Increase In Recording Density Research Articles

Detection Scheme With Joint Intertrack Interference and Media Noise Mitigations for Heat Assisted Interlaced Magnetic Recording

Heat assisted interlaced magnetic recording (HIMR) is a promising candidate for the next-generation of magnetic recording technology to further increase the area density beyond 1Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Specifically, the high temperature and low temperature tracks are written in an interlaced order to improve the recording performance. However, the inter-track interference (ITI), inter-symbol interference (ISI) and thermal jitters brought by the increased recording density and Curie temperature variations are severe in HIMR, which degrade the bit error rate (BER) performance obviously. In this study, we propose a multitrack detection scheme with joint intertrack interference and media noise mitigations. Here a multi-task neural network (MTNN) is designed to simultaneously predict ITI pattern and residual media noise, then the 2D variable equalizers corresponding to different ITI patterns are implemented and predicted residual media noises are embedded into the branch metrics of modified Bahl-Cocke-Jelinek-Raviv (BCJR) detector to mitigate ITI and whiten media noise. The simulation demonstrates that the proposed MTNN with variable equalizer and modified BCJR detector (MTNN+VE+MB) algorithm mitigates the ITI and media noise effectively. At the channel bit density of 3.10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , it provides 2.6 dB signal-to-noise ratio (SNR) gain compared to that of conventional 2D fixed equalizer with pattern dependent noise prediction detector (FE+PDNP) for the low temperature (LT) tracks with 4% Curie temperature variance.

Films of immiscible systems obtained by three-electrode ion-plasma sputtering

The data on the ion-plasma sputtering of composite targets of the systems (Fe, Co, Ni) – (Ag, Bi) whose components do not mix in the liquid state is presented. The lattice parameters and sizes of crystals of metastable phases were determined. The films deposited show the formation of supersaturated solid solutions ore nanocrystalline and amorphous phase. The decomposition of metastable phase is accompanied by precipitation of equilibrium solid solutions based on incoming elements ore an intermediate phase. We studied the thermal stability of metastable states in (Fe, Co, Ni) – (Ag, Bi) films, the electrical and hysteresis magnetic characteristics of as-deposited and heat-treated films and established the optimum film compositions with precise values of temperature coefficient of resistivity (∼ 10−5 K−1) and high coercive force (H c ∼ 150 kA/m). Such films can be promising for use as thin-film precision resistors and magnetic information carriers with an increased recording density.

Multitrack Detection With a Two-Dimensional Soft-Transition Assisted Multitask Neural Network for Heat-Assisted Interlaced Magnetic Recording

Heat-assisted interlaced magnetic recording (HIMR) further increases recording density compared to heat-assisted magnetic recording due to the interlaced track layout. However, the smaller circular thermal profile of the top low-temperature-written tracks results in severe transition curvatures, which causes nonlinear distortions of the readback signal and degrades the bit error rate (BER) performance. Moreover, the increased recording density causes severe two-dimensional (2-D) intersymbol interference (ISI) along both down track and cross track directions. To mitigate the effect of nonlinear distortion and 2-D ISI in an HIMR system, we model a 2-D soft-transition, information-assisted, multitask neural network and modified Bahl–Cocke–Jelinek–Raviv (BCJR) detector (2DST-MTNN-MB) algorithm to detect three tracks simultaneously. The readback signal and 2-D soft-transition information are fed into the multitask neural network to obtain equalized signals and soft bit estimates of three tracks simultaneously. Then, the signal of the current track and soft estimates of the side tracks are embedded into the branch metrics of the modified BCJR detector for data detection, and the low-density parity check decoder is cascaded for error correction. The simulation results show that the 2DST-MTNN-MB algorithm provides 5.0 dB signal-to-noise ratio gains with reduced computation complexity compared with a single-track, single-task neural network and one-dimensional BCJR detector algorithm for the low-temperature-written track at channel bit density of 3.51 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , thereby narrowing the gap of BER performance between high- and low-temperature-written tracks.

Multitrack Detection With Two-Dimensional Hybrid Equalizer for High-Density Bit-Patterned Media Recording

Bit-patterned media recording (BPMR) is one of the most promising candidates to further increase recording density beyond 1 Tb/in2. For high-density BPMR systems, severe intertrack interference (ITI) and media noise caused by decreased track pitch bring severe deterioration to the readback signals. To mitigate the impact of ITI and media noise on bit error rate (BER) performance, we developed a multitrack detection scheme with a two-dimensional (2-D) hybrid equalizer, which utilizes 2-D distinct equalizer coefficients corresponding to the different estimated ITI patterns. By analyzing the probability density distribution of the log likelihood ratio (LLR) output by the decoder, a LLR threshold is implemented to evaluate the reliability of estimated ITI patterns. Then, the 2-D variable equalizer trained with different ITI patterns is implemented for the reliably estimated ITI patterns; otherwise a 2-D fixed equalizer trained with a pseudorandom bit sequence is utilized. Hence, compared to the conventional 2-D fixed equalizer, the 2-D hybrid equalizer decreases the mean squared error of equalization with reduced error propagation risk. The simulation results indicate that the 2-D hybrid equalizer provides increased signal-to-noise ratio gains compared to the 2-D fixed equalizer when the media noise and track misregistration become more severe.

Physical properties and structure of vapor-quenched immiscible alloys

The method of modernized ion-plasma sputtering produced metastable states, including nanocrystalline and amorphous phases in films of Fe-Ag, Fe-Bi, Fe-Ag-Bi, Fe-Co-Ag and Ni-Ag alloys whose components do not mixed in the liquid state. The periods of the crystal lattices and the dimensions of the crystallites of the nonequilibrium phases in the fresh-sputtered state and after the heating are determined. The temperatures of the beginning and the end of the decomposition of metastable phases are established when heated at a constant rate. The electric and hysteretic magnetic properties of films in freshly dusted and thermally processed states are measured. The compositions and conditions for obtaining films with low values of the temperature coefficient of electrical resistivity (~ 10-5 K-1) and high coercive force (HC ~ 150 kA/m) are established. Such films can be promising for use as thin-film precision resistors and magnetic information carriers with an increased recording density.

Reduction of Intergranular Exchange Coupling for CoPt–B2O3 Granular Media by Employing a RuCoCr-Oxide Buffer Layer With Oxide of Various Melting Points

CoPt-oxide granular films have been widely used for perpendicular magnetic recording media. To further increase recording density of the media, enhancement of magnetocrystalline anisotropy $(K_{u})$ to overcome the thermal stability issue and reduction of media noise to increase signal-to-noise-ratio are essential. The requirements can be realized through the promotion of columnar growth of magnetic grains which are phase separated with oxide boundary material and intergranular exchange decoupling 1), respectively. Generally, for the grain boundary materials of the granular media, oxides with amorphous phase which do not dissolve into CoPt metal have been utilized. The oxide is expected to segregate into the trench of Ru underlayer so that CoPt grains can grow heteroepitaxially on the bump of Ru underlayer. Many authors have proposed various kinds of oxide materials to be segregated into the grain boundaries, like SiO 2 2), TiO 2 3), Ta 2 $\mathrm {O}_{5}^{3)}$, Nb 2 $\mathrm {O}_{5}^{4)}$, WO 3 4), MgO 5), Cr 2 $\mathrm {O}_{3}^{5)}$, and $\mathrm {Y}_{2} \mathrm {O}_{3}^{5)}$. Previously, we had found that a granular medium with high magnetocrystalline anisotropy $(K_{u})$ was successfully realized when B 2 O 3 was applied for the grain boundary material 6). However, the separation between magnetic grains at the initial growth region in the Ru trench by low melting point oxide such as B 2 O 3 is not sufficient 7). Therefore, we have carried out an investigation on the deposition of a buffer layer (BL) with non-ferromagnetic metal and oxides of various melting points $(T_{m})$ on the Ru underlayer to grow the non-ferromagnetic metal and oxide on the Ru bump and trench, respectively, aiming to intergranular exchange decouple the magnetic grains. In this paper, we will discuss about the effect of utilizing BL with oxide of various $T_{m}$ on the intergranular exchange decoupling of CoPt$- \mathrm {B}_{2} \mathrm {O}_{3}$ granular media in relation with magnetic properties.

浮動ヘッドスライダの接触過程におけるディスク微小うねり励振振動の解析

Vibration characteristics of a flying head slider in hard disk drives at touchdown attract strong interest because head‒disk spacing must be decreased to less than 1 nm in order to increase recording density. This study first evaluates head‒disk interfacial force based on rough surface adhesion contact models and a simple air-bearing force model. Then, microwavinessexcited vibration of single-degree-of-freedom slider model at touchdown was numerically simulated in more detail than the previous study. It was found that the slider exhibits a MW-excited vibration having low frequency components of less than 100 kHz at touchdown and shifts to severe contact state near the boundary of static instability onset. When a small static unstable region is generated by decreasing air-bearing stiffness ratio, the occurrence of the low frequency spacing variation is interrupted by temporal flying state (loss of contact). When the static unstable region further increases by decreasing air-bearing stiffness ratio, the spacing variation with low frequencies can appear only at the beginning of contact and immediately jumps to a light contact state and then shifts to a perfect flying state during a long input power range and eventually jumps to a heavy contact state. These calculated results seem to correspond to the actual slider dynamic behaviors at touchdown in the inner, outer and middle diameter regions. The possibility of surfing recording is discussed from the analytical and experimental evidences.

無潤滑ディスク DLC の摩擦特性の温度依存性に関する研究

To further increase recording density of disk storage drives, the heat assisted magnetic recording (HAMR) can solve the coercive force problem of thermal fluctuation and is regarded as the key technology for achieving recording density of more than 1Tb/in2 (1)(2). In this technology, it is suggested that there are serious issues for the diamond like carbon (DLC) overcoat on the disk surface. Because that is heated to high temperature with laser beams(2). In this study, first, we measured temperature dependence of friction properties of non-lubricated DLC surface on magnetic disks. Second, we considered the influence of the accumulated smear on test pin surface during the friction test. As a result, it was estimated that the smear on pin surface accumulated by the laser irradiation was originated from the hydrocarbon contamination adsorbed on DLC surface.

High density holographic versatile disc (HVD) system using collinear technologies

Collinear Holography was proposed by OPTWARE Corporation, in which the information and reference beams were aligned co-axially and modulated by the same SLM. With this unique configuration the optical pickup can be designed as small as the DVD's, and can be placed on one side of the recording disc. A 2-dimensional digital page data format was used and the shift-multiplexing method was employed to increased recording density of HVD. As the servo technology is being introduced to control the objective lens to be maintained precisely to the disc in the recording and the reconstructing process, a vibration isolator is no longer necessary. In HVD, the pre-formatted meta-data reflective layer was used for the focus/tracking servo and reading address information, and the dichroic mirror layer was used for detecting holographic recording information without interfering with the preformatted information. Experimental and theoretical studies suggest that the holographic material is very effective to increased recording density of the system. HVD will be compatible with existing disc storage systems, like CD and DVD, and enable us to expand its applications into other optical information storage systems.

A Study of Timing Recovery for High-Recording-Density Tape Systems

Today's tape storage systems are widely used as a low-cost solution for data backup and archiving for the rapidly growing amount of digital information produced in various fields. Following the Information Storage Industry Consortium (INSIC) Roadmap [1], tape cartridge capacity continues to scale by doubling the capacity every two years. Developments in read channel technology are expected to play an important role in improving data-storage capability. The introduction of powerful error-correction codes, such as low density parity check (LDPC) codes to the tape storage system (and which have already been applied to current HDD systems) is anticipated to achieve system operation under low SNR conditions, enabling additional increases in areal recording density. However, at a low SNR, a loss-of-lock (LOL) in the phase-locked loop (PLL) will frequently occur in the current timing recovery scheme. Previous studies of the iterative timing recovery scheme [2], [3] enable systems to operate robustly even at low SNR conditions. However, due to the complexity of these systems, the cost of hardware makes it difficult to introduce iterative timing recovery schemes into multichannel tape systems. In this work, it is shown that erroneous feedback in the PLL caused by misdetection of the detector is the major cause for LOL in tape systems at high recording densities. To cope with these misdetections of the detector while maintaining an acceptable latency in the feedback loop, a new timing recovery scheme that utilizes the soft-output information of the detector was studied. The results show that LOL rate performance can clearly be improved by such schemes, with only a small increase in implementation complexity.

High density collinear holographic data storage system

Holographic data storage system (HDSS) has been a good candidate for a volumetric recording technology, due to their large storage capacities and high transfer rates, and have been researched for tens of years after the principle of holography was first proposed. However, these systems, called conventional 2-axis holography, still have essential issues for commercialization of products. Collinear HDSS, in which the information and reference beams are modulated co-axially by the same spatial light modulator (SLM), as a new read/write method for HDSS are very promising. With this unique configuration, the optical pickup can be designed as small as DVDs, and can be placed on one side of the recording media (disc). In the disc structure, the preformatted reflective layer is used for the focus/tracking servo and reading address information, and a dichroic mirror layer is used for detecting holographic recording information without interfering with the preformatted information. A 2-dimensional digital page data format is used and the shift-multiplexing method is employed to increase recording density. As servo technologies are being introduced to control the objective lens to be maintained precisely to the disc in the recording and reconstructing process, a vibration isolator is no longer necessary. Collinear holography can produce a small, practical HDSS more easily than conventional 2-axis holography. In this paper, we introduced the principle of the collinear holography and its media structure of disc. Some results of experimental and theoretical studies suggest that it is a very effective method. We also discussed some methods to increase the recording density and data transfer rates of collinear holography.

Magnetoelectric assisted 180° magnetization switching for electric field addressable writing in magnetoresistive random-access memory.

Magnetization-based memories, e.g., hard drive and magnetoresistive random-access memory (MRAM), use bistable magnetic domains in patterned nanomagnets for information recording. Electric field (E) tunable magnetic anisotropy can lower the energy barrier between two distinct magnetic states, promising reduced power consumption and increased recording density. However, integration of magnetoelectric heterostructure into MRAM is a highly challenging task owing to the particular architecture requirements of each component. Here, we show an epitaxial growth of self-assembled CoFe2O4 nanostripes with bistable in-plane magnetizations on Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) substrates, where the magnetic switching can be triggered by E-induced elastic strain effect. An unprecedented magnetic coercive field change of up to 600 Oe was observed with increasing E. A near 180° magnetization rotation can be activated by E in the vicinity of the magnetic coercive field. These findings might help to solve the 1/2-selection problem in traditional MRAM by providing reduced magnetic coercive field in E field selected memory cells.

Patterned Bit Cell Arrangement and Broadening of Switching Field Distribution Caused by Magneto-Static Interactions

Patterned media technology is a promising approach for future high density magnetic data storage. The bit-to-bit spacing becomes smaller as recording density increases. As a result, the magneto-static field (Hd) from the surrounding bits becomes stronger, and contributes more to the broadening of the switching field distribution (SFD) of targeted bit. This paper reports the investigations between possible bit-cell arrangements and SFD broadening caused by the magneto-static field. The possible bit-cell arrangements are categorized into three groups-square arrangement, isosceles triangular arrangement, and equilateral triangular arrangement. The influences of manufacturing induced deviations, including bit-cell position offset and bit-cell size variation, over the Hd are also simulated. The results suggested that isosceles triangular arrangement can reduce the SFD broadening, without increasing the requirement on track positioning accuracy.

Analysis on dynamic characteristics in time domain of slider with ultra thin spacing

With increasing recording density in computer hard disks, the flying height of the magnetic head becomes 10 nm or less. The numerical method of the control equations needs to be optimized consistently with the decrease of the flying height. The iterative process is usually divergent with traditional methods which cannot ensure the accuracy of the analysis results and the stability of numerical calculations. Firstly, a new scheme is presented to solve the ultra-thin lubricating control equations in this paper. The equilibrium equation is obtained by taking the Couette flow terms and time term as dominating terms in iterative process. Moreover, the weighting flow factor is introduced for the abrupt change of film thickness in order to reduce calculated deviation. Secondly, the rules for variation of flying pose of the magnetic head with time are obtained by the time domain analysis. The results show that the magnetic head can recover to the equilibrium position after being slightly influenced by outside disturbance. And the dynamic lubrication properties of the head suffer heavily from the vibrations in the height direction and fluctuation in the pitch direction. Finally, the analysis of the sine roughness-passing capacity of the head indicates that the head can surpass small sine waves and the equilibrium position can be retrieved after a period of decaying oscillation. The proposed research ensures the accuracy and stability of the numerical calculation of the control equations, and the dynamic characteristics and flying stability of a magnetic head is studied systematically. It can also be the theoretical basis for the analysis of the flight characteristics of the slider.

Half-data-page insertion method for increasing recording density in angular multiplexing holographic memory

We have developed a method to use a half-size data page between two full-size data pages to increase the recording density in angular multiplexing holographic memory up to 1.5× as much as the conventional angular multiplexing sequence. In our recording sequence, the full- and half-size data pages are alternately multiplexed. This is because each plane wave from various points in a data page has different angular selectivity. A half-size data page has higher angular selectivity than a full-size data page. The required angular intervals were estimated by numerical simulation taking holographic medium tilt into account. Also, an angular multiplexing experiment using the half-data-page insertion method resulted in a low bit error rate of the order of 10(-3), which is sufficient for practical use.

Multicomponent gas mixture air bearing modeling via lattice Boltzmann method

As the demand for ultrahigh recording density increases, development of an integrated head disk interface (HDI) modeling tool, which considers the air bearing and lubricant film morphology simultaneously is of paramount importance. To overcome the shortcomings of the existing models based on the modified Reynolds equation (MRE), the lattice Boltzmann method (LBM) is a natural choice in modeling high Knudsen number (Kn) flows owing to its advantages over conventional methods. The transient and parallel nature makes this LBM an attractive tool for the next generation air bearing design. Although LBM has been successfully applied to single component systems, a multicomponent system analysis has been thwarted because of the complexity in coupling the terms for each component. Previous studies have shown good results in modeling immiscible component mixtures by use of an interparticle potential. In this paper, we extend our LBM model to predict the flow rate of high Kn pressure-driven flows in multicomponent gas mixture air bearings, such as the air–helium system. For accurate modeling of slip conditions near the wall, we adopt our LBM scheme with spatially dependent relaxation times for air bearings in HDIs. To verify the accuracy of our code, we tested our scheme via simple two-dimensional benchmark flows. In the pressure-driven flow of an air–helium mixture, we found that the simple linear combination of pure helium and pure air flow rates, based on helium and air mole fraction, gives considerable error when compared to our LBM calculation. Hybridization with the existing MRE database can be adopted with the procedure reported here to develop the state-of-the-art slider design software.

B-2-4 熱アシスト磁気記録におけるヘッドディスクインターフェイスに関する研究 : DLC保護膜の構造におよぼす加熱の影響(B-2 ヘッド・ディスク・インタフェースに作用する相互作用他,口頭発表:ヘッド・ディスク・インターフェイス)

To further increase recording density of disk storage drives, thermally-assisted magnetic recording can solve the fundamental issues of thermal fluctuation and is regarded as the key technology for achieving recording density of more than 1 Tb/in^2. In this technology, it is suggested that there is critical issues for the ultra-thin liquid lubricant films and diamond-like carbon (DLC) films on the disk surfaces, because they are heated to high temperature with laser beams. In this study, we focused on the thermal stability of the three types of DLC films deposited by chemical vapor deposition (CVD), ion beam deposition (IBD) and filtered cathodic vacuum arc (FCVA) deposition. We also compared the optical properties of heated three DLC films characterized by ellipsometry, electron spectroscopy for chemical analysis and raman spectroscopy. As a result, the effects of heating temperature on the structure of DLC films were clarified.

Performance of Advanced Data-Detection Schemes on Barium-Ferrite Particulate Media

Noise-predictive maximum likelihood (NPML) detection has become state of the art in modern hard-disk drive (HDD) systems. Tape drive systems, in contrast, have typically adopted a traditional partial-response maximum likelihood detection approach, a fact that mainly reflects the large disparity in areal recording density between tape drives and HDDs. However, tape drive systems maintain a growth rate in areal recording density that is similar to that of HDDs, which implies that in future tape storage systems also advanced detection schemes need to be employed. This paper investigates the performance achieved by NPML, in particular, data-dependent NPML for data detection on barium-ferrite (BaFe) particulate tape media. Using actual readback waveforms, it is found that these advanced detection techniques are fairly efficient in improving the bit-error-rate performance of the read channel. Therefore, they enable, in combination with the BaFe media, a significant increase in linear recording density over that of state-of-the-art tape systems.

Magnetism of nanostructured materials for advanced magnetic recording

Abstract The continuous increase in magnetic recording density has so far been achieved by scaling the geometrical dimensions. However, it is now obvious that conventional magnetic recording is reaching its scaling limit. To realize ultrahigh recording densities of 1 Tbit/in2 and beyond in the future one of the most straightforward concepts is bit-patterned composite media with a regular arrangement of magnetic nanodots, where each nanodot is composed of a hard magnetic and a soft magnetic part and represents one bit. The hard magnetic component guarantees thermal stability for smallest dot sizes whereas the soft magnetic component reduces the coercivity and thus enables writeability with conventional heads. In this review the requirements for ultrahigh density magnetic recording are presented which seem to be optimally fulfilled by L10-FePt (hard magnetic)/Fe (soft magnetic) nanocomposites.

Design of a dual stage actuator tape head with high-bandwidth track following capability

As tape recording density increases, high-frequency track following capability of the tape head actuator becomes more important. This paper details the mechanical design and experimental analysis of a dual state actuator tape head. The dual stage actuator design combines a traditional voice coil motor for low frequency track following with a piezoelectric actuator for high-frequency track following. Non-parametric and parametric modeling techniques are used in the analysis of the dual stage actuator. The performance of the dual stage actuator head shows good correlation with the model and will enable improved track following capability for future high-performance tape drives.