Hard disk drives (“HDDs”) are widely used to store digital data or electronic information for enterprise data processing systems, computer workstations, portable computing devices, digital audio players, digital video players, and the like. Generally, HDDs store data on a disk with a surface of magnetic material. A transducer head, e.g., read-write head, includes a writing component that magnetically polarizes areas or bits of the magnetic material with one or two polarities to encode either binary zeros or ones. Thus, data is recorded as magnetically encoded areas or bits of magnetic polarity. The direction of the magnetization points in different directions, which can be referred to as a positive state and a negative state. Each bit can store information (generally binary information in the form of either a 1 or a 0) according to the magnetic polarization state of the bit. Typically, bits are arranged along respective radially-adjacent (e.g., concentric) annular tracks of a disk. A single disk can include space for millions of tracks each with millions of bits. A transducer head also includes a reading component that detects the magnetic polarity of each bit or area and generates an electrical signal that approximates the magnetic polarity. The signal is processed to recover the binary data recorded on the magnetic material.
The amount of data that an HDD can store and the cost of storing data is a largely a function of the areal density of the HDD. The areal density is a measure of the bits of data encoded per unit area. For example, areal density may be measured as gigabits of data per square inch (Gb/in2). The HDD typically stores more data as the areal density increases. In addition, the cost per byte of data stored typically decreases as the areal density increases. Therefore, increasing the areal density of a HDD is highly desirable.
The areal density of the HDD is dependent in part on the strength of a magnetic field generated by the write head that polarizes the magnetic material. Generally, a stronger magnetic field concentrated in a smaller area results in higher areal density. Unfortunately, write heads tend to degrade (e.g., unintentionally write to or erase) areas of magnetic material adjacent intended tracks, decreasing areal density and increasing the signal-to-noise ratio of the read head signal. The magnetic polarities of adjacent magnetic material areas are a component of noise. Noise includes background phenomena that are added to the read head signal that are unrelated to the data bit being read. As the areal density increases, the noise increases, which can increases the difficulty of writing data to a magnetic medium and recovering data from the read head signal.
Read-write heads may have defects that increase the tendency of the heads to degrade tracks adjacent intended tracks. Some read-write heads have magnetic insulating shields that act to reduce degradation of adjacent tracks. However, the efficacy of such shields can vary from part-to-part. Additionally, the magnetic insulating shields, or other components of the head, may be defective or have less-than-optimal performance. Degradation of adjacent tracks also varies based on variations in the magnetic properties of the material of the disk, internal stresses of the disk, and other factors.
Because the adjacent track degradation characteristics of HDDs vary from part-to-part, HDDs are tested to ensure that the adjacent track degradation characteristics of the HDD meets minimum standards. Some conventional tests include the asynchronous writing of bit patterns along a single or multiple tracks. Conventional asynchronous track tests include writing a bit pattern in a track and writing different bit patterns in the same or different tracks to detect adjacent track degradation. Some HDD manufacturers employ a single pattern synchronous track test to detect degradation of regions adjacent the synchronous track. Single pattern synchronous track tests include preparing background around a pre-determined location of the disk (e.g., erasing areas or writing regular tracks of information adjacent the pre-determined location) and repeatedly writing a non-alternating bit pattern or single bit pattern along a central part of the prepared area (commonly called an “aggressor” track). The adjacent tracks or surrounding regions of the magnetic medium of the HDD are then read by the read head of the HDD, and a determination of the adjacent track degradation characteristics of the HDD is made based on mapping of the read head signals.