A comprehensive picture of magnetic recording: theory and experiment part II: Analog recording (audio and video)

In Part I of this paper we described a computer model for the simulation of magnetic recording and applied this model to the simulation of digital recording. The second part covers the application of the model to the remaining two major techniques of magnetic recording: Analog audio and analog video recording. Theoretically determined correlations between hardware and medium parameters and the recording properties of the recording systems are discussed and compared with experimental data. It is shown that the model describes even such complex techniques as the overwrite recording in HiFi-VHS video recording to a quantitative degree.     

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.     

Multilevel read-only optical recording methods

The advantages of read-only storage is the predominance of optical recording relative to magnetic and other rewritable methods. Multilevel (ML) read-only technology has been a trend to improve the data capacity and transfer rate. Based on the principle and coding method of ML, this paper demonstrates some ML read-only recording methods, of which a new ML read-only recording is developed. This recording method integrates amplitude modulation achieved by the reaction mechanism of physics and chemistry of photoresist with the run-length-limited technology. The discs can be achieved using standard photoresist mastering and replication techniques with great compatibility to conventional binary read-only discs.     

Micromagnetic modeling for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is one of the candidate systems beyond the perpendicular recording technology. Here, a micromagnetic model and a heat transfer model are introduced to study the heating and cooling processes in the HAMR media; then, by integration of the SPT head and the laser heating source, the recording performance is simulated and investigated on a single track at an area density of 1 Tb/in². In the HAMR system, the temperature in the medium under the laser wave guide is increased by heating, and decreased by air bearing and heat conduction when the write process really occurred. The target of this study is to find the proper design of the head-laser assembly for optimum recording. It is found that the proper distance between the laser wave guide and the head's main pole rear/front edge is only 41.4/1.4 nm for optimum recording performance.     

Magnetic properties of soft layer/FePt--MgO exchange coupled composite perpendicular recording media

The magnetic properties of exchange coupled composite （ECC） media that are composed of perpendicular magnetic recording media FePt MgO and two kinds of soft layers have been studied by using an x-ray diffractometer, a polar Kerr magneto-optical system （PMOKE） and a vibrating sample magnetometer （VSM）. The results show that ECC media can reduce the coercivities of perpendicular magnetic recording media FePt-MgO. The ECC media with granular-type soft layers have weaker exchange couplings between magnetic grains and the magnetization process, for ECC media of this kind mainly follow the Stoner Wohlfarth model.     

Micro magnetic structure of magnetic clusters in CoCrTaPtB recording media

Micro magnetic structure in CoCrTaPtB recording media with H_c>3 kOe was studied by magnetic force microscope and LLG simulation. Two types of the characteristic magnetic structure were observed. One is the convergent-type magnetic structure, which is observed as localized magnetic cluster on bit. The other is the vortex structure, which is responsible for the recording bit peroration. These micro magnetic structures cause the medium noise in high-density recording and must be suppressed by adjusting coercive force H_c and inter-granular interaction to explore high density recording media.     

Extensions of perpendicular recording

Gains in storage density in magnetic recording have fundamentally been achieved by scaling—reduced geometrical dimensions under the assumption that the recording physics does not change if all dimensions are scaled appropriately. It is becoming clear that evidence of the breakdown of scaling is now seen. We will here discuss ways to break the constraints on magnetic recording set by scaling. In particular, we will discuss energy-assisted recording, domain-wall-assisted recording, and bit-patterned media, with some emphasis on recording system considerations.     

Single-layer and multilayered films for perpendicular recording based on a Co-Cr alloy

Results of materials research into the dependence of the magnetic properties of Co-Cr recording media on the structural properties are discussed in relation with results from recording experiments. The magnetic properties depend strongly on the preparation conditions. An increase in substrate temperature results in an increase of the coercivity, but also gives rise to inhomogeneities of the Co-Cr film in the growth direction and to lateral inhomogeneities due to Cr segregation. The inhomogeneities in the growth direction are suppressed by the application of a non-magnetic, amorphous Ge layer between the substrate and the Co-Cr layer, resulting in a magnetic layer with a large perpendicular anisotropy. From the slope of the hysteresis loops we obtain, using a model proposed by Kooy and Enz, an estimate for the mean size of the stripe domains, which is found to increase with increasing layer thickness and decrease with increasing substrate temperature. The change in the size of the stripe domains is consistent with the trends calculated from noise spectra obtained in recording experiments. Multilayers of thin Co-Cr layers alternated with non-magnetic Ge layers are also discussed.     

Simulation on near-field light generated by metal nano-dot on GaAs substrate for heat source of heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is promising for achieving more than 1 Tb/inch² recording density. A near-field transducer (NFT), which forms a hot spot of 10–100 nm in diameter on a recording medium, is necessary in HAMR. In this study, localized surface plasmons generated by a metal nano-dot in a novel device for a heat source of heat-assisted magnetic recording were analyzed using a simple model in which a metal hemisphere was formed on a GaAs substrate and a quasi-electrostatic approximation. The scattering and absorption efficiencies as well as the enhancement factor were investigated for several kinds of metal. As a result, their dependence on the wavelength and the polarization direction of the incident light was clarified.     

Below-diffraction-limited hybrid recording using silicon thin film super-resolution structure

We report on new experimental results for below-diffraction-limited hybrid recording. In our experiments, by means of focused laser assisted magnetic recording, the magnetic domains within TbFeCo thin films are obtained under an external perpendicular direct magnetic field. For a single magnetic medium, the domain size is mainly determined by the focused spot, which is about 620~nm for the laser wavelength λ =406~nm, and a numerical aperture of the lens of 0.80. However, when a silicon thin film structure is inserted between the substrate and the magnetic medium, the recording domains can be reduced obviously. By optimizing the experimental condition, even the size can be reduced to about 100~nm, which is below the diffraction limit, i.e. about 1/6 of the spot size. This is very useful for improving the hybrid recording density in practical applications.     

Past and present of perpendicular magnetic recording

Past and present of perpendicular magnetic recording (PMR) is described. PMR was born at a university institute 30 year ago and hard disk drive (HDD) industry is now converting the products from longitudinal magnetic recording (LMR) to PMR. The way to the present, however, was not flat. The research of PMR experienced the death valley in the 1990s, but was overcome by its unique research system. The commercialization of PMR was much later than it was expected in the beginning, which might have affected the position of HDDs in storage systems. The market of HDDs is expected to steadily grow towards the future being supported by PMR.     

Some results with a modified self-consistent magnetic recording channel model

Self-consistent iterative simulation of the magnetic recording channel has been established as a powerful tool for the workers in this field. Though many papers have been published, even the one-dimensional model - which is especially suited for thin metallic films - can be improved or supplemented. The model presented here contains a new method of considering head sensitivities without transformation into the frequency domain and a detailed impulse analysis. With slight modifications it can be used for anisotropic materials.     

Overwrite recording on different recording media

A contact printing model is introduced to explain the re-recording dip in overwrite recording. The first signal on a recording medium is transferred to the medium surface, where the second signal increases the susceptibility to transfer the S-1 signal magnetically with opposite phase. The output signal of S-1 will be compensated by the transferred signal, and will be zero, which is supposed to be the re-recording dip. The contact printing model is confirmed by experiments.     

The limits to magnetic recording — media considerations

At present longitudinal magnetic recording systems are the basis of all low cost high-density information storage systems. During the recent past the data density stored on rigid disk media which is the higher density format have increased at the rate of 60% per annum compound. However, very recently due to the introduction of new advanced GMR spin-valve heads this rate of advance has increased to 100% per annum in laboratory demonstrations. Hence, it is pertinent at this time to enquire as to where the fundamental physical limitations of longitudinal magnetic recording may lie. In this context there are two principle areas of interest: the first of these is limitations to data rate. These are concerned with the fundamental physics of the maximum rate at which a magnetic moment may reverse from one direction to the other. The theoretical calculation of these limits is complex and not well understood but the limits of our understanding will be reviewed in this paper. Secondly, and of principle concern is the limit to the density at which information can be stored in a magnetic thin film. This latter limitation is based around the signal to noise ratio and also the question of the stability of increasingly small written bits. Signal to noise considerations are extremely complex and derive from factors such as the shape of bits and cross-talk between neighbouring bits or even neighbouring tracks. In this article the fundamental origins of noise will be reviewed in terms of the basic physics that gives rise to variation in transition shapes. Cross-talk and cross-track interference will not be discussed as these are generally addressed through issues associated with the resolution of the servo-mechanism that positions the head above a track and is not associated with the fundamental physics of the medium itself. Thermal stability of a bit of information is of critical importance particularly as media is made ever thinner and will form a major aspect of the discussion in this work. Finally, possible material physics solutions to some of these limitations will be presented in terms of measurable parameters which to some limited degree may be controlled by process conditions.     

Drive-based recording analyses at >800 Gfc/in using shingled recording

Since the introduction of perpendicular recording, conventional perpendicular scaling has enabled the hard disk drive industry to deliver products ranging from ∼130 to well over 500 Gb/in² in a little over 4 years. The incredible areal density growth spurt enabled by perpendicular recording is now endangered by an inability to effectively balance writeability with erasure effects at the system level. Shingled magnetic recording (SMR) offers an effective means to continue perpendicular areal density growth using conventional heads and tuned media designs. The use of specially designed edge-write head structures (also known as ‘corner writers’) should further increase the AD gain potential for shingled recording. In this paper, we will demonstrate the drive-based recording performance characteristics of a shingled recording system at areal densities in excess of 800 Gb/in² using a conventional head.Using a production drive base, developmental heads/media and a number of sophisticated analytical routines, we have studied the recording performance of a shingled magnetic recording subsystem. Our observations confirm excellent writeability in excess of 400 ktpi and a perpendicular system with acceptable noise balance, especially at extreme ID and OD skews where the benefits of SMR are quite pronounced. We believe that this demonstration illustrates that SMR is not only capable of productization, but is likely the path of least resistance toward production drive areal density closer to 1 Tb/in² and beyond.     

Theory of longitudinal digital magnetic recording on thick media

A theory of the digital magnetic recording process has been produced which facilitates the prediction of the recording properties of thick particulate and other media. It allows studies to be made of recorded patterns through the depths of media and the influence of these on overall recording properties. The theory is capable of dealing with particulate, thin-film and multilayer media.     

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.     

Writability issues in high-anisotropy perpendicular magnetic recording media

Challenges and recent developments associated with writability issues in high-anisotropy perpendicular recording media are reviewed. The writing field is limited by the high coercivity caused by the high anisotropy. Some new alterna- tives are proposed to solve the writability issues, including texture-tilting-assisted, domain-wall-assisted, energy-assisted magnetic recording technologies, and so on, In addition, we propose new alternatives for the next-generation of magnetic recording media.     

Sputtering gases and pressure effects on the microstructure,magnetic properties and recording performance of TbFeCo films

The MsHc value is considered to be a key factor in high-density recording, and controlling the microstructure on the magnetic underlayer was found to be an effective way of increasing the MsHc of the amorphous TbFeCo magneto-optical (MO) medium. In this paper, we investigate the TbFeCo film's magnetic properties and the effects on the microcolumnar structure, which depends on the sputtering conditions of using various sputtering gases including Ar, Kr, and Xe, and the recording characteristics of TbFeCo memory layers. With heavy sputtering gases such as Kr or Xe, the columnar structure can be prepared in a TbFeCo film at a pressure lower than 1.0 Pa. The columnar structure of a recording layer can be effectively formed thanks to the effects of the magnetic underlayer, which has a fine surface even in the sputtering process in which Xe gas is used. The above applies to the sputtering process in which Ar gas is used. Also, when Xe gas is used in the sputtering process, coercivity Hc is increased through the formation of a well-segregated microcolumnar structure built on domain wall pinning sites, and we obtain a large MsHc and a high squareness ratio of the Kerr-hysteresis loop. Our results indicate that processing a TbFeCo film with heavy sputtering gases is suitable for tiny mark stability because the temperature gradient of Hc is increased. The objective of the low-pressure sputtering process using Xe gas to produce the columnar structure is to achieve ultra-high-density recording with tiny mark stability in the TbFeCo medium. This has been confirmed with magnetic force microscope (MFM) images of stable tiny marks recorded on TbFeCo film.     

Fabrication challenges for patterned recording media

Lithographically patterned recording media are one of the approaches to achieving Tb/in² and beyond recording densities. This will require fabrication of sub-10 nm discrete magnetic islands covering a full disk with tight spacing and size distributions and a narrow switching field distribution. To become an economically successful technology, this will need to be achieved with high throughput and low cost. The technology to fabricate such patterned media will need to be developed, and may require innovative solutions such as self-assembly and nanoimprinting, along with improved magnetic thin films for achieving high anisotropy and narrow switching field distributions.