Chun Lian Ong

Perspectives of magnetic recording system at 10 Tb/in 2

The magnetic recording technology started to face superparamagnetism at the areal density of a few 10 s Gb/in2. To postpone this physical limitation, the recording configuration has to be changed by either increasing the write capability or reducing the grain number per bit. It had been switched from conventional longitudinal recording to perpendicular recording and will continue changing to other new recording configurations in the future. Beyond 1 Tb/in2 areal density, the recording system is possible to be either the energy-assisted magnetic recording (EAMR) or the bit patterned media (BPM). At 10 Tb/in2 areal density, one of them alone may not be enough and most people believe that EAMR and BPM have to be combined together to make it happen. In this paper, we explore the possible recording configurations at 10 Tb/in2 and calculate the corresponding design parameters of the recording system.

3-D Sensitivity Function of Shielded Reader by Reciprocity Principle

The sensitivity function to recognize the gapped or shielded reader structure is critical for quantitative analysis of the readback waveform. Although the 3-D finite-element method (FEM) model can capture the full features of geometry and material properties of the reader, it is still preferred to have the precise analytical model as a fast tool for reader design and optimization. This work proposes an analytical model to describe the readback sensitivity function of a shielded reader by the reciprocity principle whereby the media flux absorptions by the sensor free layer and shields are based on the magnetic fields generated when they are treated as the writing elements. Using this approach, the reader geometry and also the material properties of shields and free layer can be included in the model. The proposed 3-D sensitivity function of the model allows us to examine the criteria for read head design and evaluate the reading performance through the analysis of readback waveforms.

Trapping Electron Assisted Magnetic Recording

Moving towards 10 Tb/in2 areal density, finding a proper recording scheme with enough write-ability is the most challenging task of a magnetic recording system. Some recording schemes with enhanced write-ability, such as HAMR, MAMR, graded media, etc., have been proposed to achieve higher recording density. Here we propose a new alternative approach for enhanced writing-trapping electron assisted magnetic recording (TEAMR). In the TEAMR configuration, an electrical bias is applied to the main pole of the write head with the disk media and the other parts of head slider grounded. As the main pole area is very small, the electrostatic force produced by electrical potential is a few orders smaller than the air bearing force at the rear pad. Therefore, it will not affect the flying performance of the head slider. At the nanometer head media spacing, a very strong electrical field is produced in the head media interface. This strong electrical field will cause free electrons to accumulate (be trapped) at the interfacial surfaces of metallic magnetic grains. These trapped electrons are localized in the surface atoms of magnetic grains and will alter the valance-electron band filling of those surface atoms. For many magnetic materials, the extra band-filling electrons reduce the magnetic anisotropy energy and make it easier to be magnetically switched. In this work, the TEAMR effect was proved by the experiment study on Co alloy based commercial disk media. The first principle calculation on L10 ordered FePt crystal shows that the magnetic anisotropy can be reduced to zero with around 0.38 electrons trapped into 1 unit cell of FePt. Further increase in trapped electrons will change the magnetic easy axis from out-of-plane to in-plane, which is considered as a negative magnetic anisotropy. With the magnetic anisotropy reduction at the surface atoms of each grain, micromagnetic simulation result shows that the effective switching field can be reduced to around 11% of anisotropy field for a 1.6 ? 1.6 ? 3.2 nm3 FePt grain. Thus TEAMR can be another good candidate for energy assisted recording requiring very little modification to the current perpendicular magnetic recording system.

Dedicated Servo Recording System and Performance Evaluation

The perpendicular magnetic recording (PMR) in hard disk drives is approaching its physical limitation. The emerging technologies, such as heat assisted magnetic recording and microwave assisted magnetic recording have been proposed to record on magnetic media with thermally stable smaller size grains at higher areal density (AD). However, in the media fabrication, achieving well-isolated small size of grains is more challenging than obtaining high Ku material as recording media. Reducing the number of grains per bit is a major path for keeping AD growth of PMR in recent years. To minimize the SNR penalty at a smaller grain number per bit, pushing more on track density is the right approach. With the 2-D magnetic recording (TDMR) readers for inter-track interference cancellation, the off-track read capability is improved significantly for allowing a narrower track read. In the drive working environment, when the external vibration or other mechanical disturbance happens during the writing process, it creates more track squeeze at adjacent tracks and leaves a very narrow track at some locations of the track. When the track width is narrower than the squeeze to death width in the 747 curve, it causes hard failure in the channel. To solve the track squeeze problem, this paper proposes to add an additional magnetic recording layer in between the data recording layer and the soft underlayer of conventional PMR media. This additional recording layer is used to record servo information only. The continuous positioning error signal is able to improve the servo performance and to provide the real-time monitoring of the positioning error. When it is under bad servo conditions, the writing process can be stopped to avoid nontolerable track squeeze. The continuous servo signals are designed to be of moderate intensity at very low frequency, and its impact on data signal has been minimized. The linear density gap between the dedicated servo media and the conventional PMR media is able to be controlled within 3%. As the dedicated servo system keeps only around 100 wedges of track ID and sector ID at the data layer, the surface area saving at the data layer can break even in capacity. The dedicated servo technology together with TDMR readers is the key technology to achieve ultrahigh track density during both writing and reading processes.

Additional Reader Noise in TDMR Reader for ITI Cancelation

Shingled writing eliminates the writer width limitation and enables further bit aspect ratio reduction to extend the areal density growth of current perpendicular magnetic recording. The achievable track density of shingled magnetic recording (SMR) is determined by the reader width rather than the writer width. However, continuously scaling of reader width increases the reader resistance and results in higher reader noise. It would be an issue to maintain enough signal-to-noise ratio of reader. The 2-D magnetic recording (TDMR) was proposed to extend the density growth by advanced signal processing technique. In TDMR configuration, there are multiple readers to read multiple tracks simultaneously. Recently, the TDMR reader was proposed as a solution of intertrack interference cancelation, which allows to use wider reader to reproduce signals from narrow tracks. In this paper, an additional reader noise is observed when the reader was positioned to read two tracks at the same time. The formation of the reader sensor appears to be closely related to the current reader structure with hard bias magnet. This issue highlights that a solution is required to enable TDMR, probably a reader structure without hard-biased.

Writer Footprint Measurement on Spinstand and Media Transition Curvature Characteristics

The head media integration played a very important role to push up the areal density of perpendicular recording successfully. Besides the success of good sensitivity and high resolution reader at desired feature size of recording bit, the advanced writing process is the key enabler to sustain the areal density growth continuously. The sharper field gradient of writer and the narrower switching field distribution of recording media improve the transition jitter and promote the SNR of recording media. Without reducing grain size, the areal density of perpendicular recording has been increased by a few times. In this work, we investigate the performance of writing process via the measurement of head field footprint in recording media on the Spinstand. The feature size variations of head footprint at different writing current and different flying height (FH) illustrate the effective writing process visually. The transition curvature with transition width value at the trailing edge of writer pole is the key indicator to show the linear density capability related to writing process.

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.

Patterned media and energy assisted recording studied by drag tester

We developed a static drag tester for characterization and read/write of patterned and continuous magnetic disk media. To enable write synchronization for patterned media application, a scheme for write synchronization was developed and results for synchronized writing of patterned bits using a read-while-write approach are presented. In addition, a method for characterization of energy assisted media by drag tester was also explored.

Slider Optical Constant Distribution,

The slider material is a composition of alumina (Al2O3) and titanium carbide (TiC) with a big difference in the optical constants. The random grain size, shape, and composition of TiC make the effective optical constant of slider air bearing surface (ABS) in the size of test spot vary largely. The spot-by-spot scanning of optical constant over slider ABS by ellipsometer indicates that the n and k correlate each other linearly. As the commercia flying height (FH) testers use the averaged n and k value to calculate FH, there is a concern of FH testing error if the n and k value at the testing spot derivates from the average value. This work discovered that the FH testing error is mainly determined by the n and k correlation rather than the absolute value of n and k dispersion. The laser phase detection method improves the repeatability of FH testing significantly, but the n and k correlation is the key to estimate the accuracy of FH measurement.

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The dedicated servo media (DSM) is the conventional perpendicular magnetic recording (PMR) media with an inserted servo recording layer between the data recording layer and the soft underlayer. By using the conventional PMR media as the reference, the design consideration on the layer structure, the preparation, the magnetic properties, the microstructure, including the c-axis orientation and the growth of the layers, the surface roughness, and the recording performance for the DSM are reported.