Density Magnetic Recording Research Articles

Study on Flow Induced Vibration of Head Disk Assembly Mechanisms in Actual Hard Disk Drives

It is important to clarify air flow induced bibration characteristics in hard disk drives in order to achieve ultra-high magnetic recording density. We studied flow induced flutter vibration in actual 2.5 inch HDD which has one disk and two magnetic heads. In this study, the disk flutter vibration as well as static pressure distribution between the top cover and disk were measured, changing the track following and seek modes of the magnetic head slider. From the experimental results, it was found that the disk flutter amplitude increases and its frequency decreases, when the magnetic head is operating on a disk. We also found that the mean pressure distribution was changed under various head operating conditions. As a result, it suggests that a disk flutter amplitude increase is due to the change of mean pressure distributions and disk flutter frequency decrease is caused by the effect of head-disk coupling vibration system.

5715 スライダヒステリシス挙動におよぼす潤滑膜材料の影響(S84-1 情報機器コンピュータメカニクス(1),S84 情報機器コンピュータメカニクス)

In order to achieve a magnetic recording density of 1 Tb/in^2, the spacing is expected to be less than 2-3nm. However, a critical issue in achieving such an ultra-low spacing is the dynamic instability of the head disk interface (HDI).The goal of this research is to clarify the effects of the lubricant materials on slider hysteresis behavior in detail. In this study, the difference in the touchdown velocities was monitored by varying the lubricant film materials of the disks ; further, the correlation between the observed phenomenon and the variation in the experimental parameters was investigated. Considering the obtained experimental results, it was suggested that the variation in the touchdown velocity is due to a variation in the intermolecular forces.

Ultrathin CoPt-pinned current perpendicular to the plane spin valves

The magnetics and magnetotransport of current perpendicular to the plane (CPP) giant magnetoresistive (GMR) spin valves utilizing thin CoPt hard magnet pinning layers on Cr seed layers are investigated. 50-Å-thick CoPt18 films grown on >20Å Cr seed layers exhibit a coercivity of ∼1.5kOe and a resistivity of 31μΩcm. The low critical thickness of the CoPt hard magnet compared to IrMn or PtMn antiferromagnets makes it an attractive pinning material for small gap CPP sensors required for high magnetic recording densities. Furthermore, CoPt∕Ru∕CoFe trilayers exhibit strong antiparallel coupling and the low CoPt resistivity minimizes parasitic resistance yielding a higher magnetoresistance. Moreover, free layers with coercivities and coupling fields in a range of 5–10 Oe have been measured, demonstrating that free layer softness does not suffer from the proximity to the CoPt hard magnet due to the relatively thick spacer layers compatible with CPP GMR sensors. In particular we deposited CoPt18 pinned antiferromagnetically coupled spin with a variety of reference layers. The films were patterned into pillars with diameters ranging from 50 to 200 nm by using a combination of electron beam lithography and ion milling. The devices exhibit a magnetoresistance up to 3.6% and a resistance area product of only 20mΩμm2.

3-D FEM analysis of the effects of SPT head dimensions on recording in discrete and continuous media

Due to the considerable increase of magnetic recording density, the deviation of the head from the centerline of the track becomes a serious problem for the cross-track interference. In this paper, a three-dimensional read/write simulation system was used to investigate the effect of the width of the write head on the behavior of the flux and magnetization for continuous track medium and discrete track medium. It is shown that the magnetization in a neighboring track of the discrete track medium is smaller than that of the continuous track medium. The output of the read head using the discrete track medium is larger than that of the continuous track medium when a suitable read head width is chosen so that the cross talk is negligible.

Analysis of thermal magnetic noise in spin-valve GMR heads by using micromagnetic simulation

The size of the sensor in spin-valve GMR heads has been reduced to increase the areal magnetic recording density. Thermal magnetic noise, which arises from thermal fluctuation, becomes the main source of head noise and a limitation on recording density. Insufficient abutted permanent magnetic biasing yields an asymmetrical waveform both for signal output and thermal magnetic noise. This is due to the same mechanism as that of Barkhausen noise. In contrast, it has been found that small Hex, which is the exchange coupling strength between the bottom pinned layer (Pin1) and the antiferromagnetic biasing layer, emphasizes thermal magnetic noise without affecting signal output. In a head with small Hex, the magnetization near the air bearing surface and the top of the Pin2 layer tilts in the direction of the track width and randomly flips in the opposite direction. In synthetic ferrimagnetic heads, thermal magnetic noise chiefly depends on Hex rather than unidirectional anisotropy, Hua. The value of Hua does not directly affect thermal magnetic noise. The results indicate that Hex must be considered for reducing thermal magnetic noise.

Magnetic dot arrays with multiple storage layers

An approach to increasing the data storage density of magnetic recording was investigated wherein dot arrays are combined with multiple magnetic storage layers. The latter consists of two magnetically decoupled perpendicular Co∕Pd multilayer stacks. As a result of the difference in the coercivity between the two stacks, the orientation of the remanent magnetization of each stack can be set independent of the orientation of the other layer. Therefore, each dot allows storing four different magnetization states, which give rise to four different readback signal levels. Thus, the investigated media structure allows doubling the storage density in magnetic recording applications. It was found that even for thick spacer layers a weak magnetostatic coupling of the storage layers is present preferring ferromagnetic alignment of the storage layers.

Angular dependence of switching properties in single Fe nanopillars

The continued increase in areal densities in magnetic recording makes it crucial to understand magnetization reversal in nanoparticles. We present finite-temperature micromagnetic simulations of hysteresis in Fe nanopillars with the long axis tilted at angles from 0° to 90° to the applied sinusoidal field. The field period is 15 ns, and the particle size is 9×9×150 nm. The system is discretized into a rectangular pillar of 7×7×101 spins each with uniform magnetization. At low angles, reversal begins at the endcaps and proceeds toward the center of the particle. At 90° reversal proceeds along the entire length of the particle (save at the ends). The switching field was observed to increase over the entire range of angles, consistent with recent experimental observations. A second, lower-resolution micromagnetic simulation with 1×1×17 spins, does not agree with experiment, but shows behavior very similar to that of the Stoner–Wohlfarth model of coherent rotation.

Methods for preparing patterned media for high-density recording

The areal bit density of magnetic disk recording has made a colossal increase over the last decades. Extrapolation leads to recording parameters not likely to be achieved without changes in the present way of storing magnetic data. One of the potential solutions is the use of patterned media, which should consist of a regular two-dimensional array of single domain dots with large uniaxial magnetic anisotropy. Such media will shift the super paramagnetic limit positively in comparison with the present thin film media.Theoretically, a bit density in the order of Tbpsi is possible. This paper will discuss the most valuable methods for preparing patterned media.

ナノモーションアクチュエータの機構系と制御系の同時最適設計(S62-1 機構・制御(1),S62 情報機器コンピュータメカニクス)

A highly precise tracking actuator for high magnetic recording density was developed. The tracking actuator was consisted of a displacement amplitude mechanism and a layered piezoelectric element (PZT). When the actuator was designed to increase its working distance, its resonance frequency was always reduced for a target value. The resonance frequency was not designed to independent the working distance. We applied a simultaneous optimal design method to the tracking actuator design. The crossover frequency of the open loop transfer function with the optimized actuator increased up to 23 % for the original one.

Fabrication of FePt nanoparticles for self-organized magnetic array

A self-organized array of magnetic nanoparticles can be potentially used to increase the storage density in magnetic recording. One challenge in this approach is to obtain long-range order assemblies of the nanoparticles. One method to solve this problem is to pattern a substrate having circumferential patterns, whose dimensions are within the coherent length of the self-organized array. By patterning the disk into topographically confined circumferential patterns with such dimensions, thermally stable magnetic nanoparticles may be used to fabricate magnetic recording disks. The circumferential patterns in this case are formed on the disk substrate with dimensions of 100–500 nm and depths of 5–20 nm, prepared by electron beam lithography and reactive ion etching techniques. The monodispersed FePt nanoparticles were synthesized by thermal decomposition of iron pentacarbonyl and reduction of platinum salt simultaneously in the presence of surfactant molecules, achieving a size distribution of 3.15±0.20 nm. Our initial experimental results showed that the monodispersed FePt nanoparticles were successfully deposited into the circumferential patterns and they self-organized into a superlattice. The self-assembly of the FePt nanoparticles in the circumferential pattern strongly relied on the line edge roughness (LER) of the circumferential pattern. This suggests that we need to fabricate the circumferential pattern with LER at the length scales of approximate an individual nanoparticle size 3–4 nm in order to improve the assembly quality further. This is another challenge for today’s advanced lithography and etching processes.

3212 高速回転円板間流れに起因するディスクフラッタ及びアーム振動の実験解析

Recently, it is important to clarify air flow induced vibration issues for HDD in order to achieve ultra high magnetic recording density. We studied flow induced disk flutter dynamics as well as arm off-track vibration between corotating disks by using LDV. In particular, the effect of arm on disk flutter and arm off-track vibration was investigated. It is found that the arm which is inserted between co-rotating disks suppresses the disk flutter vibration, compared with the case without arm. The arm shape and thickness, however, do not affect the disk flutter dynamics quantitatively. In comparison with upper and lower disks, lower disk flutter was larger than upper disk flutter. In addition, it could be observed that arm off-track vibration was the largest when the arm was located on outer track of the disk.

Effective permeability and imaging of multilayered soft films for perpendicular recording media

Summary form only given, as follows. A finite thickness soft magnetic film has an effective permeability for an equivalent semi-infinite layer that is lower than the bulk (infinite film) material permeability. Thinner soft magnetic film layers show significant variation of effective permeability with wave number or magnetic recording density. When the magnetic material is divided between several equally thick soft layers, the effective permeability of the composite films is reduced, particularly at lower wave numbers This effect increases with more layers in the composite soft magnetic film. A thin exchange-decoupling layer (3 nm thick) separates the magnetic layers. Multilayered soft magnetic films are one approach to reduce the domain noise in the soft magnetic imaging layer used in perpendicular magnetic recording media. However, the disk designer must be careful with the number and thickness of the layers in the composite film to avoid significant reduction in the imaging capability of these films. This imaging is important in writing perpendicular media since the total write field is that produced by the magnetic recording head plus the image of the head field from the soft magnetic underlayer. This imaging factor determines how strong the imaging field will be in enhancing the applied field from the head to write on the perpendicular magnetic media. At lower recording densities, (e.g. in servo writing) the imaging factor can be significantly reduced in soft magnetic multilayered films making writing high coercivity media more difficult.

Improved GMR characteristics of specular spin valves by nano-oxide layer formation with mixed gases

Summary form only given. The enhanced MR ratio of over 10% and good thermal stability are strongly required for spin valve (SV) used in a read head in order to realize ultrahigh magnetic recording density beyond 40 Gbit/in/sup 2/. Recently, very applicable SVs with a pinned layer containing a nano-oxide layer (NOL) were suggested as a new technology for the improvement of MR ratio, where NOL enhances specular electron scattering. Several oxidation methods for NOL have been studied on the NOL interface control and MR improvement. Of these, plasma oxidation and ion beam oxidation can be hard to control the formation rate of very thin NOL. Accordingly natural oxidation is a good way to control the formation rate because of low oxidation rate. However, we found that natural oxidation in pure O/sub 2/ atmosphere did have a good reproducibility in our experiments. In order to improve the reproducibility of uniform NOL, we have investigated the natural oxidation conditions by mixed gases (O/sub 2//N/sub 2/ and O/sub 2//Ar) for the NOL formation. The SV samples used have the structure of Ta(5 nm)/NiFe(2.5 nm)/IrMn(7 nm)/CoFe(2.5 nm)/NOL/CoFe(1.5 nm)/Cu(2.6 nm)/CoFe(1.5 nm)/NiFe(4.5 nm)/Ta(5 nm).

Head-disk interface considerations at 10-nm flying height

As magnetic recording density increases toward hundreds of Gb/in/sup 2/, both the magnetic spacing and head-disk clearance decrease to <10 nm. By one estimate, the magnetic spacing for 1 Tb/in/sup 2/ is about 6 nm and the read width is /spl sim/30 nm. There are at least two different approaches to achieving this. The first is an extension of the traditional flying interface and the second is contact recording. In the former case, one needs to be concerned about maintaining adequate clearance both at sea level and at higher elevation, whereas in the latter case wear and corrosion of the heads and disks may pose major challenges. An example of how head-disk interference takes place in a disk drive is given for an experimental 10-nm flying slider. The effects of radial flying height profile, takeoff height of the disk, and disk curvature on mechanical spacing are presented. The results of changes occurring on the air bearing surface and the disks after long-term flyability tests are discussed.

Chemical ordering at low temperatures in FePd films

We demonstrate that, if a high degree of short range order is present in FePd disordered films, a high value of the long range order parameter S can be obtained by using postgrowth ion irradiation at very low processing temperatures. FePd films deposited monolayer by monolayer at room temperature on MgO(001) substrates exhibit a very low degree of long range order (S∼0.1) but a high degree of short range order as demonstrated by extended x-ray absorption fine structure measurements. Irradiation with 130 keV He+ ions at low fluences (2.0×1016 ions/cm2) leads to a large increase in the long range order parameter and to a large increase in perpendicular anisotropy for irradiation at substrate temperatures lower than 200 °C. This could have a great impact on the current race toward high magnetic recording density media.

Development of novel PZT thin films for active sliders based on head load/unload on demand systems

Micromachined active sliders based on head load/unload on demand systems is an interesting concept technology for ultra-high magnetic recording density of more than 100 Gb/in2. The active sliders that we proposed use PZT thin films as a microactuator and control the slider flying height of less than 10 nm. It is necessary to develop high performance microactuators in order to achieve active sliders operating at very low applied voltage. This paper describes the development of novel PZT thin films for active sliders. The sol–gel fabrication process for PZT thin films is developed and the fundamental characteristics for the PZT thin films are investigated. It is confirmed that the PZT thin films have good ferroelectric properties. Furthermore, novel thin film microactuators are proposed. The feature is that the sol–gel PZT thin films (thickness 540 nm) are deposited on the sputtered PZT thin films (thickness 300 nm) fabricated on bottom Pt/Ti electrodes. Therefore, the novel thin films consist of a thermal SiO2 layer and the sputtered and sol–gel PZT thin films layers sandwiched with upper Pt and bottom Pt/Ti electrodes on a Si slider material. Fabricating the diaphragm microactuator, the piezoelectric properties for the novel composite PZT thin films are studied. As a result, the piezoelectric strain constant d31 for the novel PZT thin films is identified to be 130 × 10−12 m/V. This value is higher than conventional monolithic PZT thin films and it is found that the novel composite PZT thin films have the good piezoelectric properties. This suggests the feasibility of realizing active sliders operating at lower voltage under about 10 V.

Quantum limit of magnetic recording density

Macroscopic quantum tunneling imposes an upper limit on the areal density of magnetic disk drives. We derive a general expression for this limit, and apply it to Co-alloy-based longitudinally recorded media, as well as to potential future media based on the high anisotropy material FePt. We estimate the temperature at which the switching rate due to quantum tunneling becomes comparable to the thermally activated switching rate. This is the lowest temperature to which a magnetic storage device can be cooled with the expectation of increasing the areal density by enhancing stability against spontaneous switching.

Increased areal density using a 2-dimensional approach

There is a drive toward achieving much higher track density in magnetic recording. Increase in track density will produce more interaction/crosstalk between adjacent tracks. The simulated variation of bit error rate (BER) as a result of increased inter-track interference (ITI) using a Viterbi Detector is presented in this paper. Results show that for a given BER and signal-to-noise ratio (SNR), different levels of ITI can be tolerated depending on the code. Calculations suggest 2-dimensional modulation codes designed for a channel experiencing ITI can result in up to 23% gain in areal density.

Properties and Structure of SmCoAlSi-based Trilayer for Longitudinal Recording Media

The introduction of the Cr interlayer caused magnetic decoupling in SmCoAlSi / Cr / SmCoAlSi and SmCoAlSi / Cr / SmCoCuTi thin films. High coercivity of 3400∼3840 Oe and extremely fine grain size in this two kinds of films were examined. Magnetic coupling within sublayers was introduced by using different substrate bias among the SmCoAlSi (deposited with substrate bias of -150 V) / SmCoAlSi (deposited with no substrate bias) / SmCoAlSi (deposited with substrate bias of -150 V) triple layers. This multilayer exhibited high coercivity ∼2960 Oe and S* 0.96. SmCoAlSi / SmCoCuTi / SmCoAlSi trilayer improved matching between the magnetic layer and the Cr underlayer, and led to increased in-plane anisotropy, high coercivity 3620 Oe and S* 0.91. The microstructures of these four kinds of media were also examined. Results obtained suggest that it is possible to produce SmCoAlSi-based multilayer media with the combined magnetic properties required for the ultra-high magnetic recording density of ∼20 Gbit/in2.

Analysis of Contact Deformation and Stiction Between Textured Disk and Textured Slider

To meet the ever-increasing magnetic recording density, the hard disk industry is focusing on reducing flying height. Texturing the slider surface to reduce the head–disk contact area is one of the most challenging and promising techniques in the current industry. In this study, a mathematical–physical model based on an extension of the Greenwood–Tripp model is proposed for predicting and analyzing the contact deformation and stiction between both textured disk and slider. The contact deformation and stiction of the head–disk interface is analyzed in considering surface texture parameters, lubricant properties, and loading conditions.