Triple Harmonic Method Research Articles

A New Code Pattern of Triple Harmonic Method for Precise In-Situ FH Measurement in Perpendicular Recording

The thermal flying height (FH) control technology had been applied to adjust FH in hard disk drive (HDD) during read/write processes. The readback signal-based in-situ FH testing technology is the feasible way to monitor and control the FH of heads in HDD. For perpendicular recording, the soft magnetic underlayer has the effects on readback signal and change the Wallace equation into a hyperbolic formula with no explicit solution to the spacing change versus the signal amplitude. Fortunately, the hyperbolic formula can be reduced to the traditional Wallace spacing loss equation at short wavelength. In the actual application, there is a need to compare the FH change or modulation at different disk locations and the harmonic ratio method is preferred to cancel media magnetic fluctuations and a certain level of off-track. This work proposed a new code pattern (11110000) to generate three major harmonic signals: the first, the third, and the fourth harmonics. With zero of the second harmonic signal, the harmonic energies are able to concentrate to the third and the fourth harmonics with strong signal intensity. The most important feature is that both the third and the fourth harmonics can meet the short wavelength requirement with enough signal intensity. This allows the new triple harmonic method to measure FH by the ratio of the fourth over the third harmonics. With the both harmonics in the range of short wavelength, it guarantees the accuracy of FH measurement during the touchdown process.

The Limits of the Wallace Approximation for PMR Recording at High Areal Density

This paper discusses the validity of the Triple Harmonic method and the Wallace readback model for the determination of head-disk spacing variations in today's perpendicular recording (PMR) systems at >; 500 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal density. For the recording system investigated here, and in the limit of high frequency, approximately beyond the frequency corresponding to the 2 T (0011) pattern, this method fails, as head and electronic noise start to interfere. In the low-frequency range of PMR systems, the Wallace equation does not apply any longer. Accurate magnetic spacing variations can be obtained by using the FFT of the intensity ratio of the readback signals, and fitting with the appropriate algebraic expression. Using this method, a repeatability of less than 0.1 nm was obtained, by using a random data pattern of only four 512 bytes data sector long.

Two-Dimensional Flying-Height Modulation Mapping at HDD Drive Level

A measurement system for flying-height modulation of magnetic heads is proposed, and two-dimensional (2-D) flying-height modulation mapping is demonstrated at the drive level. The measurement system is based on the triple harmonic method, which uses a readback signal from the read element of the magnetic head. This technique, which maintains a constant wavelength of the pattern for fly-height modulation measurement, was proposed to achieve 2-D flying-height modulation mapping. Moreover, the frequency response of the measurement system was calibrated, and measurement accuracy was improved

Engineering issue study of triple harmonic method for in situ flying height analysis

In situ flying height testing technology is becoming more and more important in slider–disk interaction analysis and manufacturing quality control of disk drives and head-related components. Triple harmonic method is a quite promising choice for in situ flying height analysis, compared with other in situ methods reported up to now. This paper reports results of investigations on engineering issues of applying triple harmonic method for in situ flying height analysis. The paper reports results of analysis on the effects of various testing conditions on flying height testing repeatability and accuracy. Results suggest that working at reasonable high channel density and working on the ratio between third and first harmonics will be an advantage in terms of both flying height testing sensitivity and testing repeatability. Comparing with media thickness effect, the gap-length variation among different heads will be important if it is to study flying height difference among different heads and the testing is at high channel density. Also, it is suggested to work at AC erased track, in order to reduce the non-linearity caused by hard transition.

Write-Induced Pole-Tip-Protrusion and Its Effect on Head-Disk Interface Clearance

Write-induced pole-tip-protrusion (WPTP) and its effect on head-disk interface (HDI) clearance are studied. Drives are placed in a vacuum chamber and the contact altitude is used in this study together with triple harmonic method. It is found that write-induced PTP is a function of the write-signal frequency and the write duration of the head. The magnitude of the WPTP is on the order of nanometers. For a state-of-the-art HDI design with nominal flying height around 10 nm, WPTP can reduce the HDI clearance dramatically. It can cause write modulation and altitude failure. In order to improve the HDI reliability, WPTP must be taken into account.

Tribo-magnetics and nanometer spaced head-disk systems

As technology moves toward 5/spl sim/7 nanometer spaced head-disk systems for high density magnetic recording, the mechanical interaction between head-slider and disk-media will strongly affect the magnetic integrity of magnetic head-disk systems. It is time to pay more attention to the study of the relationship among magnetic data on disk media, readback signal from magnetic head, and mechanical interaction between head-slider and disk-media. Therefore, the concept of tribo-magnetics is proposed in this work to study the relationship between mechanical interaction in head-disk interface and the magnetic performance of the magnetic head-disk systems. The work reported in this paper covers 3 major parts of tribomagentics: tribo-decay, tribe-noise and tribomagnetic testing. Results indicate that head-disk interaction can disturb the stability of the recorded magnetic transitions and lead to a decay of readback signal-tribo-decay. Furthermore, the MR/GMR heads are sensitive to the energy transferring between head and disk when the head interacts mechanically with disk media. As a result, tribo-noise will be generated and the reading capability of the head will be affected. Head-disk spacing is one of the most important head-disk interface parameters. The relationship between head-disk spacing and the read/write performance of the head-disk system can he used to characterize the dynamic variation of head-disk interface. Two new methods are reported in this work to increase the testing sensitivity and the testing range of in-situ measurement of the head-disk spacing, including a triple harmonic method for high sensitivity in-situ measurement and a scanning carrier current method for load/unload applications which requires larger testing range.