Hyung Jai Lee

Study of Lithographically Defined Data Track and Servo Patterns

We report on fabrication of discrete tracks on perpendicular magnetic recording (PMR) media with an e-beam lithographical process. We studied the recording performance of the e-beam media on a spinstand in parallel with conventional PMR media. Discrete track media show significant reduction in adjacent track erasure (ATE). We studied and quantitatively measured the source of the ATE improvement, and developed a triple track geometrical model to calculate achievable track density for both discrete track recording (DTR) and continuous media. From the model, we identify two factors of DTR that contribute to reaching a higher TPI. Using the same fabrication technique, we also studied servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts with servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better position error signal sensitivity and support higher TPI in the future.Discrete tracks are fabricated on conventional PMR media with an e-beam litho graphical process. The recording performance is studied on a spinstand in parallel with conventional PMR media. Discrete track media shows significant reduction in adjacent track erasure (ATE). The source of the ATE improvement is studied and quantitatively measured. A triple track geometrical model is developed to calculate achievable track density for both DTR and continuous media. From the model, we identify two factors of DTR, which contribute to reaching a higher TPI. Using the same fabrication technique, we also study servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts and servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better PES signal sensitivity and support higher TPI in the future.

Fabrication of Flyable Perpendicular Discrete Track Media

Discrete track media offers many potential recording advantages over conventional continuous media in hard disk drives. In this study, we present a novel fabrication process for discrete track perpendicular magnetic media via electron beam lithography, ion milling, and the use of a protective Al sacrificial layer. Physical characterization of the media confirms the process is able to produce patterned tracks with no damage to the media. Spin stand analysis verifies the disks are flyable and capable of recording sharp transitions without any degradation in the magnetic signal

Write Synchronization in Bit-Patterned Media

Writing bit-patterned media has been a challenging problem because it is necessary to synchronize write timing with physical bit-pattern locations on disk. This timing requirement must be addressed not only in disk drives but also in testing environments such as a spin stand. This paper first describes a technique for synchronizing write timing on a spin stand. Testing results show that, with data block length similar to a disk drive sector, the write timing jitter due to synchronization error is much smaller than the magnetic transition jitter. We then discuss the potential timing jitter sources in a disk drive, including disk speed variation, head vibration, and write clock stability. Experimental data show that the head vibration is potentially the most difficult jitter source for the write clock to handle. The possibility and difficulty in applying coding techniques to correct write timing error is also investigated. While it is possible to create codes for this purpose, practical considerations can limit its usefulness.

Recording Performance Study of PMR Media With Patterned Tracks

Discrete track recording technology offers a potential advantage in reducing adjacent track erasure/interference. As nano-imprinting technology advances, fabrication of such media has been demonstrated. In recent years, discrete track recording has generally been viewed as one of the next promising technologies for areal density advancement. In this study, we evaluated the recording performance of PMR media with patterned tracks. To accurately assess the advantage of discrete track performance and compare with current continuous media, both recording performances were measured on one single track. The head flying height is monitored on the patterned and continuous media regions. At 100-nm data track width, patterned tracks show noticeably better signal-to-noise ratio and significantly lower adjacent track erasure compared with continuous media at the same track width. Such measured performance advantages are critical to increase track density beyond 300-400 ktpi

Understanding Adjacent Track Erasure in Discrete Track Media

We present a micromagnetic modeling analysis on recording performance, in particular adjacent track erasure (ATE), in discrete track media. We have compared the ATE performance between the continuous and discrete track media, and we have modeled two types of perpendicular media: one with zero intergranular exchange coupling and the other with moderate intergranular exchange coupling. We found that in the medium with moderate intergranular exchange coupling, the ATE has significantly been suppressed; that is, the track pitch can be significantly smaller with the same recording head and recording condition for the same ATE level. This gain becomes less significant if the medium has zero intergranular exchange coupling.

Lithographically patterned servo position error signal patterns in perpendicular disks

For discrete track media to become a viable alternative, it is essential to produce accurate servo patterns in a cost effective manner. This study presents a spin stand analysis and comparison of position error signals generated from various lithographically defined servo patterns and servo bursts written on continuous regions of the same perpendicular recording media. It is demonstrated that the edge variation on the patterned servo elements is much less than that found on conventional servo bursts. In addition, evidence shows that despite some amplitude loss resulting from the removal of magnetic material, patterned servo bursts produce good quality position error signals when compared to servo bursts written on continuous media.

Micro-track profiles of ESD damaged AMR and GMR heads

We systematically studied the micro-track profiles of ESD damaged AMR and spin valve GMR heads, and correlated these with the dynamic electric performances of each head. Spin valve GMR heads and two types of AMR heads made by different structure and material are ESD (HBM) stressed and studied. We observed a double peak in micro-track profile after more than 10% change of MR resistance. This means that the centers of AMR and GMR sensors become less sensitive before the total melting of MR element during ESD zapping. We also observed double peak micro-track profile after pin reversal of SV head. We attribute this to changes in the domain configuration caused by partial reversal of pinned layer moment along MR (GMR) stripe.

Effect of crystal texture on the magnetic properties of thin HCP Co-Ni films

In order to study the effect of crystal texture on the coercivity and squareness ratio of ferromagnetic thin films, Co-20 wt. % Ni films of various thicknesses (500A \sim1.0\mu m) were sputter deposited on glass, single and polycrystalline substrates using a R.F. diode. The crystal texture and preferred orientation of sputtered Co-Ni films were determined by x-ray diffraction technique. The chemical composition and distribution of non-metallic elements of sputtered films were studied by energy dispersive analysis of x-ray and scanning Auger microprobe respectively. Scanning and transmission electron microscopes were used to study the structure and morphology of the films. The sputtered Co-Ni films were found to have a simultaneous presence of [0002] and [ 10\bar{1}0 ] orientations with a stable HCP structure. The change of preferred orientation of Co-Ni films with thickness (500A to 1μm) resulted in dramatic changes in coercivity, squareness ratio and the shape of the M-H hysteresis loops of the films. It appeared that the magnetic behavior of these films was predominantly controlled by the texture of the films and was less sensitive towards the thickness.

The effect of air-bearing surface roughness on avalanche test

The traditional method of glide height measurement is used for avalanche height evaluation. The air-bearing surface (ABS) of the avalanche head is assumed to be perfectly flat. This assumption, however, is not appropriate for today’s drive. In addition, a glide avalanche head normally has a different surface finish than that of a real recording head. So what an avalanche head is “seeing” on the disk surface may not be what a real head “sees” in the drive. Therefore, it becomes extremely important that one can correctly report the true avalanche height. In this study, we proposed one possible way of doing so by correctly calibrating the flying height of a glide head and the defining glide avalanche contact as “0.02% bearing depth.”

Improved recording performance using side-shielded pole head in PMR drive

A recording performance for the side shielded pole head (SSPH) with trapezoidal pole tip is compared to a conventional single pole head (CSPH) with the SiO/sub 2/ dispersed double-layered perpendicular magnetic recording (PMR) media. The SSPH showed 3 dB higher SNR, 22.5% enhanced D/sub 50/, and about 10 dB better NLTS compared to the CSPH design. Noise analysis showed the SSPH had lower noise than the CSPH. The lower transition noise and better NLTS is attributed to reduction of transition jitter in SSPH. The perpendicular magnetic recording drive having a storage capacity of 80 GB/platter has been successfully demonstrated with SSPH. Finally, the on-track bit-error rate (BER) measured for 10 sectors of a linear density of 653.2 kb/in at 7200 rpm was about 10 /spl times/ 10/sup -6.7/ for the SSPH design, about two orders improvement over the CSPH design.