Adjacent-track interference in ultrahigh-density perpendicular recording system

The role of exchange interactions and thermal relaxation in advanced three-layer exchange-coupled composite perpendicular recording media with graded anisotropy was investigated through studies on magnetic viscosity, magnetic hysteresis reptation, and spin-stand adjacent track interference. For this purpose, thickness series in the NiW seed and the three magnetic layers were made and studied. For each sample, both magnetic viscosity and reptation were evaluated using a magnetometer over a series of initial magnetic states. Magnetic viscosity, which is the change in the magnetization of the sample with time, showed a distinct second-order dependence on a logarithmic time-scale. In general, viscosity appeared to be more strongly influenced by exchange-coupling interactions in the media than the thermal stability. Magnetic reptation, which is the change in the magnetization when the external field was repeatedly cycled between a nonzero field and zero field, i.e., remanence, showed two types of distinct reptation phenomena—field reptation and remanence reptation—depending on whether the magnetic state was evaluated with the external field present or removed, respectively. Both field and remanence reptation showed a second-order logarithmic dependence on the number of cycles. A comparison of viscosity and field reptation revealed a correlation, indicating that the origin of field reptation in perpendicular recording media can be associated with viscosity. Such a correlation could not be established between viscosity and remanence reptation. Spin-stand signal amplitude change of written tracks, due to adjacent track interference, also followed a second-order logarithmic dependence on the number of adjacent writes. When compared appropriately, the signal amplitude change showed an excellent correlation with remanence reptation across all media samples. This suggests that adjacent track interference in magnetic recording can be associated with remanence reptation evaluated using a magnetometer, despite their vast difference in time-scales. Moreover, this correlation was observed irrespective of the thermal stability of the media, which corroborates the hypothesis that in composite media, exchange interaction effects have a more dominant role than thermal relaxation effects in influencing switchability vis-à-vis adjacent track interference. In consequence, remanence reptation can be a useful technique to study the interplay of exchange and thermal effects in magnetic media.

In the last decade, interest in increasing digital storage density on flexible media has resulted in an improvement from less than 1.5 . 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> bits/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to more than 1.5 . 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> bits/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This remarkable increase has been possible due to new and improved recording configurations as well as enhanced media and head designs. In order for flexible media magnetic recording technology to maintain its competitive edge, recording densities will have to increase in the next decade by at least another order of magnitude. This, clearly, can be achieved only by pushing both linear and track densities simultaneously toward their respective limits. While linear densities on digitally recorded flexible media have been investigated up to densities approaching 4 . 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> flux changes/mm, track densities on flexible media have attained values exceeding 4 tracks/mm only in a few isolated cases. In fact, it is commonly assumed that it would be much easier to increase linear densities than track densities because of dimensional substrate stability as well as defect and noise considerations. On the other hand, it is essential to understand the factors that influence track densities since in terms of recording parameters this should be the most direct way to further increase storage density. In this paper we have studied the defect behavior of flexible media at high track densities (i.e., very narrow trackwidths) and at varying linear densities. In particular, we have investigated the relationship between drop-out length and trackwidth as a function of recording density for trackwidths of 12.5, 25, 55, 112, and 150 μm, respectively, using commercially available media. The analysis of the defect statistics leads us to conjecture that the depth extension of the defects as well as their area must be taken in account in defect modeling. A qualitative description of the effect of such defects will be discussed together with its meaning for high density recording.

As the areal density of future magneto-optical (MO) disk drives is increased, adjacent track interference on readout will become a more significant engineering problem. Many issues are involved in the analysis of this problem, including tracking servo misregistration, the transverse dimension of written domains, the reading laser beam intensity profile (especially aberrations), the nature of the optical-transfer function for reading information to each side of the track center, and diffraction effects involving disk grooves and the linearly polarized read beam. The change from pulse-position to pulse-width modulation significantly increases the recording system sensitivity to adjacent track crosstalk (ATC). We conducted a variety of experimental tests of ATC to separate and understand the above effects. Two numerical scalar diffraction models of MO readout have been used to make quantitative predictions of signal amplitude as a function of off-track displacement and written frequency. Sensitive measures of read-beam aberration have been developed from conventional disk tests involving ATC and on-track signal analysis. In this paper, we report on the closure of readout models with experimental measurements and the development of methods for choosing the appropriate balance of bit and track density for future MO drives.

Inter-track interference free coding and equalization

Areal densities for data tape systems have been increasing exponentially ever since the first product introduction in the early 1950's. Increases in linear density have provided the major part of the increases in recording density. Now, however, the time has come to push harder on track densities. The signal-to-noise issues for higher track densities are relatively well understood. However, the use of full-time track following, with multi-channel heads, adds additional issues to the list of tape design objectives for future systems of very high track density. The ultimate drive error performance will now be impacted by lateral tape motion, relative azimuth angle between tape and head, and track spacing variation due to dimensional changes in the tape. Controlling lateral tape motion and relative azimuth angle requires improvements in tape edge quality, the overall tape-path design, and the writing of the original servo signals on the tape. Controlling the track spacing problem will require better understanding of tape dimensional stability and how it is influenced by environmental changes, by pack stresses, and by tape design and processing. We conclude with suggestions of how to achieve the improvements necessary to stay on the required areal density trend line.

Abstract— Possible evidence for the presence of 248Cm in the early Solar System was reported from fission gas studies (Rao and Gopalan, 1973) and recently from studies of very high nuclear track densities (≥ 5 × 10g cm−2) in the merrillite of the H4 chondrite Forest Vale (F. V.) (Pellas et al., 1987). We report here an analysis of the isotopic abundances of xenon in F. V. phosphates and results of track studies in phosphate/pyroxene contacts. The fission xenon isotopic signature clearly identifies 244Pu as the extinct progenitor. We calculate an upper limit 248Cm/244Pu &lt; 1.5 × 10−3 at the beginning of Xe retention in F. V. phosphates. This corresponds to an upper limit of the ratio 248Cm/235U ≤ 5 × 10−5, further constraining the evidence for any late addition of freshly synthesized actinide elements just prior to Solar System formation. The fission track density observed after annealing the phosphates at 290 °C (1 hr, which essentially erases spallation recoil tracks) is also in agreement with the 244Pu abundance inferred from fission Xe. The spallation recoil tracks produced during the 76 Ma cosmic‐ray exposure account for the very high track density in merrillites.

Differences between heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording at narrow track widths are compared. We can see that HAMR appears to have similar adjacent track interference, but increased readback signal from the adjacent tracks (side reading). This background interference signal appears to be a consequence of the media architecture, and can be decreased if the media soft underlayer spacing to the hard layer, and transducer is reduced. The impact is also reduced for very high linear densities, due to a smaller fraction of unbiased flux from the neighbor tracks reaching the test track. The use of two reader multiple sensor magnetic recording at high track density appears to address some of this issue, and we can demonstrate viable track writing for HAMR beyond 1M tracks per inch using this readback technique.

A high density recording plated disk, which consists of a chemically plated Co-Ni-P medium and a spin-coated and lubricated SiO 2 overcoat, has been developed using techniques based on extensive studies on error defects, wear durability and wet durability throughout all the production processes. The plated disks were evaluated by practical level tests. The average numbers of error defects were about 50 per surface at 620FRPM (15,700FRPI) linear density measured with 18.5 μm track width Mn-Zn ferrite heads. The plated disk wear durability was sufficient for 20,000 contact start/stop (CSS) cycle tests and 108times seek motion test at 0.2 μm flying height with Mn-Zn ferrite heads. Wet durability was determined for humidity tests under 40 °C, 80% R. H. for 7 months. Errors were not increased after the tests. Based on these excellent evaluation results, the plated disks are used in an 8 inch high performance 400 MB disk drive developed by NTT. Further, higher density recording characteristics of the plated disk were investigated using narrower gap length Mn-Zn ferrite heads at smaller head-medium spacings. As a result, 2,900 FRPM (74K FRPI) linear recording density was obtained as D 50 with a 0.03 μm thick Co-Ni-P medium disk and a 0.2 μm gap length head at 0.12 μm head-medium spacing.

Microwave-assisted magnetic recording (MAMR) is one of the promising candidate technologies for next-generation hard disk drives. In MAMR, the high-frequency field from a spin-torque oscillator (STO) in the recording head is utilized to reduce the magnetization switching field of the medium. Micromagnetic modeling has shown the potential of MAMR; however, in terms of experimental results with actual heads and media, only little evidence of media magnetization reversal and improvement on overwrite has been reported. In this paper, we focus upon the experimental results of MAMR gain defined as a difference in the read-write performance when the STO is activated compared with when not activated. In conclusion, we show clear bit error rate improvement by activating the STO, giving an areal density increase of around 5–10% after considering the penalty associated with the magnetic core width increase. We also confirm that the magnetization switching in the STO is fast enough up to 2.3 GHz, and that there is no negative impact on the adjacent track interference nor on the far track interference.

In this work, the performance of an antisymmetric dual magnetoresistive (A-DMR) head, having a track-width of W/sub t/=2 /spl mu/m, a stripe height of h=2 /spl mu/m and a gap length of 80 nm, is presented and compared to theory. A thin film inductive head that incorporates high magnetization FeAlN poles, with a gap length of 0.25 /spl mu/m, was used to record thin film disk media with an areal magnetic moment of 0.5 memu/cm/sup 2/ and a coercivity of 3 kOe. The A-DMR roll-off and spectral curves that are fit in amplitude and shape with a 2-D analytical curve, indicated D50=190 kfci. The theoretical curves fit the experimental data with two fitting parameters, the spacing parameter of the spacing loss term and a parameter /spl zeta/=/spl zeta/ (W/sub t//h) that can be calculated with a 3-D micromagnetic model. The cross-track profile, off-track response and adjacent track interference are analyzed in the time domain and compared with previously reported theoretical results, where in this case the write head track-width is 2.5 /spl mu/m. The head showed some adjacent track interference and pulse amplitude asymmetry at lower linear densities, mainly due to a lower magnetic transverse bias level of one of the sensor layers. In spite of the good linear resolution of these heads, they offer limited sensitivity in comparison to spin valve heads.

Hard disk drives (HDDs) have been investigated to determine how to increase their areal recording density. Recently, however, HDDs have been having problems with noise, thermal fluctuations, and writability simultaneously, commonly known as a trilemma. Microwave-assisted magnetic recording (MAMR) is a promising recording method to solve this trilemma [1-4]. In MAMR, a spin torque oscillator (STO) is placed between a conventional write head and a trailing shield. Because the AC field from the STO decreases the switching magnetic fields of the medium, the writability can be improved. As a result, the trilemma can be solved. Therefore, a high frequency AC field with strong amplitude is needed. In this study, we examined methods to increase the AC-field frequency by using a micro-magnetic simulator that takes into account magnetic interactions between the write head, the medium, and the STO.

The analytical model for the intrinsic coercive squareness parameter, Sint* was applied to hard–soft stacked exchange-coupled composite media, and correlations with the microstructure and switching were studied. Thickening the hard magnetic layer in the composite stack led to a decrease in Sint*, as did thickening the NiW seed layers. However, this decrease was masked by thermal effects at the normal magnetometry time-scales of measurement. Upon thickening the soft layer in the composite stack, Sint* increased sharply at first and then only slightly. In contrast, the extent of incoherent switching, estimated from the peak value of the minor loop slope, increased slowly at first, and then sharply. The changes in Sint* and switching are correlated to the microstructure, particularly, grain size effects for the NiW series and growth effects for the hard–soft composite media series. Media signal-to-noise ratio at low recording frequencies, and adjacent track interference also show correlations with Sint*.

Traditionally, channel generated bit error rates (BERs) at a certain signal to noise ratio (SNR) have been modeled by a Gaussian distribution function. This approach has been suitable for a peak detection channel as the analysis is performed in only one bit. In the case of a partial response maximum likelihood (PRML) channel, one data bit is determined by multiple code bits. There is a burst of errors detected if any one of the involved code bits gives an error. The chi-square distribution function, which is a sum of several independent Gaussian-squared distributions, is another method to model BERs of a PRML channel. Particularly, different PRML codes could be modeled by a chi-square distribution with different degrees of freedom. In this paper, the relationship of SNR to BER is generated using the chi-square method. The measured on-track rms SNR, read sensitivity function, and media magnetization are the input parameters. Old data and adjacent track interference are considered as random variables. At specified track pitch and read write misregistration, the probability distributions of these random variables are defined by the read sensitivity function and media magnetization. For a given interference, the off-track SNR is calculated. Error rate at a given interference is then derived from off-track SNR by using the SNR to BER relationship defined by a chi-square distribution. The BER at a specific track pitch and read–write misregistration is computed by integrating the error rate at a given interference with the probability distribution of the interference. This model describes the statistical nature of the BER in magnetoresistance head applications and provides very good correlation to experimental data.

Two-track squeeze and adjacent track interference (ATI) are major barriers to increasing track density in hard disk drives (HDD). These depend on skew angles made by a magnetic head and circumferential direction on a magnetic disk. This paper describes relationships between the skew angle and the magnetic core width (MCW) which affects two-track squeeze and ATI performance. We propose a design concept of a track pitch profile at different skew angles considering MCW. Equivalent robustness of ATI performance on different skew angle conditions is obtained with the optimized track pitch.

In the light intensity modulated overwriting method with exchange coupled multilayers, the erasing area surrounds the writing area because the erasing temperature is lower than the writing temperature. Therefore, the data on the adjacent track might accidentally be erased when informations are recorded on a track at a high track density. This Thermal Cross-Talk(TCT) can be effectively reduced by controlling write pulse shape and by applying a UV-resin coating on the top of the disk. By such measures, a high track density up to 1.35μm track pitch was proved feasible.