitry 26 which implements an appropriate error correction code with respect to bits being sent to the disk drive for storage and with respect to possibly corrupted bit sequences read from the disks. A program controlled microprocessor unit 28 governs the delivery and receipt of bits, instructions, and status information via link 22 and also communicates via a link 30 with a host computer 32. Link 30 carries (i) operating system commands from host computer 32 requesting the reading or writing of data, (ii) the data itself, and (iii) status information from disk controller 24.

Referring to Fig. 2, each surface of one of the magnetic disks 40 is organized in concentric tracks 42. Near the center 44 of the disk is an inner guard band 46 that separates the outer tracks (used for customer data) from inner tracks 48 used for diagnostic and other purposes. The read/write head assembly 20 (Fig. 1) includes at least one thin-film head 50 mounted on a slider 52 which is supported on an arm 54. The slider flies above the disk with the poles of the head positioned to read or write on a desired track.

Operation

Referring to Fig. 3, when the operating system in host computer 32 asks disk controller 24 to read data in a specified block of the disk, controller 24 passes the instruction along to the disk drive 12. The head is then moved to the proper track and performs a read operation (block 60, Fig. 3) to read the information in the specified block. The information read from the disk is then returned to the disk controller. If the number of errors in the information is within the capacity of the error correction circuitry to handle (block 62), the errors are corrected (64) and the correct data is passed back to the host computer. On the other hand, if the errors are too numerous, the error correction circuitry indicates that condition to microprocessor 28. Microprocessor 28 then sends to disk drive 12 an error recovery instruction which tells the disk drive to take steps to attempt to improve the efficiency of the intended read operation.

In response to the error recovery instruction, microprocessor 16 executes a sequence of error recovery procedures, which may include, for example, adjusting the head position slightly to try to improve its centering over the track. The sequence of procedure also includes a head conditioning procedure which includes the following sequence of steps.

First, the identity of the head which failed to perform an acceptable read operation and its present track location are stored temporarily (66). Then the head is repositioned (68) over a diagnostic read/write track located inside the guard band. Next the head is caused to perform a write operation 70 on the diagnostic track. As explained above, this write operation typically results in a small change in the domain configuration (and hence the permeability of the head poles) which may be sufficient to enable the head to perform an acceptable read operation. The head is then repositioned (72) at the original track location. Once the disk drive has completed all of the error recovery procedures, it reports that fact to the disk controller, which then again causes the disk drive to attempt the same read operation (60) as before. The error correction step (62). When the errors are found to be uncorrectable for a fifth time (74), the block is declared uncorrectable (76).