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.     

Recording performance of discrete track patterned media fabricated by focused ion beam

We report on the recording performance of discrete track patterned media fabricated by focused ion beam (FIB). We investigated performance over a small area by spinstand read/write testing. Discrete track patterned regions show smaller magnetic track width and better signal separation between adjacent tracks and therefore higher track density than that of nonpatterned continuous media as a result of reduced side fringe effect and edge noise. We found that, at a designed groove depth of 4-8 nm, the shallow FIB etched grooves already provide good isolation between adjacent tracks, indicating the superiority of ion beam induced modification of magnetic properties in film media over physical modification of disk surface topography. This has implications for discrete track recording and media fabrication.     

Fabrication and MFM study of 60 nm track-pitch discrete track media

Discrete track magnetic recording media with a 60 nm track pitch and prewritten servo patterns were fabricated and tested for read/write performance, and a feasibility analysis of the embedded servo was performed. The fabrication process consisted of ultraviolet nanoimprint lithography (UV-NIL) and sequential ion beam etching on a conventional perpendicular magnetic recording medium. Magnetic patterns were written to the fabricated tracks at 700 kilo flux changes per inch (kFCI) using a spin stand and were read using magnetic force microscopy (MFM), with a resulting signal-to-noise ratio (SNR) of 12.15 dB. The servo pattern was also visualized with MFM. These results demonstrated the feasibility of writing to a 30 nm wide discrete data track and the workability of the embedded servo pattern.     

Model for demagnetization-induced noise in thin-film magneticrecording media

A linear, statistical model is described which predicts the power spectrum of measured noise in bulk-demagnetized (i.e. AC-erased) thin-film magnetic recording media. It is shown that the noise is the result of magnetic flux which is ascribed to erasure-induced transitions along the track length in the medium. The noise power spectrum for a rigid disk medium is shown to correspond to the power spectrum of Poisson-distributed induced transitions along the track length, while noise along the track width is sufficiently described in terms of a uniform, average magnetization with small variance. Experimental data from two thin-film disks are used with the model to estimate the Poisson parameter for each disk. It is demonstrated that AC-erased noise from particulate media can be considered as a limiting case of the Poisson model     

Side-Track Erasure Processes in Perpendicular Recording

In perpendicular recording, substantial erasure of the stored data patterns can occur during the writing process. Among all those erasure processes, side-track erasure (STE) is one of the critical issues in drive head/media integration. Unlike the adjacent track erasure (ATE) process, the locations of the STE affected areas are often many tens of tracks away from the central writing track location. In this work, we report on an experimental investigation and quantification of the general attributes and the origins of the STE processes in various situations. Particularly, we thoroughly characterize some distinctive signatures and behaviors of STE processes by employing both the amplitude- and bit-error-rate-based STE measurement methods in combination with a novel magnetic force microscope characterization technique     

Magnetic recording on domain walls in a single crystal film

The existing magnetic data recording media employ polycrystalline tracks, on which data are encoded using domains with different orientations of magnetic moments. We have numerically simulated data recording on the track in a single crystal film, in which the domain structure is formed using an intrinsic magnetostatic field of the crystal and the Bloch domain walls play the role of information bits.     

Magnetic recording theories: Accomplishments and unresolved problems

The published theoretical treatments on the magnetic recording process are reviewed with particular emphasis on the validity of the models and the assumptions on which they are based. It is concluded that the existing theories fairly well describe the geometrical aspects of recording-transducer to medium spacing and gap lengths-but are deficient in describing analytically the magnetic state of the recording medium before and after demagnetization. Calculated magnetization distributions by harmonic analysis for different recording media are in support of the fact that the usually assumed linear and arctangent magnetization transitions are only rough approximations of the magnetic state of a recording medium between regions of opposite magnetization. Additional shortcomings of our theoretical understanding are indicated by the assumptions of uniform magnetization through the recording medium thickness, neglecting the perpendicular component of the magnetization, and not taking into account finite track widths and magnetostatic interactions between adjacent transitions.     

磁头与离散磁道式磁盘瞬态接触行为研究

 离散磁道式磁盘在与磁头瞬态接触过程中极易损坏.为改善离散磁道式磁盘的瞬态接触状况,采用有限元仿真方法,建立了平整化前后离散磁道式磁盘与磁头的瞬态接触模型,分析了平整化前后离散磁道式磁盘接触应力分布特点,研究了磁头冲击速度、径向速度、磁盘表面摩擦系数等接触条件及平整化对离散磁道式磁盘最大等效塑性应变、塑性应变总体积的影响.结果表明：磁头冲击速度、寻道速度增大均可导致磁盘最大等效塑性应变、塑性应变总体积增大;摩擦系数增大可增大磁道最大等效塑性应变、减小塑性应变总体积;在接触初期,平整化离散磁道式磁盘可以减小磁道最大接触应力,缓解应力集中现象;在接触全过程中,平整化离散磁道式磁盘可以减小磁道最大等效塑性应变及塑性应变总体积;平整化所用2种弹性模量等力学特性不同的填充材料,即磁道材料与类金刚石碳,对于磁道接触状况的改善作用区别较小.以上结论可为降低离散磁道式磁盘的破坏程度提供理论指导．     

Measurement of Down-Track Correlation for Perpendicular Magnetic Media With Various Inter-Granular Exchange Coupling

In order to attain high track density, track-edge noise was studied in conjunction with the fine magnetic structure of perpendicular media. The track-edge noise was increased in media with stronger exchange coupling. The down-track correlation length was measured and compared with the cross-track correlation length. The track-edge noise and correlation length depended upon the inter-granular exchange coupling of the media. Magnetic isolation of grains and steep head field gradient along the cross-track direction is expected to improve the write resolution at the track-edges.     

Bounds on the capacity of a peak power constrained Gaussian channel

The authors present an upper bound and conjecture a lower bound on the information theoretic capacity of a digital magnetic recording channel. The channel is modeled as a linear time-invariant filter with a peak constrained input, and with additive non-white Gaussian noise at the output. The bounds are evaluated numerically using typical head and media parameters. The effects of write current, track width, media noise, and electronics noise on the channel capacity are examined     

The vibrating head magnetometer--Instrument for the measurement of magnetic transitions in high coercive force films

The magnetization pattern recorded in high coercive force media, such as magnetic tapes or discs, is measured with the vibrating head magnetometer (VHM) by vibrating a recording head parallel to the surface of the medium and along the track length. The VHM can be used with small, irregular-shaped samples. It can be used to measure precisely the shape of recorded magnetic transitions. Application of the VHM to the precise location of dropouts on prerecorded samples and to the alignment of a recording head with respect to a prerecorded track is described. Measurements have been made with the VHM of the half peak width and relative peak signal of isolated transitions in Co alloy films. The half peak width was found to be proportional to(t/H_{c})^{0.5}.     

Distortion of inductive read head signal due to side-writtenmagnetization

We apply a Fourier series model to calculate distortion in an inductive read head signal due to side-written magnetization patterns in thin-film magnetic recording media. This model predicts that a magnetization pattern consisting of in-track longitudinal and transverse track edge magnetizations will produce a signal distorted by the flux from the side magnetization. By using parameters obtained from a direct magnetic image of the patterns, this analysis predicts the morphology and magnitude of signal distortion as a function of pattern dimensions. The implication of this distortion for higher bit densities suggested for the future was found to be significant. The proposed method will offer reliable means to quantitatively assess the effect of side writing at such densities     

Side fringing fields and write and read crosstalk of narrow magnetic recording heads

In magnetic recording systems the side fringing fields of magnetic recording heads are responsible for crosstalk from adjacent tracks and eventually for partial erasure of adjacent tracks, thereby limiting the attainable track density. In this paper we derive analytical expressions for the magnetic field near the side of a recording head and calculate the cosine transform of the longitudinal field component, with the head side angle and gap length as parameters. The field of a head of zero width is also considered. Due to the side fringing field the written track is somewhat wider than the geometrical head width; the increase in width being approximately proportional to the maximum field strength in the recording medium and the head-to-medium distance. The amplitudeuof the read crosstalk signal from an adjacent, infinitesimally narrow track is calculated and it appears that it can be approximated byu/u_{0} = 0.5 exp (-2pi x/lambda), where u0is the on-track signal (with zero head-to-medium spacing),xis the distance between track and head side, and λ is the wavelength. Maximum track densities are calculated for a specified crosstalk-to-signal ratio and a given head width and wavelength. For a wavelength of 10 μm, a head width of 5 μm, and a crosstalk of -20 dB, the track density is limited to about 130 tracks/mm, assuming a track width equal to the head width. When the track is taken to be 5 μm wider than the head to account for the effects of the write process, no guardband at all is needed for -20 dB crosstalk and the limit to the track density is 100 tracks/mn.     

Influence of orientation ratio on reverse erase-edge noise andtrack-edge dipole distribution

The cross-track profile of media noise is measured on a precision spinstand for oriented and nonoriented media. These data are correlated with magnetic force microscopy (MFM) images to determine the location of track-edge noise with high spatial resolution. A significant component of track-edge noise is located in a narrow band at the edge of bits recorded in opposition to the previously saturation-erased direction. This reverse erase-edge noise (REEN) increases as orientation ratio increases. The magnitude and distribution of REEN is consistent with a reverse-dc-erase mechanism. δM data indicate a greater influence of magnetostatic and/or exchange coupling for the oriented media. Together with larger on-track reverse-dc-erase noise and higher supralinear transition noise, these results suggest enhanced collective magnetization reversal for the oriented media relative to the nonoriented media. MFM images also reveal the presence of narrow magnetic-dipolar strips at the track edges. These dipolar strips are generated by cross-track components of the head field. The track-edge dipole moment decreases as orientation ratio increases due to preferential alignment of easy axes along the down-track direction. These dipoles contribute to base line shift and are not a significant source of media noise     

Micromagnetic Modeling of Track Squeeze and Erasure Versus Skew Angle

Micromagnetic modeling of adjacent track squeeze in perpendicular magnetic recording, and its effect on track density was studied as a function of skew angle. Based on amplitude 747 curve of track squeeze and reader off track capability of 10%, a loss of 6.5% of track density was predicted by micromagnetic model for head of 150 kTPI track density. The analysis also shows that erase width is smaller than magnetic write width and erase width formula based on pole geometry overestimates the effect of track density loss due to skew angle.     

Design of a manufacturable discrete track recording medium

The potential benefits of patterning discrete tracks onto a disk for magnetic data storage have long been investigated. A practical process for manufacturing a cost-effective discrete track recording (DTR) medium has prevented such a disk from being introduced into a product. In this paper, a process utilizing nano-imprint lithography techniques to create a land and groove structure on the surface of a disk substrate will be described. Design considerations for the geometry of the structure, as well as of the magnetic write and read widths of the head, are discussed. Data showing the magnetic characteristics and recording performance of a DTR medium are also presented.     

Thermal stability improvement for longitudinal magnetic recording media

This paper reports and discusses the effects of M/sub r/t cancellation and bottom/top magnetic layer thickness ratio on thermal stability and recording characteristics of low-noise synthetic antiferromagnetic (SAF) longitudinal recording media. It is shown that by proper design and process, SAF media with low M/sub r/t cancellation can deliver excellent overall recording performance with acceptable thermal stability. We present a unique approach to significantly improve the thermal stability of SAF media with low M/sub r/t cancellation by adjusting the bottom/top magnetic layer thickness ratio. Increasing the bottom/top magnetic layer thickness ratio significantly improves amplitude decay rate and media coercivity temperature dependence with little sacrifice in recording performance.     

Theory of playback process with soft magnetic underlayer

Based on the Tsuboi-Fushida-Wallace spacing loss law, reciprocity principle, and the principle of an image plane, a two-dimensional, head independent theory of the playback process has been developed for perpendicular magnetic recording (PMR) media with a soft magnetic underlayer (SMUL). An assumption that the reversible magnetic permeability of the recording media is equal to one gives results in a simple form. The article, while not addressing the effect of underlayer on write performances, demonstrates complications caused by the presence of the underlayer during readback.     

3 Gb/in2 recording demonstration with dual element headsand thin film disks

We have successfully demonstrated magnetic recording at an areal density of 3 Gb/in² with narrow track inductive-write MR-read dual element heads on low noise Co alloy thin film disks. In this demonstration, the write head is a ten turn thin film inductive head with thick and narrow P2 pole-tips. The read head is a shielded ~1 μm trackwidth MR sensor soft-film biased in the read region for linearization and exchange-biased at the tail regions for magnetic stabilization. During recording tests, the heads were flown over low noise Co-alloy media at a clearance similar to that in the previous 1 Gb/in² recording experiment. Results showed good writability from the narrow track write head in terms of overwrite and hard transition shift. Readback yields symmetrical signals as large as 600 μV (p-p) and rolloff measurements showed 50% densities as high as 5000 fc/mm. Track profile and microtrack profile measurements showed the write and read trackwidths to be ~1.4 μm and ~1.1 μm respectively, with tight side-writing and side-reading characteristics. An overall assessment of the parametric recording results suggested areal density feasibility up to as high as 3 Gb/in². This projection was confirmed by error rate performance testing using a PRML channel with a digital filter and write precompensation. At a data rate of 4-5 Mb/s and at very low ontrack error, a linear density as high as 185 Kbpi and an optimized track pitch as narrow as 1.5 μm were achieved, corresponding to an areal recording density of ~3.1 Gb/in²     

A Spacecraft Model Flying Around a Levitated Globe Using?YBCO Bulk Superconductors

A small spacecraft model flying around a levitated globe has been designed based on magnetic force. The globe can be levitated in mid-air by the interaction between the magnets fixed at the North and South poles and the YBCO bulk superconductors installed on the globe frame near each of the poles. The magnetic track for the spacecraft is located at the equatorial circumference. When the small spacecraft with YBCO bulks is being cooled to LN₂ temperature at a certain distance apart from the track, then the spacecraft can be automatically suspended in mid-air near the track and moved along the track without any obvious friction after a push by a hand. The model can be used as a demonstration of magnetic levitation, suspension, side suspending, magnetic bearing, flywheel energy storage, flying spacecraft principle, and can also be used as an educational experiment for the diamagnetic effect and applications of superconducting materials to students.