Magnetic Recording Systems Research Articles

Lubricant Depletion Characteristics Induced by Rapid Laser Heating in Thermally Assisted Magnetic Recording

In this study, fundamental research was conducted on lubricant depletion due to rapid laser heating in thermally assisted magnetic recording (TAMR) systems. The effects of lubricant film thickness on lubricant depletion were investigated using a conventional lubricant, Zdol2000. The lubricant depletion characteristics resulting from laser heating were found to largely depend on the lubricant film thickness. Lubricant depletion mechanisms were also studied. In this study, it was found that lubricant depletion is predominantly due to lubricant evaporation for cases in which the lubricant film thickness is greater than one monolayer. In addition, the effect of laser irradiation duration on lubricant depletion was investigated using both continuous and 300-kHz pulsed laser irradiations, since actual TAMR systems require pulsed laser irradiation lasting a few nanoseconds. The differences between the fundamental lubricant depletion characteristics of continuous and pulsed laser irradiations were elucidated on the basis of the experimental results obtained in this study. Furthermore, it has been suggested that diamond-like-carbon (DLC) thin films on the disk surfaces are also affected by rapid laser heating, because the temperature of the films increases to more than 500 K upon laser heating. Therefore, we also studied the damage to DLC thin films due to rapid laser heating and found that the reflectivity of the DLC thin films changed; this change was presumably due to the decrease in the DLC density as well as the asperity in the laser-irradiated region.

Two-Bit-Per-Dot Patterned Media for Magnetic Storage

Bit-patterned recording media are made of arrays of physically separated magnetic nanodots, each nanodot carrying one bit of information. We demonstrate that by stacking perpendicular-to-plane and in-plane magnetized thin films within each magnetic dot, it is possible to obtain a 2-bit-per-dot multilevel magnetic recording system without penalty to the readout signal-to-noise ratio. The stray field above the dots was measured and analyzed by electron holography for different magnetic configurations and compared with numerical simulations. The perpendicular-to-plane magnetized layer mainly influences the stray field above the center of the dots, whereas the in-plane magnetized layer determines the stray field above the gap between dots. The areal density of information can thus be doubled with this media by making an optimum use of the whole area of the medium.

Switching field distribution and transition width in energy assisted magnetic recording

In order to achieve higher areal density, magnetization transition width must be reduced. This requires small media switching field distribution (SFD). Here we explore SFD and transition width in heat assisted magnetic recording and microwave assisted magnetic recording. We reveal that for energy assisted magnetic recording, additional SFD broadening exists as compare to conventional perpendicular recording. We show the effect of SFD broadening on transition width and the implications to the magnetic recording system as the areal density increases.

Joint TA Suppression and Turbo Equalization for Coded Partial Response Channels

Thermal asperities (TAs) cause a crucial problem in magnetic recording systems because they can cause a burst of errors in data detection process. Without a TA detection and correction algorithm, the system performance can be unacceptable, depending on how severe the TA effect is. This paper proposes an iterative scheme to suppress the TA effect. This scheme jointly performs TA suppression and turbo equalization on the partial-response channels with error-correction codes. At each turbo iteration, an improved readback signal is obtained by subtracting the reconstructed TA signal from the TA-affected readback signal, where the reconstructed TA signal is computed based on a least-squares fitting technique with an aid of soft decisions from previous turbo iterations. Simulation results indicate that the proposed scheme outperforms the conventional receiver with separate TA suppression and turbo equalization, and is only marginally more complex than the conventional receiver.

Grating Coupler Design of an Integrated HAMR Head Light Delivery System Using Topology Optimization

This research is objected to design the grating structure for the purpose of efficient light delivery for the heat assisted magnetic recording (HAMR) system. The parameter study is firstly performed to determine the proper size of waveguide components and the topology optimization method is employed to design the grating structure. The formulation of the optimization problem is set to maximize the efficiency of the optical delivery. In the topology optimization result, the coupling efficiency is improved more than 20 times in comparison with the initial coupling efficiency and it is verified that the suggested optimization process can be applied to the grating structure design in the HAMR head effectively.

Trapping Electron Assisted Magnetic Recording

Moving towards 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal density, finding a proper recording scheme with enough write-ability is the most challenging task of a magnetic recording system. Some recording schemes with enhanced write-ability, such as HAMR, MAMR, graded media, etc., have been proposed to achieve higher recording density. Here we propose a new alternative approach for enhanced writing-trapping electron assisted magnetic recording (TEAMR). In the TEAMR configuration, an electrical bias is applied to the main pole of the write head with the disk media and the other parts of head slider grounded. As the main pole area is very small, the electrostatic force produced by electrical potential is a few orders smaller than the air bearing force at the rear pad. Therefore, it will not affect the flying performance of the head slider. At the nanometer head media spacing, a very strong electrical field is produced in the head media interface. This strong electrical field will cause free electrons to accumulate (be trapped) at the interfacial surfaces of metallic magnetic grains. These trapped electrons are localized in the surface atoms of magnetic grains and will alter the valance-electron band filling of those surface atoms. For many magnetic materials, the extra band-filling electrons reduce the magnetic anisotropy energy and make it easier to be magnetically switched. In this work, the TEAMR effect was proved by the experiment study on Co alloy based commercial disk media. The first principle calculation on L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ordered FePt crystal shows that the magnetic anisotropy can be reduced to zero with around 0.38 electrons trapped into 1 unit cell of FePt. Further increase in trapped electrons will change the magnetic easy axis from out-of-plane to in-plane, which is considered as a negative magnetic anisotropy. With the magnetic anisotropy reduction at the surface atoms of each grain, micromagnetic simulation result shows that the effective switching field can be reduced to around 11% of anisotropy field for a 1.6 ? 1.6 ? 3.2 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> FePt grain. Thus TEAMR can be another good candidate for energy assisted recording requiring very little modification to the current perpendicular magnetic recording system.

High Density Perpendicular Recording With Wrap-Around Shielded Writer

Wrap-around shielded writers combining both trailing shield and side shield have been studied both by modeling and by experiments. WAS design is demonstrated to have superior performance over its predecessor trailing shielded writer in perpendicular magnetic recording systems. While maintaining the down-track performance, the side shield significantly reduces the side fringing field, thus enabling high track densities. In this paper, various aspects of WAS design are investigated. Issues unique to side shielded design such as near and far track erasure are also discussed in detail. An outlook of future PMR writer and recording systems design is presented at the end.

An Analytical Approach for Performance Evaluation of Bit-Patterned Media Channels

In this work, a new analytical approach is used to evaluate the error performance of bit-patterned media (BPM) magnetic recording channels that employ one-dimensional (1D) and two-dimensional (2D) generalized partial response (GPR) equalizers to combat the significant inter-track interference (ITI) expected in BPM magnetic recording systems. The probability density function of ITI is obtained analytically and is used to estimate the bit error rate (BER) from the Viterbi detector. The proposed method takes into account most of the important factors affecting the BER such as ITI, un-equalized intersymbol interference (ISI), colored noise and the distance and the multiplicity of error events. In this work, it is shown that for 1D channels, modeling ITI and un-equalized ISI by Gaussian PDFs leads to inaccurate BERs and that the non-Gaussian distribution of the ITI and un-equalized ISI must be taken into account for more accurate BER estimates. This method provides fast and accurate estimates of BERs for moderate to high signal-to-noise ratios (SNRs). By using this analytical method, time-consuming numerical simulations for error performance evaluation can be avoided.

Bit patterned media based on block copolymer directed assembly with narrow magnetic switching field distribution

Electron-beam (E-beam) directed assembly, which combines the long-range phase and placement registration of e-beam lithography with the sharp dot size and spacing uniformity of block copolymer self assembly, is considered highly promising for fabricating templates that meet the tight magnetic specifications required for write synchronization in bit patterned media magnetic recording systems. In our study, we show that this approach also yields a narrower magnetic switching field distribution (SFD) than e-beam patterning or block copolymer self-assembly alone. We demonstrate that the pattern uniformity, i.e., island diameter and placement distributions are also important for achieving tight magnetic SFDs.

Timing Recovery Strategies in Magnetic Recording Systems

At some point in a digital communications receiver, the received analog signal must be sampled. Good performance requires that these samples be taken at the right times. The process of synchronizing the sampler with the received analog waveform is known as timing recovery. Conventional timing recovery techniques perform well only when operating at high signal-to-noise ratio (SNR). Nonetheless, iterative error-control codes allow reliable communication at very low SNR, where conventional techniques fail. This paper provides a detailed review on the timing recovery strategies based on per-survivor processing (PSP) that are capable of working at low SNR. We also investigate their performance in magnetic recording systems because magnetic recording is a primary method of storage for a variety of applications, including desktop, mobile, and server systems. Results indicate that the timing recovery strategies based on PSP perform better than the conventional ones and are thus worth being employed in magnetic recording systems.

FePtCu Films for Heat-Assisted Magnetic Recording Media Prepared from Cu/Pt/Fe Trilayers

L10-ordered FePtCu ternary alloy thin films with (001) preferential orientation were fabricated from Cu/Pt/Fe trilayers to maintain their Curie temperature at a level suitable for the operating temperature of recording media in a heat-assisted magnetic recording (HAMR) system. It was confirmed that the addition of Cu to FePt films reduces the Curie temperature of the FePtCu films to a level lower than that of FePt films. The Cu content ratio of FePtCu films was controlled by adjusting the Cu layer thickness of the Cu/Pt/Fe trilayers. With increasing Cu thickness, their Curie temperature dropped below 300 degrees C and their perpendicular coercivity decreased to a sufficiently low level of about 1kOe.

Magnetization process of heat assisted magnetic recording by micro-magnetic simulation

Magnetization reversal in a uniform magnetic field and one bit recording process by a thin film head in the heat assisted magnetic recording system of TbFeCo medium are studied by using the micro-magnetic simulation and the heat equation. The Landau-Lifsitz-Gilbert equation is solved for magnetic parameters at temperatures as the time goes by. It is found that magnetization proceeds as a progressive wave, although this behaviour may not limit the recording speed. The recording bit is expanded by the thermal fluctuation. The expansion can be suppressed as the medium thickness increases, because the thermal fluctuation is small in the thick medium. So the control of the medium thickness is important very much to achieve a high-density heat-assisted recording.

Study on Lubricant Depletion Induced by Laser Heating in Thermally Assisted Magnetic Recording Systems: Effect of Lubricant Thickness and Bonding Ratio

In this study, we have carried out fundamental research on lubricant depletion due to laser heating in thermally assisted magnetic recording. In particular, we investigated the effects of lubricant film thickness and lubricant bonding ratio on lubricant depletion. Conventional lubricants Zdol2000 and Ztetraol2000 were used. The lubricant depletion characteristics due to laser heating were found to depend largely on the lubricant film thickness and material. That is, for films thicker than one monolayer, the lubricant depletion depth increased with the laser-irradiation duration, whereas the thickness of the lubricant after laser irradiation on the diamond-like carbon (DLC) films tended to remain at a constant film thickness of one monolayer. The lubricant depletion width gradually increased as the laser irradiation duration increased. The increasing trends for the lubricant depletion width were quantitatively very similar and almost independent of the initial lubricant film thickness. However, for lubricant films with thicknesses less than one monolayer, the lubricant depletion depth was very small. The lubricant depletion width increased remarkably to several hundred micrometers as the laser irradiation duration increased. The lubricant depletion depth and width were much smaller for Ztetraol2000 than Zdol2000. In addition, the lubricant-bonding ratio was found to greatly affect the lubricant depletion characteristics due to laser heating. In other words, the lubricant depletion depth and width decreased as the bonding ratio increased. The lubricant depletion mechanism involves the evaporation of mobile lubricant molecules when the maximum attained temperature is less than 100 °C. Another suggested lubricant depletion mechanism involves the thermocapillary stress effect, which is induced by the disk surface temperature gradient resulting from the non-uniformity of the laser spot intensity distribution.

Light Delivery System for Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) technology needs a light delivery system with an incorporated near-field transducer (NFT) as a minute heat generator. A novel system for light delivery with a triangular spot size converter (SSC) and an optical fiber is proposed. The advantages of using a triangular SSC are optical efficiency and integration tolerance between the optical fiber and the NFT. It also has good affinity to conventional head fabrication process. The optical fiber enables flexible laser location. It was confirmed that fabricated triangular SSC can propagate light and reduce beam spot. Computer simulations and experiments demonstrated possibility of integration of light delivery system between NFT, SSC, and optical fiber. Flyability of a pico slider with an optical fiber was also demonstrated. It shows that the optical fiber does not have a big impact on flyability below 10 nm flying height and off-track vibration.

Numerical Investigation of Thermal Problems in Heat-Assisted Magnetic Recording System

This paper describes the numerical investigation of two thermal related problems in heat assisted magnetic recording: 1) the dependence of the recorded mark width on the laser power and 2) the temperature distribution of the lubricant layer and its variation with time and the change with and without the heat sink layer. It is found that the mark width increases with the increase of laser power, but the increase is not linear due to the Gaussian distribution of the laser intensity. It is also found that in certain power range, a smaller laser spot will generate a wider mark width at the same laser power. For lubricant cooling process, the temperature in lubricant layer decreases quickly after the laser is removed. Within 0.5 ns, the maximum temperature in lubricant layer drops 35.6degC when there is no heat sink layer applied under the recording media layer. With heat sink layer applied under the media layer, the temperature drop is even faster and decreases by 57.0degC in 0.5 ns. Hence, the heat sink layer is useful to quickly cool the lubricant layer to a thermally stable range.

Perspectives of Magnetic Recording System at 10 Tb/in$^{2}$

The magnetic recording technology started to face superparamagnetism at the areal density of a few 10 s Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . To postpone this physical limitation, the recording configuration has to be changed by either increasing the write capability or reducing the grain number per bit. It had been switched from conventional longitudinal recording to perpendicular recording and will continue changing to other new recording configurations in the future. Beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal density, the recording system is possible to be either the energy-assisted magnetic recording (EAMR) or the bit patterned media (BPM). At 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal density, one of them alone may not be enough and most people believe that EAMR and BPM have to be combined together to make it happen. In this paper, we explore the possible recording configurations at 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and calculate the corresponding design parameters of the recording system.

A Study of LDPC Coding and Iterative Decoding System in Magnetic Recording System Using Bit-Patterned Medium With Write Error

In this paper, low-density parity-check (LDPC) coding and iterative decoding system is studied in the magnetic recording system using a bit-patterned medium (BPM). The magnetic recording system using BPM has a serious problem of requiring precise write synchronization to correctly write the recording sequence on each intended island. In this paper, an error locator using the parity-check matrix of LDPC code is utilized to curb the influence of write error on the iterative decoding process. The performance of the proposed system with the error locator is evaluated by computer simulation for magnetic recording system using BPM with synchronization write error at a recording density of 2 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and it is compared with a conventional Reed-Solomon (RS) encoding and decoding system. The results show that the proposed system provides better performance.

Read Channel Modeling for Detection in Two-Dimensional Magnetic Recording Systems

In this paper, we describe a read channel model for detector design for two-dimensional magnetic recording (TDMR) system, a novel strategy for recording at upto 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We describe a scheme for (1) modeling of the recording medium, (2) modeling of the write/readback process, and (3) an experimental method for the characterization of noise in the TDMR channel, occurring due to irregularities in the bit-boundaries in the recording medium, that can be used for detection purposes.

TDMR Platform Simulations and Experiments

Two-dimensional magnetic recording (TDMR) is a novel architecture for magnetic recording systems proposed to achieve densities towards 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . TDMR differs from other solutions in that it does not require a complete redesign of the head or medium for its gains, relying instead on powerful 2-D codes and signal processing to reliably store and retrieve the information. To explore the viability of TDMR via simulation, models for the 2-D medium, writing, and readback are needed. In this paper, we propose models for these processes and discuss ongoing progress in 2-D experimental waveform retrieval that will be used to validate the models. We show some initial results achieved using these models which demonstrate the viability of TDMR and that will provide an indication of what densities may be possible with TDMR, when the 2-D codes and detectors are completed.

Highly Parallel FPGA Emulation for LDPC Error Floor Characterization in Perpendicular Magnetic Recording Channel

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Low-density parity-check (LDPC) codes offer a promising error correction approach for high-density magnetic recording systems due to their near-Shannon limit error-correcting performance. However, evaluation of LDPC codes at the extremely low bit error rates (BER) required by hard disk drive systems, typically around <formula formulatype="inline"><tex Notation="TeX">$10^{-12}$</tex> </formula> to <formula formulatype="inline"><tex Notation="TeX">$10^{- 15}$</tex> </formula>, cannot be carried out on high-performance workstations using conventional Monte Carlo techniques in a tractable amount of time. Even field-programmable gate array (FPGA) emulation platforms take a few weeks to reach BER between <formula formulatype="inline"><tex Notation="TeX">$10^{-11}$</tex></formula> and <formula formulatype="inline"><tex Notation="TeX">$10^{-12}$</tex></formula>. Thus, we implemented a highly parallel FPGA processing cluster to emulate a perpendicular magnetic recording channel, which enabled us to accelerate the emulation by <formula formulatype="inline"><tex Notation="TeX">$&gt; 100 \times$</tex></formula> over the fastest reported emulation. This increased throughput enabled us to characterize the performance of LDPC code BER down to near <formula formulatype="inline"> <tex Notation="TeX">$10^{- 14}$</tex></formula> and investigate its error floor. </para>