Nonlinear Transition Shift Research Articles

Optimization of Precompensation for NLTS in Perpendicular Magnetic Recording

Nonlinear transition shift (NLTS) and nonlinear amplitude distortion (NLAD) in perpendicular magnetic recording were investigated for CoPtCr-SiO2 media with various SiO2 contents that have various inter-granular exchange coupling. The tighter inter-granular coupling increases both NLTS and NLAD. The NLTS and NLAD were discriminated and extracted NLTS showed that perpendicular media exhibited the opposite shift to longitudinal media as theoretically expected. Careful precompensation optimization is required to obtain a good error-rate performance at high-linear densities

Decoding Behavior Study of LDPC Codes Under a Realistic Magnetic Recording Channel Model

Low-density parity check (LDPC) codes have shown near-capacity performance in additive white Gaussian noise (AWGN) channels. However, in magnetic recording systems, the readback signals suffer from various impairments (namely, transition noise, nonlinear transition shift, and partial erasure) in addition to AWGN. In this paper, we describe an FPGA-based advanced magnetic recording channel simulator (that includes these impairments) and an LDPC coding system. The error correcting performances of LDPC codes in the presence of such realistic impairments are investigated down to bit error rate (BER) of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> and frame error rate (FER) of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup>

垂直磁気記録における非線形ひずみの解析

In perpendicular magnetic recording, reduction of nonlinear distortion is necessary in order to achieve a higher areal density as well as to improve the signal-to-noise ratio. CoPtCr-SiO2 perpendicular magnetic recording media with various SiO2 contents were used to examine nonlinear distortion mechanisms and optimization of pre-compensation was investigated by the 5th harmonic method. The results showed that the nonlinear distortion was caused by nonlinear transition shift (NLTS) and nonlinear amplitude distortion caused by transition percolation. This was supported by micro-track model calculations. Therefore, the pre-compensation values should be optimized by taking both nonlinearities into account.

High-performance CoPtCrO perpendicular media: optimizing exchange coupling and anisotropy orientation dispersion

A systematic investigation of the structural, magnetic, and recording properties of CoPtCrO media with different intergranular exchange coupling and anisotropy orientation dispersion is performed. Reduced exchange coupling between grains results in lower media transition jitter and nonlinear transition shift. By narrowing the Co anisotropy orientation dispersion, a substantial reduction of both transition and DC noise is achieved. Media with high squareness, coercivity, nucleation field, low exchange and narrow Co c-axis dispersion have been fabricated. Such media exhibit excellent recording performance and are promising for high-density perpendicular recording.

Control of transition shift using DC-bias current

A new method to eliminate the nonlinear transition shift originated from DC-like magnetic fields is proposed. It was clarified that transition shift of recorded bits observed for all 1's recording was caused not only by DC-erased magnetization of a medium but also an external DC field. The influence of the external field was not negligible small even for a cusp-field single-pole head with high immunity from a data-erasure. It was confirmed that the shift of the transition points could be well compensated by introducing an appropriate DC-bias current superimposing on a write current.

Simulation study of nonlinear transition shift in perpendicular magnetic recording media with increased M–H loop slope

The increase in nonlinear transition shift (NLTS) for double-layered perpendicular magnetic recording media with increased M–H loop slopes was studied using micromagnetic simulation. The simulation indicated increased NLTS for the media, but NLTS was reduced for a medium with a higher coercivity. It was concluded that increased NLTS of the media is mainly caused by the decreased write head field gradient corresponding to the decreased write field due to the increased recording sensitivity of the media. It was indicated that use of media with a higher coercivity could reduce NLTS, and an increased write head field gradient directly reduces NLTS of the media.

Exchange-assisted nonlinear bit shift reduction at high linear densities in synthetic ferrimagnetic media

Synthetic ferrimagnetic media comprised of two magnetic layers tend to exhibit larger nonlinear transition shift (NLTS) compared to single magnetic layer conventional media. NLTS is reduced by increasing the antiferromagnetic exchange field (Hex) and especially by employing a lower layer with high magnetization and thickness product (Mst). Due to the lower switching field of the stabilization layer, the position of the transition during writing is farther downstream than that of the upper layer. We suggest that, at high densities, the pinning of the lower layer transition below the nearest neighboring upper layer transition leads to a reduction of demagnetizing fields and therefore NLTS.

Dynamic Head Characterization in the Presence of Reader Nonlinear Distortion

Nonlinear response in magnetostrictive sensors significantly affects the spin-stand parametric measurements used to characterize writer and reader performance of recording heads. A simple production level test defined as the nonlinear distortion (NLD) ratio was used to quantify the degree of reader nonlinearity. A positive NLD value represents the presence of reader distortion while a value of zero represents no distortion. On a large population of heads tested, the median NLD value was 0.25, which is equivalent to an amplitude loss of 10% for an isolated pulse due to reader saturation. A detailed analysis of the measured data on nonlinear transition shift, partial erasure, and PW50 of these production heads show that greater than 50% of the measured variance is due to readback nonlinear distortion rather than true performance variation of the device.

Recording Characteristics at a Channel Rate Over 300 Mb/s in a Helical-Scan Tape System

Recording characteristics at high-linear density were investigated using a spin-stand tape tester. Low-noise metal obliquely evaporated tape with a 40-nm magnetic layer was used. Read signal level and nonlinear transition shift of a thin-film head was compared with that of a metal-in-gap head. A prototype helical-scan tape drive with a thin-film head and a giant magnetoresistive head was developed to investigate the recording characteristics at channel rates over 300 Mb/s. A sufficient signal-to-noise ratio of 18 dB was obtained at a channel rate of 300 Mb/s with a read track width of 0.8 /spl mu/m and a linear density of 376 kFRPI.

Front-End Write Process Model for High Data Rate Longitudinal Magnetic Recording

In drive designs with write data rates >1 Gb/s, transitions sent to the write head collide at 1T with reflections from earlier transitions. This increases the need for write-driver voltage headroom, efficient interconnects, and efficient magnetic materials. Design alternative studies in this regime require detailed models of all components in the write process. We modeled nonlinear bit transition shift (NLTS) and spectral signal-to-noise ratio (SNR) measurements using our preamp/interconnect/writer/media model and compared it to measurements of the actual devices that were modeled. We describe methods for reconciling write driver models with varying head and interconnect impedance, verification performed with scanning Kerr effect microscopy (SKEM), and magnetic models with instantaneous nonlinear effects included.

垂直磁気記録における媒体Ｍ−Ｈループの傾きと記録特性

The contribution of the M-H loop slope to the recording properties of double-layered perpendicular magnetic recording media was studied. Experimental results confirmed that the sharpness of the magnetization transition corresponded to the steepness of the M-H loop. The write sensitivity, S/N, and modulation overwrite also increased along with the recording resolution in media with steeper M-H loops. Though the nonlinear transition shift increased in media with an increased loop slope, the shift was dominated by the phase distortion, which could be minimized by phase precompensation.

Experimental study of reader nonlinearity in perpendicular recording using pseudorandom sequences

The reader nonlinearity is characterized using the pseudorandom sequence method. A simple model is developed and simulated waveforms are used to substantiate the physical interpretation of the echo pulse that corresponds to the reader nonlinearity. It is shown that the measured reader saturation effect increases relatively with the linear density when the linear density is not extremely high. Depending on the extent of the reader saturation, the measurement of nonlinear transition shift (NLTS) can move from positive to negative even though the intrinsic NLTS is positive in perpendicular recording. The optimal write precompensation is dependent on both NLTS and reader saturation. It is also shown that as the linear density goes higher, the amplitude exhibits more asymmetry due to the intrinsic reader asymmetry and the reader asymmetry can change sign.

R/W properties and thermal stability of synthetic ferrimagnetic media

The thermal stability and read-write properties of synthetic ferrimagnetic media (SFM) are compared with conventional media (CM) of the same stability factor KuV/kT and Mr/spl delta/ (remanent magnetization and thickness product). SFM are found to be more stable than the equivalent CM, suggesting that the thermal stability advantage does not arise simply from an increased effective volume. The overwrite properties (OW) and nonlinear transition shift (NLTS) are better for CM. However, for media with the same signal decay rate, the OW is similar over a wide head field range and the NLTS match at larger fields.

Flat-top pole write head design

A write head design with pedestal-defined throat and flat-top pole is presented. The flat-top pole design enables critical dimension control and extendibility to narrower feature size. Write head performance improves further by implementing a single-layer six-turn, closely packed coil with yoke length of 14 /spl mu/m and incorporating a sputtered high moment material on both top and bottom poles. With write current of 40 mA, better than 35 dB overwrite and less than 16% (-16dB) nonlinear transition shift can be achieved on the medium with coercivity of 4400 Oe and Mrt of 0.32 memu/cm/sup 2/ at areal densities up to 60 Gb/in/sup 2/.

Perpendicular recording heads for extremely high-density recording

The basic design and recording performance of advanced probe heads with trapezoidal pole tips are described. Eighty advanced probe heads were chosen to study the effects of different media designs on parametrics and signal-to-noise ratio (SNR) performances. Excellent writability of the advanced probe heads is demonstrated by a series of overwriting (OW) and nonlinear transition shift (NLTS) saturation measurements. Write currents as low as 5 mA/sub o-p/ are required for recording. Excellent OW can be obtained with both high and low OW ratios. Better than -14 dB of NLTS was measured at linear densities as high as 950 KFCI. While OW was observed to be insensitive to media designs under study, NLTS and media SNR were observed to be strongly dependent on the details of media designs. Adjacent track erasure margins of the advanced probe heads were observed to be excellent on both media under study. Minimal side-writing has been observed using the advanced probe heads with trapezoidal pole shape. The advanced probe heads have also been successfully integrated into 80 GB/P perpendicular magnetic recording drives. Finally, linear densities as high as 830 KBPI and track densities as high as 188.7 KTPI were achieved on optimal head/media combinations. Perpendicular magnetic recording at areal densities as high as 145.7 Gb/in/sup 2/ have thus been demonstrated.

The digital-recording performance of obliquely oriented metal-evaporated tape in the reverse direction

We have investigated the signal-to-noise ratio (SNR) and the nonlinear transition shift (NLTS) properties of recording in both the forward and the reverse directions by using obliquely oriented metal-evaporated (ME) tape. The pulse width at 50% threshold of the reverse direction is 28% wider than that of the forward direction, but the SNR after equalization is almost the same by reducing linear recording density by 14% from that of the forward direction. It is possible, therefore, to use the high-density read/write performance of obliquely oriented ME tape in a linear tape system, which would yield a less-than 7% loss of total recording capacity.

Effect of reader saturation on the measurements of NLTS in perpendicular recording

In perpendicular recording, the magnetostatic interaction delays the second transition in a dibit, giving rise to a positive nonlinear transition shift (NLTS) in the written magnetization pattern. However, when the reader becomes saturated, the measured NLTS can take on negative values. Two different techniques to measure NLTS are simulated using a self-consistent recording model: (1) the fifth-harmonic cancellation and (2) dipulse extraction from pseudorandom sequence. The measured NLTS from both techniques is shown to be affected by reader saturation.

Demonstration and characterization of 130 Gb/in2 magnetic recording systems

We have successfully demonstrated longitudinal recording at areal density of 130 Gb/in2 at a data rate as high as 170 Mbps (21 MB/s) and at a bit-aspect-ratio (BAR) of 2.9, using merged inductive-write/spin-valve-read heads on low noise thin film disks. The heads were fabricated with the standard photolithography and wafer pole trimming used in our currently available commercial products. The reader is a bottom synthetic spin valve (BSSV) with a 0.09 μm gap, and the writer has a conventionally trimmed pole with 0.09 μm gap. The reader magnetic read width (MRW) was measured at 0.10 μm. At read bias of ∼4 mA we measured reader sensitivity as high as 20 mV/μm. The write head was also optimized for tracks as narrow as 0.14 μm operating at overwrite (OW) of 36 dB and nonlinear transition shift (NLTS) better than −25 dB at 610 kBPI, without precomp. Using conventional media we measured total spectral SNR∼18 dB. The media to electronics noise ratio was 4.8, showing that we are still operating in a media noise limited system. Low-flying air bearing surface (ABS) design and advanced pole tip recession (PTR) control were required to achieve magnetic spacing of about 15 nm.

High linear density study of advanced single-pole head

In this work, we present the general head design and magnetic recording performance of our second-generation advanced probe heads. These heads require as low as 5-mAo-p write currents for optimal recording. We demonstrate superior writeability via a series of overwrite (OW) and nonlinear transition shift (NLTS) saturation measurements. Excellent OW can be obtained with both high and low OW ratios. Media with VSM coercivities as high as 5500 Oe can be recorded with OW of 32 dB. We measured better than -12 dB of NLTS at a linear density of 1500 kfci. In this work, we present data where the magnetic read width (MRW) for the same read head is smaller when measured on perpendicular media compared to longitudinal media. We also show that the external field sensitivity of the advanced probe heads is excellent as larger than 32 and 23 Oe of external fields are required to erase a low-density signal on 3380- and 2200-Oe disks, respectively. Overall, the basic architecture of this advanced probe head is extendable to 0.12-/spl mu/m write track-width, while we discover no remanence in the pole.

Effect of head field rise time in perpendicular recording

An analytic recording model for the transition parameter and the nonlinear transition shift (NLTS) with the rise-time effect has been developed. In this model, the Schwartz-Christoffel transformation is used to calculate the combined image field from the single-pole writer and the soft underlayer. It is shown that both medium velocity and finite head-field rise-time increase the transition parameter. The NLTS for a dibit pattern increases with linear density, but decreases with medium velocity. A large transition parameter arises because a finite rise time reduces the record field-gradient as the bit time is reduced. For a dibit, the demagnetizing field from the first bit increases the transition interval of the dibit, while the write-time difference of the second bit and the reference bit decreases the interval.