Density Magnetic Recording Research Articles

Suppressing FePt order-disorder structural phase transition temperature by Ag doping: First principles investigation for enhancing magnetic recording density

ABSTRACTIn this work, we used density functional theory to study the structures, formation energy and order–disorder structural transition temperature (To) of L10 FePt with Ag additive. The results show that an Ag atom substituting for a Fe site is more energetically preferable than that of substituting for a Pt, thus Ag atom should replace Fe site. We found that To is significantly suppressed by Ag impurity because of the increasing in configurational entropy, whereas the magnetism tends to decrease due to the absence of some Fe atom in doping processes. The decrease in To could open the opportunity to prepare FePt with lower temperature and small grain size for the high density magnetic recording media application.

Advanced Dual-Free-Layer CPP GMR Sensors for High-Density Magnetic Recording

All-metal current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) read sensors with a shield-to-shield spacing (S2S) of 16–21 nm and a narrow track width of down to 25 nm were fabricated using ferromagnetic CoFeMnSi Heusler-alloy-based spin valves. Room temperature GMR ratios from these read sensors are obtained of up to 6% and 14–24% ( $\Delta \textrm {RA}= 7.1$ –12.0 $\text{m}\Omega \mu \text{m}^{2}$ ) at $\mathrm {S2S} = 16$ and 21 nm, respectively. Studies and results of electron transport and CPP GMR support the sustainability and scalability of the CPP GMR process for Tb/in2 the areal density of magnetic recording. A universal parameter defined as magnetic resistivity for a sensor device, $\Delta \textrm {RA}/\mathrm {S2S}$ in ohm micrometers, is proposed to gauge the practically and rationally applicable CPP GMR for the read sensor process. The investigation of the CPP GMR operation range and micromagnetic simulation demonstrates the feasibility of the CPP GMR read sensors at $\mathrm {S2S} = 21$ nm for sustaining 1.0 Tb/in2 and of those at $\mathrm {S2S}= 16$ nm for marginally supporting 2.0 Tb/in2 the areal density of magnetic recording, The future path to and potential of the technology for ever increasing areal density beyond 2.0 Tb/in2 are addressed with emphasis on the importance of further enhancing the CPP GMR for process margin improvement.

The Design of Protograph LDPC Codes for 2-D Magnetic Recording Channels

This paper investigates the performance of the protograph-based low-density parity-check (LDPC) codes for 2-D intersymbol interference (ISI) channels with magnetic recording density. The protograph LDPC codes have been shown to possess simple hardware implementation and excellent error performance over additive white Gaussian noise channels and over partial response channels. We propose a design scheme and construct two new protograph LDPC codes for 2-D ISI channel, with the help of extrinsic information transfer chart analysis and asymptotic ensemble weight distribution analysis. The theoretical analyses and simulated results show that the proposed codes outperform the previously optimized irregular LDPC code in both low- and high-SNR regions. Thus, the proposed codes are good alternatives compared with other error-correction codes for the use in magnetic recording systems.

Modeling Perpendicular Magnetic Multilayered Oxide Media With Discretized Magnetic Layers

Micromagnetic models derived from the Landau-Lifshitz-Gilbert equation and its derivatives are important tools that can be used to understand error rate performance in magnetic recording systems. With increasing linear density in perpendicular magnetic recording (PMR), exchange coupled composite (ECC) media is required and optimizing the cap layer is essential to achieve good recording performance [1]. It is important to model the cap layer and thin oxide layers accurately to obtain a reasonable evaluation of recording performance, especially when the bit length is approaching the grain pitch. The often-used macro-spin model may not be sufficient to describe the semi-continuous nature of the cap layer. The macro-spin model may lead to discrepancies with the experimental data in many key areas such as resolution, non-linear effects [2], and particularly SNR at high linear densities. Here, we evaluate the performance of a fully discretized cap model for ECC media. We compare simulation results of two discretized cap models to the traditional macro-spin model as well as to experimental data.

Modeling interface exchange coupling: Effect on switching of granular FePt films

To raise the areal density of magnetic recording to ∼1 Tbit/in2, there has been much recent work on the use of FePt granular films, because their high perpendicular anisotropy allows small grains to be stable. However, their coercivity may be higher than available write-head fields. One approach to reduce the coercivity is to heat the grain (heat assisted magnetic recording). Another strategy is to add a soft capping layer to help nucleate switching via exchange coupling with the hard FePt grains. We have simulated a model of such a capped medium and have studied the effect of the strength of the interface exchange and thickness of hard layer and soft layer on the overall coercivity. Although the magnetization variation within such boundary layers may be complex, the net effect of the boundary can often be modeled as an infinitely thin interface characterized by an interface exchange energy density—we show how to do this consistently in a micromagnetic simulation. Although the switching behavior in the presence of exchange, magnetostatic, and external fields is quite complex, we show that by adding these fields one at a time, the main features of the M-H loop can be understood. In particular, we find that even without hard-soft interface exchange, magnetostatic coupling eliminates the zero-field kink in the loop, so that the absence of the kink does not (as has sometimes been assumed) imply exchange coupling. The computations have been done with a public-domain micromagnetics simulator that has been adapted to easily simulate arrays of grains.

Long Range Ordering of 5-nm-Sized Dot Arrays with a Pitch of <10 nm along EB-Drawn Guide Lines Using PS-PDMS Self-Assembly

We demonstrate the possibility of forming long-range-ordered self-assembled nanodot arrays with dots size of 5 nm and pitches of 10×7.5 nm2using guide line templates and low molecular weight (MW) (4,700–1,200 g/mol) poly (styrene)-poly (dimethylsiloxane) (PS-PDMS) for application in ultrahigh density patterned media. The self-assembled PDMS nanodots are controlled in a long range by varying the heights and gaps of the guide lines. Adopting the 14-nm-high resist guide lines with suitable gaps, the 5-nm-sized and 10×7.5 nm2-pitched self-assembled nanodots were ordered in maximum 7 dot arrays with long-range order. The experimental results demonstrate that the method is possible for achieving patterned media with magnetic recording densities of 8.6 Tbit/in.2using low MW PS-PDMS and slim guide lines.

Maximum magnetic recording density and switching field distribution

An analytical calculation of the parameters of magnetic recording on a square lattice of magnetic granules of the patterned medium has been carried out. The maximum density of the longitudinal and perpendicular recording, which is limited by the thermal instability of the magnetic state and by the magnetic dipole interaction of magnetic granules (bits), has been calculated. The switching field distribution function with the parameters determined by the magnetic dipole interaction of granules and their geometry has been determined for perpendicular recording on rod-shaped granules.

Additional Reader Noise in TDMR Reader for ITI Cancelation

Shingled writing eliminates the writer width limitation and enables further bit aspect ratio reduction to extend the areal density growth of current perpendicular magnetic recording. The achievable track density of shingled magnetic recording (SMR) is determined by the reader width rather than the writer width. However, continuously scaling of reader width increases the reader resistance and results in higher reader noise. It would be an issue to maintain enough signal-to-noise ratio of reader. The 2-D magnetic recording (TDMR) was proposed to extend the density growth by advanced signal processing technique. In TDMR configuration, there are multiple readers to read multiple tracks simultaneously. Recently, the TDMR reader was proposed as a solution of intertrack interference cancelation, which allows to use wider reader to reproduce signals from narrow tracks. In this paper, an additional reader noise is observed when the reader was positioned to read two tracks at the same time. The formation of the reader sensor appears to be closely related to the current reader structure with hard bias magnet. This issue highlights that a solution is required to enable TDMR, probably a reader structure without hard-biased.

Analysis of Heat-Assisted Magnetic Recording to Density of 4 Tb/in<inline-formula> <tex-math notation="LaTeX">$^{\textbf {2}}$ </tex-math></inline-formula>

Thermal performance of media is a key factor limiting heat-assisted magnetic recording density. In this paper, the effects of near-field optical transducer tip size on the thermal profiles of media are studied, and the results show that even with a tip size of 10 nm, the obtained cross-track thermal spot size and down-track thermal gradient still cannot meet the requirements of 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for continuous-wave laser heating. Pulse laser heating can improve the thermal distribution significantly, and the requirements for 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> can be met at a pulsewidth of 100 ps. Dynamic micromagnetic recording simulation with Landau-Lifshitz-Bloch equation is conducted for pulse laser heating recording. The results indicate that 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> density is realizable for FePt recording media. It is also pointed out that, for short-pulse laser heating recording, media with large magnetic damping constant is important.

Keynote: Thin Film Magnetic Heads: Break-through of Electroplating into Electronics, Quantum Jump in Magnetic Storage

When IBM announced the random access magnetic disk storage system RAMAC 305 in 1957, it was hailed as a major break-through in data storage. By the early 1960s engineers realized that within a decade or so the system would not be able to achieve the higher areal density needed to meet the ever increasing demand for greater storage capacity. The limitation was that the hand-wound magnetic head could not be made any smaller to further increase the areal density on the discs. Many proposals were made and patents filed for small heads, but the technology to produce the required miniature 3-D structures did not exist.The first break-through came when we demonstrated that it is possible to electroplate the 3 µm thick, precise patterns of copper and permalloy required for a magnetic head through a photoresist mask on a thin, conductive seed layer. This was followed by exploratory investigations to achieve the required magnetic material properties of the metallic alloys and the invention of appropriate plating tools to build the head structures. Working side by side with the design engineers, functional requirements of the head could be balanced against fabrication process capabilities to produce a testable device.Our presentation will describe the structure of the inductive read/write heads, MR read-only heads and integrated MR read/inductive write heads. We will show that viable heads would not have been possible without several studies and inventions relating to plating solution chemistry, processing, materials and very uniform, high intensity, reproducible agitation in plating tools. Furthermore, all the process steps had to be carefully chosen to make sure no process step damaged any of the previously fabricated structure or interfered with subsequent steps. We were learning process integration before that concept even had a formal name. Once the prototype heads were built at the Yorktown Research lab and accepted for IBM’s next generation storage system, the next step was to design and build a pilot line in San Jose, IBM’s manufacturing site for magnetic storage. Electrochemical processes with the precision required for the thin film head had never before been run on a production line. Establishing the line required frequent visits to San Jose and often daily phone contact with their engineers. Furthermore, several of the engineers who were hired to run the San Jose line spent their first year with IBM in Yorktown doing research related to head fabrication, thus facilitating the transfer of this knowledge to San Jose.The five prong approach that prevailed from the first prototype to the final product was to concurrently develop the fabrication tools and to engage in studies related to the materials, the process, process control and process integration. Our preference was to use concrete data wherever possible, but to make the best engineering judgment if such data was not available. Studies were carried just far enough to obtain the information required to control the process and build the head. When time permitted, or if new problems surfaced, we could further pursue these studies. Where problems could be clearly defined and did not need head building capability, we tried to get professors and students interested in working on them.We have demonstrated that even in the absence of a full set of concrete data it is possible to build a relatively complex device by using the available information in combination with the best engineering judgment and then coming back to refine the technology into a full-fledged manufacturing process with all the necessary controls for optimum mass production.When the RAMAC 305 was first introduced, the disk size was 24 inches in diameter and the density of magnetic recording was 2 kilobits/in2. Today the disk size may be as small as two inches in diameter and the density is approaching a trillion bits per square inch – an increase of nearly 6 orders of magnitude in 50 years. In the same time, the cost of storage has dropped by 6 orders of magnitude. The successful use of electrochemistry in building the thin film head has underscored the importance of this field in both science and engineering. Electrochemical technology today is treated as a desirable fabrication approach in silicon chip fabrication, packaging, chip package attachment and MEMS. The thin film head has been a key factor in enabling the computers that now play a vital role in every aspect of our lives.

Ordering of self-assembled 5-nm-diameter poly(dimethylsiloxane) nanodots with sub-10 nm pitch using ultra-narrow electron-beam-drawn guide lines and three-dimensional control

We demonstrate the possibility of forming long-range ordered self-assembled arrays of 5-nm-diameter nanodots with pitch of 10 × 7.5 nm2 using guide line templates and low molecular weight (MW) (4700–1200 g/mol) poly(styrene)-poly(dimethylsiloxane) (PS-PDMS) for application in ultrahigh density patterned magnetic recording media. We propose a three-dimensional control which involves control of the height of the guide lines, the thickness of the PS-PDMS films, and the gap between the guide lines in order to produce 5-nm-diameter, sub-10 nm pitched nanodots with long-range order along the guide lines. Adopting a 13-nm-thick PS-PDMS film and 14-nm-high resist guide lines, the 5-nm-diameter and 10 × 7.5 nm2-pitched self-assembled nanodots were ordered in 4–7 dot arrays with long-range order. The experimental results demonstrate that the method is suitable for the production of patterned media with magnetic recording densities of 8.6 Tbit/in.2 using low MW PS-PDMS and slim guide lines.

HAMR Performance and Integration Challenges

Heat-assisted magnetic recording (HAMR) is a fast evolving technology, and has been established as the next enabler of higher areal density in magnetic recording. After achieving high areal density capability, HAMR drive integration was recently demonstrated. In this paper, we discuss some of the recent learning from component performance and drive integration. We identify a key challenge in HAMR integration: erasure due to thermal background heating. The background heating was introduced to improve near-field transducer reliability and reduce laser power requirement. We present experimental and modeling data on the impact of thermal background, both in the cross-track (adjacent-track erasure) and down-track (self-erasure) dimensions.

Experimental Study of Head-Disk Interface in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) has been proposed to achieve magnetic recording density. In HAMR systems, a laser is used to locally heat the magnetic disk to the Curie temperature, which is ~ 400°C-600°C, to assist in the data writing process. The high temperature working condition is a great challenge for the head-disk interface. Lubricant can be depleted by evaporation or decomposition. The protective carbon overcoat can be graphitized and oxidized. The surface quality, such as its roughness, can be changed as well. The near field transducer (NFT) structure is also vulnerable under the large number of heating cycles. All of those effects of the heating will present challenges for the reliability of the HAMR performance. In this paper, an experimental HAMR platform is introduced. The free space laser exposure of the media is calibrated based on the magnetization decay of special data tracks. Lubricant loss is determined from the optical surface analyzer. The lubricant changes as a function of different thermal load cycles are investigated as well. The disk topography is measured by an atomic force microscope after the mobile lubricant is removed. The hard overcoat's topography dependence on the laser exposure conditions is also studied. With regard to the media disk, we establish three distinct laser power thresholds for lubricant loss, disk surface degradation, and magnetization decay. As the laser power increases, the lubricant starts to change first before the magnetization starts to decay, indicating that lubricant could be the most vulnerable component to the laser heating in HAMR systems. Then, the disk overcoat's surface changes, becoming rougher after the magnetization starts to decay. Furthermore, the lubricant degradation is more serious under cyclic loading conditions. With regard to the transducer, the metal film of the NFT gets removed due to high energy laser ablation, and then the NFT loses its capability to focus the light, as required for HAMR systems.

Study of microstructure and magnetic properties of L10FePt/SiO2thin films

Achieving magnetic recording densities in excess of 1Tbit/in2 requires not only perpendicular media with anisotropies larger than 7 MJ/m3, making FePt alloys an ideal choice, but also a narrow distribution below 10 nm for a reduced S/N ratio. Such grain size reduction and shape control are crucial parameters for high density magnetic recording, along with high thermal stability. Previous work has shown that the L10 FePt grain size can be controlled by alloying FePt with materials such as C, Ag, and insulators such as AlOx, MgO. Au and Al2O3 also act to segregate and magnetically decouple the FePt grains. Better results were obtained with C with respect to the uniformity of grains and SiO2 with respect to the shape. We present our results on co-sputtering FePt with C or SiO2 (up to 30 vol %) on MgO (001) single crystal substrates at 350 and 500 oC. With C or SiO2 addition we achieved grain size reduction, shape control and isolated structure formation, producing continuous films with high uniformity and a narrow grain size distribution. These additions thus allow us to simultaneously control the coercivity and the S/N ratio. We also will report structural and microstructural properties.

L10 FePtX-Y media for heat-assisted magnetic recording (Phys. Status Solidi A 7∕2013)

Heat-assisted magnetic recording (HAMR) is a new technology that will increase the achievable areal density in magnetic hard disk drives. In order to write bits into HAMR media, a laser locally heats beyond the Curie temperature and rapid cooling in the presence of the head field produces transitions in the magnetic orientation. Chemically ordered FePt exhibiting high magnetic anisotropy, will be used for the new thermally stable recording media. As discussed in the Review Article by D. Weller et al. (pp. 1245–1260), HAMR media are optimized by growing granular L10 FePt on a relatively thin thermal resistor seed layer on top of a thicker thermal conductor or heatsink layer. In addition soft underlayers are suggested to optimize the magnitude and angle of the magnetic write field. The achievable areal density depends on the near-field transducer dimension and the thermal conductivity of the media. Besides grain size, shape and distribution requirements, the thermal properties of both the magnetic and the seed layers need to be improved and optimized to achieve the required signal-to-noise ratio for higher areal densities in future magnetic recording, which will be in the range of 1.5-5 Tb∕in2.

A Study of Linear Density Dependence of Media Noise Power in Perpendicular Magnetic Recording

Media noise power saturates at high linear density in perpendicular magnetic recording. It has been reported that one of the root causes is transition noise saturation due to unsaturated magnetization governed by transition width. However, noise saturation phenomena cannot be fully explained by this mechanism alone. For example, low noise media which saturate at lower linear density show better read-write performance. In this study, we show that noise saturation can be explained by a combination of a readback effect and a noise spectrum that changes with density.

Pattern Transfer of 23-Nm-Diameter Block Copolymer Self-Assembled Nanodots Using CF&lt;sub&gt;4&lt;/sub&gt; Etching with Carbon Hard Mask (CHM) as Mask

The self-assembly of block copolymer (BCP) has demonstrated as promising alternative technology to overcome the limitation of conventional lithography owing to its ability in forming nanostructure with size 3-100 nm. In this study, we investigated a technique to transfer self-assembled nanodots of Poly(styrene-b-dimethyl siloxane) (PS-PDMS) BCP to Si. The pattern transfer of PS-PDMS nanodots with the pitch of 33 nm and the diameter of 23 nm using CF4 etching with Carbon Hard Mask (CHM) as Mask is demonstrated. Si nanopillar with height of 51 nm was fabricated. This result improves the potential use of PS-PDMS BCP self-assembly technique for fabrication nano-electronic devices, such as quantum dot solar cell and ultrahigh density of magnetic recording.

6PM2-C-3 熱アシスト磁気記録におけるレーザ照射による潤滑膜減耗特性(6PM2-C OS6 マイクロナノトライボロジー(3))

To achieve magnetic recording densities of more than 10 Tb/in^2, the head-disk interface (HDI) spacing should be less than 2-3 nm. Thus far, thermally assisted magnetic recording (TAMR) technology has been studied and developed to achieve such ultra-high magnetic recording densities. However, the use of ultra-thin liquid lubricant films as well as DLC thin films on the disk surface in TAMR leads to critical issues with regard to the HDI. In addition, HDD technology with head-disk assembly mechanism in which He gas is enclosed has been developed. In this study, the effect of lubricant bonding mechanisms on lubricant depletion has been studied. Furthermore, the mechanisms of lubricant depletion due to laser heating have bean investigated and discussed.

Tuning the properties of magnetic thin films by interaction with periodic nanostructures.

The most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and out-of-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures.

Microfocal lens for energy-assisted magnetic recording technology

In this Letter an original method is proposed to assist optical coupling in energy-assisted magnetic recording systems by introducing a microfocal lens. Simulation results show that the lens effectively reduces the incident light spot size to subwavelength scale, which benefits the magnetic recording density. The proposed microfocal lens can be manufactured at low cost because its fabrication is compatible with the conventional magnetic read/write head process.