Adjacent track interference analysis of shielded perpendicular writers

In order to achieve high track density in perpendicular recording, write-field leakage from the main pole to adjacent tracks needs to be minimized. We used a 3-D finite-element method to investigate the optimal writer design, which minimizes fringing fields. We also confirmed the results using experimentally by studying adjacent track interference (ATI) on a spinstand. According to both modeling and experimental data, we found that ATI performance can be clearly improved with a wrap-around-shielded (WAS) writer while maintaining enough write-ability. In order to reduce the fringing field, a WAS having thick throat, high permeability and high saturated magnetic flux density was required. We also confirmed that variations of ATI performance due to flare length distributions can be effectively suppressed with the WAS writer.     

Current-confined-path (CCP) giant magnetoresistive (GMR) sensors

Current-perpendicular-to the plane (CPP) giant magnetoresistive (GMR) sensors with a current-confined-path (CCP) layer inserted within the Cu spacer have been manufactured using ultrahigh vacuum PVD sputtering, photolithography, and ion milling processes. Compared with a pure metallic CPP system, the CCP insertion layer enables a substantial increase in sensor resistance with an equivalent or better GMR ratio, and thus a significant improvement in the ΔRA amplitude. Heads with such a sensor have been tested under various bias currents, both quasistatically and on a spinstand. It was found that the resistance of the sensors increases with increasing bias current and voltage, following typical metallic behavior. Also, the CCP insertion layer enables operation at higher bias currents compared with the pure metallic sensors (without the insertion layer) and thus a higher output signal. This effect is attributed to less magnetic instability due to the reduced Ampere fields around the narrow (∼6 nm) current-confined paths. Finally, the CPP-GMR heads with CCP layer were tested under high-density recording conditions using the perpendicular recording media. Bit error rate (BER) as a function of linear density is reported. Microtrack profiles were also recorded to determine track density capability.     

Magnetic recording materials since 1975

When information is to be recorded and stored on a re-usable medium, magnetic recording, in one form or another, has been and is today the dominant technology. Magnetic particles or thin films having a coercivity of several hundred oersteds, are easily capable of retaining a magnetic pattern of recorded information (at densities of tens of thousands of bits/inch and track densities of thousands of tracks/inch) for hundreds of years and yet when desired, the pattern can be changed by simply writing the new information over the old. Since the recording process requires only a change in direction of the electron spins, the process is infinitely reversible and the new information may be read immediately with no development process being required. This paper deals with developments of the magnetic (and some important non-magnetic) properties of recording media that have occurred since 1975. It describes their current embodiment in particles of cobalt-modified iron oxides, chromium dioxide, barium ferrite and metals and in thin films of metals, alloys and oxides. The curious and currently unexplained aspects of the behavior of these important materials is noted.     

Adjacent-track interference in ultrahigh-density perpendicular recording system

In perpendicular recording system, the increase of track density is crucial to achieve ultrahigh areal density. At higher track densities, the adjacent-track interference (ATI) arises. In this work, ATI is studied by micromagnetic simulation. Two adjacent tracks are written successively. The track–track distance (TTD) and head–medium spacing are varied to analyze the write and read performance of these two tracks and to investigate the influence of ATI on recording performance. Simulation results indicate that when a track is written first, it is less vulnerable to ATI. ATI is stronger in a track with higher linear recording density. The head–medium spacing plays a significant role in the achievement of low ATI in perpendicular recording system. If the head–medium spacing is reduced to 5 nm, areal recording density above 540 Gb/in² could be realized.     

Constructive ITI-coded PRML system based on a two-track model for perpendicular magnetic recording

In this paper, we study not only the new constructive inter-track interference (CITI) code based on the equalized level but also Viterbi detection algorithm taking into account the ITI from adjacent tracks for the perpendicular magnetic recording channel without a differentiator of two-track model. Although the investigation of this paper was not practical but theoretical, the result shows that the permissible percentage of ITI for conventional Viterbi detection to attain better performance compared with the case of single track is 26%, while the percentage for the proposed one is improved up to 50%. Further investigation has to be required under a more realistic system and moreover may be expanded into patterned media perpendicular magnetic recording.     

Neutron-induced complex reaction analysis with 3D nuclear track simulation

Complex (multiple) etched tracks are analysed through digitised images and 3D simulation by a purpose-built algorithm. From a binary track image an unfolding procedure is followed to generate a 3D track model, from which several track parameters are estimated. The method presented here allows the deposited energy, that originated from particle fragmentation or carbon spallation by means of induced tracks in commercially available PADC detectors, to be estimated. Results of evaluated nuclear tracks related to ¹²C () reaction are presented here. The detectors were exposed on the ISS in 2001.     

Recording of one-dimensional holograms in MnBi films

A new system of holographic recording on moving MnBi film is described. The data to be stored are recorded in a track of non-overlapping 1-d Fourier holograms. A cavity-dumped argon laser is used for writing. The feasibility of this new system has been demonstrated by writing and erasing tracks consisting of 20 μm by 2 μm holograms. A maximum bit density of 10⁷ bit/cm² and a writing data rate of 10⁸ bit/s should be attainable with such a system.     

Characteristics of reverse overwrite process in shingled recording scheme at ultra-high track density

A systematic experimental study of the reverse overwrite (ReOVW) process in the shingled recording scheme has been conducted in conjunction with characterization of corresponding recording performances from recording heads with different geometries. It was found that there is no ReOVW reduction as the track density increases in a strict shingled recording fashion. Nonetheless, ReOVW is indeed slightly decreased from 300 to 700 kpi in a so-called one write shingled recording process. Overall our obtained data suggest that conventional magnetic recording technology might be able to extend all the way beyond an areal density of one Tbit/in² by using the shingled recording scheme.     

Optical and Thermal Simulator for Laser-assisted Magnetic Recording

“Laser-assisted magnetic recording”, in which a recording media is heated by a laser beam while writing data, is attracting attention as a technology that enables a recording density of 1 Tb/in.². There exists another technology for media in which the recording layer is constructed with many small projections that enable high magnetic coercivity. This is called “patterned media”. For developing hard disk drives using these methods, we developed a simulator that analyzes the optical intensity distribution from the optical head for laser-assisted recording and the temperature profile on the patterned media. The simulator calculates the optical model using the finite-difference time-domain (FDTD) method. The thermal analysis of the three-dimensional model allows fast calculations using the alternating direction implicit (ADI) method. The heat source distribution data for thermal analysis is calculated in order to use the results of optical analysis. The optical and thermal analyses of the laser-assisted recording model were investigated with the simulator.     

Tracks in MoS2 irradiated with 20 MeV fullerene and 1 GeV lead ions

The first results of a Transmission Electron Microscopy study of Molybdenite (MoS₂) irradiated with 20 MeV fullerene and 1 GeV lead ions are presented. It is shown that the energy deposited by electronic excitation by both types of projectiles leads to track formation. Although the value of the linear rate of energy deposition is comparable for 20 MeV C₆₀ clusters and 1 GeV Pb ions, the diameters of the tracks created by the clusters is two times greater than the diameter of the tracks induced by 1 GeV Pb ions. Furthermore, the tracks in the sample irradiated with fullerene ions present specific contrast features.     

Automation system for optical counting of nuclear tracks

An automation system consisting of the microscope, video camera and Pentium PC with frame recorder was created. The system provides counting of nuclear tracks on the SSNTD surface with a resolution of 752 × 582 points, determination of the surface area and main axis of the track. The pattern recognition program was developed for operation in Windows 3.1 (or higher) ensuring a convenient interface with the user. In a comparison of the results on automatic track counting with the more accurate hand mode it was shown that the program enables the tracks to be detected even on images with a rather high noise level. It ensures a high accuracy of track counting being comparable with the accuracy of manual counting for densities of tracks in the range of up to 2·10⁵ tracks/cm². The automatic system was applied in the experimental investigation of uranium and transuranium elements.     

Deposition of molecular probes in heavy ion tracks

By using polarized fluorescence techniques the physical properties of heavy ion tracks such as the dielectric number, molecular alignment and track radius can be traced by molecular fluorescence probes. Foils of poly(ethylene terephthalate) (PET) were used as a matrix for the ion tracks wherein fluorescence probes such as aminostyryl-derivatives can be incorporated using a suitable solvent, e.g. N,N′-dimethylformamide (DMF) as transport medium. The high sensitivity of fluorescence methods allowed the comparison of the probe properties in ion tracks with the virgin material. From the fluorescence Stokes shift the dielectric constants could be calculated, describing the dielectric surroundings of the molecular probes. The lower dielectric constant in the tracks gives clear evidence that there is no higher accomodation of the highly polar solvent DMF in the tracks compared with the virgin material. Otherwise the dielectric constant in the tracks should be higher than in the virgin material. The orientation of the molecular probes was examined by polarized fluorescence spectroscopy. It is shown that deposited molecular probes witha high aspect ratio have a preferential orientation parallel to the ion track axis.     

Observations of fission-tracks in zircons by atomic force microscope

The fission-track (FT) method is a dating technique based on the observation of damage (tracks) by spontaneous fission of ²³⁸U left in a mineral. The date is calculated from the track density and the uranium concentration in the mineral. This is possible because the number of tracks is a function of uranium concentration and time since the start of track accumulation. Usually, the number of tracks is counted under an optical microscope after etching (chemical expansion of a track). However, as FT density per unit area rises, it becomes difficult to count the number of tracks. This is due to the fact that FTs overlap one another and are unable to be readily distinguished. This research examines the potential of atomic force microscope (AFM) for FT dating using zircons, which are likely to show higher FT density than other minerals due to their high U concentrations.To obtain an AFM image for a sample prepared for FT dating, removing the static electricity of the sample is essential to avoid an unexpected movement of the cantilever. A grain should be wider than about 30 μm to bring the cantilever on the mineral surface. Polishing with a fine grained compound is very important. There is not much difference in sharpness between images by AC mode (scanning with vibrating cantilever at a constant cycle) and Contact mode (scanning with the cantilever always in close contact with the surface). To confirm how tracks can be identified with the AFM, an AFM image was compared with an image obtained with the optical microscope. When change in the number of tracks and their shapes were observed through stepwise etching, the track expanded as the etching time increased. In addition, the etching rate was slower for large tracks than those for small tracks. This implied that the AFM can be used to observe etching of zircons with different degrees of nuclear fission damage. A track that could not be seen with the optical microscope due to insufficient etching could be observed by AFM methods, indicating the possibility of FT dating with high track densities using AFM after relatively short etching periods.     

Bit-array alignment effect of perpendicular SOMA media

One effective way to overcome the superparamagnetic limit of magnetic recording system is to reduce the grain number per bit at given signal-to-noise ratio (SNR) level by using uniformed media grains. The self organized magnetic array (SOMA) is designed to have uniform grains with perfect grain array structure. It is believed that high enough SNR with small number of grains per bit can be acheived. But in the engineering application, the recorded bit on SOMA media may align with the regular array at different locations and angles due to non-grain synchronized writing, skew angle, and circular track. This induces the bit-array alignment effect and degrades system performance of SOMA media. In this paper, the micromagnetic simulation results show that the bit array alignment effect causes large level SNR fluctuation on the same media. The SOMA media is not preferred to be used in the conventional recording configuration. It is only suitable for the configuration of patterned media.     

On methodical problems in estimating geological temperature and time from measurements of fission tracks in apatite

The results of apatite fission-track modelling are only as accurate as the method, and depend on the assumption that the processes involved in the annealing of fossil tracks over geological times are the same as those responsible for the annealing of induced fission tracks in laboratory experiments. This has hitherto been assumed rather than demonstrated. The present critical discussion identifies a number of methodical problems from an examination of the available data on age standards, borehole samples and samples studied in the framework of geological investigations. These problems are related to low- (<60°C) and high-temperature (>60°C) annealing on a geological timescale and to the procedures used for calculating temperature–time paths from the fission-track data. It is concluded that it is not established that the relationship between track length and track density and the appearance of unetchable gaps, observed in laboratory annealing experiments on induced tracks, can be extrapolated to the annealing of fossil tracks on a geological timescale. This in turn casts doubt on the central principle of equivalent time.

That such uncertainties still exist is in no small part due to an insufficient understanding of the formation, structure and properties of fission tracks at the atomic scale and to a lack of attention to the details of track revelation. The methodical implications of discrepancies between fission track results and the independent geological evidence are rarely considered. This presents a strong case for the re-involvement of track physicists in fundamental fission track research.     

Advanced granular-type perpendicular recording media

We introduce our recent experimental results for three blocked layers for currently used perpendicular recording media; a recording layer (RL: for recording), a soft magnetic underlayer (SUL: magnetic flux path in writing), and a nonmagnetic intermediate layer (NMIL: underlayer of RL and separation layer between RL and SUL). For the NMIL, uniaxial crystallographic symmetry is an essential requirement for suppression of variant growth of magnetic grains in granular-type RL. From this view point, AlN with wurtzite structure and materials with pseudo-hcp structure, which means fcc structure with stacking faults, were found to be effective. For the SUL, disordered hcp CoIr with negative K_u were found to well suppress both spike noise and track erasure due to a wide distribution of magnetic flux under the return yoke in writing and formation of a Neel wall instead of a Bloch wall in the SUL. For the RL, positive-/negative-K_u stacked media with incoherent switching mode was found to be effective in order to solve the recent write-ability problem for high K_u RL material with high thermal stability. Applying all these items, an advanced medium concept with the stacking structure of “CoPtCr-oxide/CoIr-oxide/CoIr/pseudo-hcp nonmagnetic layer/substrate” is very promising from the view point of (1) switching field reduction of a RL with high K_u material, (2) conventional amorphous SUL free, and (3) conventional NMIL free.     

The role of nucleation field of media in heat-assisted magnetic probe recording

We characterize a method of heat-assisted magnetic probe recording on perpendicular media. Heating source is field emission current from a scanning tunneling microscope (STM) tip. Recording media are three kinds of magnetic films, Co/Pt, CoNi/Pt, and Co/Pd multilayers with different nucleation fields. Pulses with amplitude of 5 V were applied between the STM tip and the recording medium. Experiments show that magnetic marks with an average size of 180 nm were formed on both Co/Pt and CoNi/Pt films whose nucleation fields are greater than their saturation magnetization. No marks were observed on the Co/Pd film whose nucleation field is smaller than its saturation magnetization. A model is built to simulate the dynamic process of domain formation in probe-based magnetic recording system. Simulation results agree with experiments and it explains the effect of the nucleation field of medium in perpendicular recording.     

The search for magnetic monopoles in magnetite from North China

Using the 27 kG magnetic field produced by a superconducting solenoid we have searched for trapped magnetic monopoles in a magnetite sample from North China. We hvve scanned the nuclear emulsion both for low ionizing tracks and heavily ionizing tracks, and polycarbonate track detectors for heavily ionizing tracks of magnetic monopoles. We obtained a 90% confidence level upper limit of 1.8×10⁻²⁶ monopoles per nucleon in the magnetite sample.     

Status of ion track technology—Prospects of single tracks

A glimpse on the present status of ion track technology is provided: irradiation, etching and electro-replication of single and multiple ion tracks. The uses of an electrolytic cell for single and counted track studies are sketched. In connection with a data acquisition system such a device enables to determine in real-time: track and bulk etch rate, cone angle, and size. Processing can be terminated at a preset time, current, diameter, or length (of wires). The system can be used to rapidly screen new track materials, etchants, and electro-replication processes.     

Strain imaging of a perpendicular recording head in a Hard Disk Drive

Imaging of magnetic-field-induced strains using scanning probe microscopy enables us to observe magnetic domain structures and magnetic force distributions. This method has an ability of observing magnetic structures in deep portions along with surface structures. We observed an air-bearing surface of a perpendicular recording head in a hard disk drive in large areas including the whole yokes, and investigated characteristics of the magnetic poles and layers subjected to magnetic fields produced by writing coil currents. Attractive forces independent of the field directions acted between the upper and lower yokes made by soft material, which generated surface displacements in the ABS. The signals were second-harmonic oscillations for alternating currents without a dc bias, and the amplitude images represented the field distribution. Meanwhile, fixed magnetic charges in both ends of the read sensor, which were produced by the hard-bias film, were subjected to the fields. The fixed charges, which hardly changed by the fields, lead to strains depending on the field directions, and generated specific contrasts of one pair of bright-dark spots in the strain images. In the absence of surface underlayer, the fields by the writing coil broke into the read sensor sandwiched by the shield layers.