Head and Media Design for Curvature Reduction in Heat-Assisted Magnetic Recording
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Recording curvature in magnetic data-storage technology has long been one of the significant challenges impacting on recording performance. Despite curvature occurrence in the conventional recording techniques such as perpendicular magnetic recording (PMR), heat-assisted magnetic recording (HAMR) is demonstrated to induce much more severe curvature than PMR. HAMR curvature could cause poor bit error rate and limits the maximum areal density capacity. Here we have theoretically predicted and demonstrated various approaches for curvature reduction from the aspect of either altering the near-field transducer head design or recording medium design. Optical and thermal modeling have indicated that by utilizing a crown-shape peg to change the thermal source profile and compensate for thermal expansion and rounding effect, it could potentially improve curvature figure of merit (FOM) and achieve curvature reduction by ~45%. In terms of the recording media design, by altering the heat sink and internal layer media material or geometry, it could also achieve curvature cancellation of ~40% with increased thermal gradient. The combined approach from both HAMR head and media perspectives with balanced recording FOMs, could potentially realize significant curvature reduction to be of similar or better recording curvature level to PMR.Keywords:
Heat-assisted magnetic recording
Patterned media
Perpendicular recording
Area density
Perpendicular recording
Recording media
Area density
Patterned media
Thermal Stability
Heat-assisted magnetic recording
Micromagnetics
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The areal density of hard disk drives increases every year. Increasing the areal density has limitations. Therefore, heat-assisted magnetic recording (HAMR) technology has been the candidate for increasing the areal density. At ultrahigh areal density, the main problem of the magnetic recording process is noise. Transition jitter is noise that affects the read-back signal. Hence, the performance of the magnetic recording process depends on the transition jitter. In this paper, the transition jitter of L10-FePt-based HAMR technology was simulated at the ultrahigh areal density. The micromagnetic simulation was used in the magnetic recording process. The average grain size was 5.1 nm, and the standard deviation was 0.08 nm. The recording simulation format was five tracks in a medium. It was found that a bit length of 9 nm with a track width of 16.5 nm at the areal density of 4.1 Tb/in2 had the lowest transition jitter average of 1.547 nm. In addition, the transition jitter average decreased when increasing the areal density from 4.1 to 8.9 Tb/in2. It was found that the lowest transition jitter average was 1.270 nm at an 8 nm track width and a 9 nm bit length, which achieved an ultrahigh areal density of 8.9 Tb/in2.
Area density
Heat-assisted magnetic recording
Patterned media
Micromagnetics
Magnetic storage
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The future evolution of magnetic recording data storage toward its ultimate limit is expected to involve a combination of energy-assisted recording on bit-patterned media, according to recent publications. In this work, we assess the effectiveness of single magnetic grain reversal under heat-assisted recording conditions by analyzing macrospin magnetization dynamics with the Landau-Lifshitz-Bloch equation. The simulations reported pertain to FePtX recording media and recording system parameters constrained by expected practical limitations. The approach adopted is assessment of the patterned media writing error rate as a function of applied bias field and areal density (AD), taking account of the relevant physics of the heat-assisted recording process. Additionally, we require that long-term thermal stability of recorded information be maintained, and that sufficient thermal and effective writing field gradients to support AD targets are available. For the long-time analysis, an Arrhenius-Nèel model of single grain switching probability is helpful. In this context, an investigation of achievable areal density with respect to tradeoffs in writing error rate at practical applied fields and thermal conditions is provided.
Heat-assisted magnetic recording
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Magnetization dynamics
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Areal density increases in conventional perpendicular magnetic recording are becoming increasingly difficult to achieve. Heat assisted magnetic recording is viewed as a potential technique to extend magnetic recording into the multiple-terabit range. Over the last 5 years, we have performed extensive simulations of Heat Assisted Magnetic Recording on both granular and bit-patterned media. For this purpose, we represent the behavior of granular media near the Curie temperature with renormalized blocks of spins of order 1 nm 3 . The change in magnetization of these blocks can then be evaluated using the Landau-Lifshitz-Gilbert equation. The behavior of bit patterned media is typically evaluated using an atomistic approach. Optical spots are calculated using a finite difference time domain technique and heat flow is evaluated using the usual Fourier differential equation. We have also evaluated the performance of competing technologies including both conventional and shingled recording of both granular and bit patterned media.
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Terabit
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Continuous increment of magnetic recording density requires the realization of necessary signal-to-noise-ratio (SNR) with less number of grains per bit, a fundamental limit in this approach is that the thermal effect for continued scaling would cause spontaneous magnetization reversal of individual grains, which would result in loss of recorded data. Patterned media can significantly reduce the grain number per bit at the required SNR by its good pattern shape uniformity. This paper focused on the recording performance evaluation of bit patterned media. Using Karlqvist head and single pole head, the SNR of oriented continuous media (CM), patterned longitudinal media (PLM) and perpendicular patterned media (PPM) were investigated via finite element method. It is found that the SNR of PLM is approximately 20% higher than that of CM, with the same anisotropy orientation distribution. At each areal density, the SNR of PPM remains the highest before the areal density reaches 800Gbit/in2. By comparing the simulation results of SNR of patterned longitudinal media at different bit aspect ratio, it is indicated that with the dot size dispersion increase, the perfect media with the highest SNR was also the most susceptible to size variation. These dispersions significantly increase down-track noise and the effect was greatest at high linear densities where the signal was low. The effect of grain diameter on magnetization variance of both PLM and PPM were also investigated.
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Area density
Perpendicular recording
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Micromagnetics
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The recording physics of bit patterned media is studied for areal densities of around 2 Tbits/inch2, focused on number of grains per bit. Write error rate and signal to noise ratio in a function of number of grains are derived as analytical functions based on the statistics theory on the binomial distribution. Modeling of writing process based on the head field gradient and switching field distribution and interference fields, is presented to extract the write-head and media parameters which are necessary to attain a high areal density recording.
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Perpendicular recording
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Recording media
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The limits of areal storage density that is achievable with heat-assisted magnetic recording are unknown. We addressed this central question and investigated the areal density of bit-patterned media. We analyzed the detailed switching behavior of a recording bit under various external conditions, allowing us to compute the bit error rate of a write process (shingled and conventional) for various grain spacings, write head positions, and write temperatures. Hence, we were able to optimize the areal density yielding values beyond 10 Tb/in2. Our model is based on the Landau-Lifshitz-Bloch equation and uses hard magnetic recording grains with a 5-nm diameter and 10-nm height. It assumes a realistic distribution of the Curie temperature of the underlying material, grain size, as well as grain and head position.
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Heat-assisted magnetic recording
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Heat-assisted-magnetic recording (HAMR) is hoped to be the future recording technique for high density storage devices. Nevertheless, there exist several realizations strategies. With a coarse-grained Landau-Lifshitz-Bloch (LLB) model we investigate in detail benefits and disadvantages of continuous and pulsed laser spot recording of shingled and conventional bit-patterned media. Additionally we compare single phase grains and bits having a bilayer structure with graded Curie temperature, consisting of a hard magnetic layer with high $T_{\mathrm{C}}$ and a soft magnetic one with low $T_{\mathrm{C}}$, respectively. To describe the whole write process as realistic as possible a distribution of the grain sizes and Curie temperatures, a displacement jitter of the head and the bit positions are considered. For all these cases we calculate bit error rates of various grain patterns, temperatures and write head positions to optimize the achievable areal storage density. Within our analysis shingled HAMR with a continuous laser pulse moving over the medium reaches the best results, and thus having the highest potential to become the next generation storage device.
Heat-assisted magnetic recording
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Patterned media
Magnetic storage
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Patterned media
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Recording media
Margin (machine learning)
Heat-assisted magnetic recording
Micromagnetics
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Recording head
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