The potential of bit patterned media in shingled recording

Shingled recording on continuous and bit patterned media (BPM) is compared. From a recording viewpoint, continuous media have the advantage due to the lack of a need to synchronise writing with dot position. For BPM the write windows at 4 Tbit/in² are only a couple of nm across, requiring extremely tight manufacturing tolerances. In readback, BPM have the higher SNR over a wide range of areal densities due to the absence of transition noise and erase bands. Significant increases in areal density could be achieved using BPM, provided dot uniformity could be maintained.     

The current status and future of granular media

The current status of CoPtCr-SiO₂ granular media is briefly reviewed, and challenges relative to media technology exceeding 1 Tb/in² are discussed. It is effective to enhance grain isolation using oxide materials that easily precipitate at the grain boundary, and this technique is adopted in commercially available perpendicular recording media. Although some difficulties such as reduction of the grain size retaining the switching field remain, 1 Tb/in² could be achieved by using granular exchange-coupled composite-type media or related technologies. Discrete track media could be used for more than 1 Tb/in² recording. It could also take several years to further develop nano-processing technology and establish a cost-effective infrastructure. BPM offers great potential in achieving high recording density, although some necessary technologies are still too primitive to consider commercial production.     

Intermediate layer thickness dependence on switching field distribution in perpendicular recording media

The effect of intermediate layer (IL) thickness on crystallographic texture and magnetic properties of CoCrPtSiO₂ granular perpendicular recording media was investigated with switching field distribution (SFD) as the focus. Even though the c-axis orientation of the Co-based recording layer (RL) broadens with the reduction of IL thickness, the SFD becomes narrower. This result demonstrates that the intrinsic SFD is not directly dependent on c-axis orientation of the recording layer but instead dependent on the magnitude of exchange coupling. It is thus possible to have a medium with thin IL and narrow SFD. This is desirable for bit-patterned media (BPM), where highly exchange-coupled grains are required.     

Micromagnetic studies for bit patterned media above 2 Tbit/in.

Bit patterned media (BPM) recording is a candidate for extremely high density magnetic recording. A micromagnetic model is built up to analyze the phase diagram of the correct-write-in condition in BPM above 2 Tb/in.² fabricated by lithography or ion irradiation methods. The target of the study is to acquire the relationship between the recording performance and the magnetic properties of the media. The medium includes the polycrystalline grains and grain boundary. In BPM fabricated by lithography with FCT structure, two phase diagrams of the correct-write-in condition are found for the anisotropy angular distribution Δθ, the ratio of tetragonal anisotropy K₂₂ to uniaxial anisotropy K₁ and the uniaxial anisotropy distribution ΔK₁. In BPM fabricated by ion irradiation methods, two phase diagrams of the correct-write-in condition are analyzed for the ratio of saturation magnetization M′_s/M_s, anisotropy field H′_k/H_k and the exchange field H′_ex/H_ex in the ion irradiated region and the bit islands.     

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.     

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.     

Recording media research for future hard disk drives

Perpendicular magnetic recording (PMR) technology has recently been introduced in hard disk drives (HDDs) by almost all the HDD manufacturers. It is expected that because of PMR technology, the superparamagnetic limit can be circumvented until around 2012. At this stage, it is necessary to seek out alternatives to extend the current PMR technology, or to invent new alternatives to move beyond the PMR technology. There are several research opportunities becoming available, as we try to extend the competitive advantage of magnetic recording. This article is an attempt to provide a brief review of the research that has been carried out, interspersed with an analysis of possible new directions.     

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.     

Magnetization behavior of nanomagnets for patterned media application

Bit patterned media (BPM) which utilize each magnetic nanostructured dot as one recorded bit has attracted much interest as a promising candidate for future high-density magnetic recording. In this study, the magnetization reversal behaviors of nanostructured L1₀-FePt, Co/Pt multilayer (ML), and CoPt/Ru dots are investigated. For Co/Pt and CoPt/Ru nanodots, the bi-stable state is maintained in a very wide size range up to several hundred nm, and the magnetization reversal is dominated by the nucleation of a small reversed nucleus with the dimension of domain wall width. On the other hand, the critical size for the bi-stability of L1₀-FePt is about 60 nm, and its magnetization reversal proceeds via domain wall displacement even for such a small dot size. These reversal behaviors, depending on the magnetic materials, might be attributed to the difference in structural inhomogeneity, such as defects. In addition to the magnetic properties, the structural uniformity of the material could be crucial for the BPM application.     

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.     

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.     

Film carriers for super-high-density magnetic storage

The problem of increasing the recording density in magnetic storage devices is considered. It is shown that nanograined magnetic film media are candidate materials for magnetic data carriers. For these materials to completely meet the requirements for super-high-density magnetic carriers, appropriate structure ordering must be set in the films. To this end, it is suggested to take advantage of the high adsorbability of 3d metal nanoparticles on high-molecular compounds. To produce the carriers based on these materials with a recording density of as high as 10¹⁰ bit/cm², nanoparticles of size ≤5 nm should be embedded in a polymer matrix. To do this, it is necessary to combine chemical and physical methods for nanocomposite production.     

Perpendicular magnetic recording—Its development and realization—

The principle of conventional magnetic recording is that magnetic fields are applied parallel to the plane of the magnetic medium. As described in this paper, the invention and development of a new method of placing the magnetized information perpendicular to the plane of the magnetic recording medium is presented. The yield in the mass production of high-density hard disk drives (HDDs) for perpendicular recording is much higher than that of HDDs for conventional recording. Consequently, it is estimated that as many as 75% of the 500 million HDDs to be shipped this year will use this technology.     

Individual bit island reversal and switching field distribution in perpendicular magnetic bit patterned media

The switching of single bit magnetic islands in bit patterned media (BPM) for two cases with 10 times difference in coercivity, as well as the switching field distribution (SFD) of the islands, has been studied using magnetic force microscopy (MFM) measurements. The intrinsic SFD is measured to be ∼9-11% of the remanence coercivity (H_cr), which contributes only ∼20-50% of the total SFD broadening (∼23-41% of H_cr). High resolution MFM observations clearly showed the influence of surrounding islands on the switching behaviour and switching fields of individual bit islands, resulting in significant contributions in SFD broadening due to non-intrinsic dipolar interactions. It was further observed that single magnetic islands could be switched within a very narrow switching field range as small as 4 Oe, which indicates very sharp and uniform switching for each individual island of BPM.     

Micromagnetic recording field analysis of fast-switching single-pole-type heads for bit-patterned media

A Landau–Lifshitz–Gilbert (LLG) micromagnetic analysis of the recording field of single-pole-type (SPT) heads was carried out. The whole volume comprising the SPT head and the double-layered medium was treated micromagnetically using the finite-difference method with cubic cells as small as 5 nm, giving a total number of cells of more than 10.8 million. A parallelized fast Fourier transform (FFT) method was used to solve this large-scale problem. Dynamic recording fields were calculated for various head structures and head materials. The timing (synchronization) between the dynamic head field and land location in bit-patterned media (BPM) is discussed and the design methodology is discussed for a fast-switching SPT head.     

Research and development of metal powder for magnetic recording

Since its first application within the compact cassette in 1978, magnetic recording media using metal powder have been improved at a rate of roughly 1 dB per year as a result of advances in such fields as tape materials, tape-making technologies, etc. Today, metal tapes have a widely expanded application area, including video and data-information applications. Dowa Mining Co., Ltd., began its research regarding magnetic powder for recording media in 1978. The Company successfully developed metal powder for 8 mm video tape. Since then, Dowa has endeavored to improve the magnetic properties and reduce the particle size of metal powder. It accomplishes these goals through the full use of the Company's unique Al doping method. Especially, during the past several years, significant improvements of the magnetic properties of metal powder have been achieved. These improvements have resulted from the introduction of new technologies, including Fe–Co alloying, sintering prevention, new reaction processes, and many other new techniques. Currently, Dowa Mining is supplying a new type of metal powder for the most technologically advanced high-density recording media. Dowa's new metal powder has an axis length of 0.1 μm, H_c of 2400 Oe, and σ_s of 155 emu/g.     

Shingle magnetic recording assessment with spinstand measurement

Shingle Magnetic Recording (SMR) is a candidate to realize recording density of over 1 Tbit/in². At the cost of complex HDD firmware, we can expect many gains such as higher write-ability to break through the famous tri-lemma. In this paper, promising empirical data and concerns found on spinstand were introduced and discussed. More than 18 dB of better reverse overwrite (ROW) than conventional write recording (CWR) was confirmed at 40 nm of track pitch. Areal density capability (ADC) was improved by 25.5% by SMR. Another gain from SMR was observed at inner diameter (ID) and outer diameter (OD) that ADC loss due to skew effect was improved by using preferable side write pole edge. The dependence of ADC on magnetic read width measured with micro-track method (MRWu) indicates that side reading is limiting ADC. Reduction in side reading through narrower MRWu and inter-track interference cancellation technology are necessary to further improve ADC. BPI capability improvement by steeper down track field gradient at the track edge needs to be considered.     

Density limits imposed by the microstructure of magnetic recording media

The fundamental limit of magnetic recording density on conventional media is set by the grain size. Once this grain size limit is reached, only a reduction of the grain size allows an increased SNR and thus an increased areal density. It is shown that, whilst maintaining thermal stability, scaling demands that the required anisotropy energy density K is proportional to the areal density, or the square of the areal density if the medium thickness reaches the critical thickness (A is the exchange stiffness of the material). Recording onto materials with such a high anisotropy requires some form of a write-assist. It is furthermore shown that the grain size limit cannot be obtained with intergranular exchange present, and six different requirements are listed that constitute ideal media. An alternative path for increasing areal density of magnetic recording is to use patterned media, where each bit contains only one grain. In this case, written-in errors dominate system performance and the maximum achievable areal density is estimated to be about 6 Tbit/in². Patterned media need to exhibit narrow distributions of their physical and structural properties with standard deviations of the order of 5% or less.     

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.     

Films made of substituted azobenzene polycomplexes with cobalt for polarization holography

We have studied the possibility of recording holograms in films of the 4-methacroyloxy-(4′-carboxy-3′-hydroxy)-2-chloroazobenzene polycomplex with cobalt, with parallel and orthogonal orientation of the light beam polarization. We have shown that these films can be used as recording media for polarization holography. The characteristic features of relaxation of the diffraction efficiency of the holograms are connected with cistrans isomerization of the azobenzene groups and the characteristic features of structural rearrangement in the polymer matrix. We hypothesize that the information-related characteristics of the studied recording media can be controlled by external electric or magnetic fields, due to the presence of magnetic metal ions within the polymer film. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 830–832, November–December, 2007.