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A micromagnetic simulation-based comparative study between perpendicular magnetic recording (PMR) and heat-assisted magnetic recording (HAMR) has been performed. Signal-to-noise ratio (SNR) is adopted as the metric of comparison, and various system parameters are considered, including fly-height (FH), write-track distance, and rotary motion of the head-stack-assembly (skew). It is found that even a HAMR system with a wide 60 nm full-width at half-maximum thermal profile demonstrates better SNR performance when compared to an optimized PMR system at a magnetic FH of 8 nm. The impact of skew on a single reader system is found to be minor for both PMR and HAMR, but the impact on a dual reader 2-D recording system differs substantially. In particular, the resolution for small bit detection depends greatly on reader shield orientation to the HAMR-written curved transition beneath the read free layer.

Differences between heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) for signal processing of the written track by one and two reader multisensor magnetic recording (MSMR) are explored using spinstand measurements. Tracks written with HAMR heads are shown to have more curvature compared with those written with modern PMR writers. We introduce two signal processing techniques in order to determine the merits and explore the recording physics tradeoffs for this complex system. To first order, we see twice as much bit error rate (BER) gain for the HAMR written track—on readback—using two reader MSMR, than the PMR. This difference we assign to the lower electronic signal-to-noise (SNR) ratio for the HAMR written track. In order to replicate practical conditions, we stress the MSMR system by applying increased adjacent track squeeze and displacing the two readers from the center of the track. Under these conditions we can resolve differences between the PMR and HAMR systems. Increased adjacent track squeeze appears to trim the broad and curved track edges, until the on-track signal starts to degrade. Increased offset of the readers from the center of the track offers improved the media (spatial) SNR, but we fail to harvest all those gains into BER across the range. We hypothesize that this is due to increasing distortion, a combination of track edge noise and reading of the encroaching tracks.

We introduce and investigate a new multi-island bit representation scheme with bit-patterned media (BPM) to increase the tolerance to the reader noise and the media noise compared with the standard BPM with a single island/bit and to granular media. The key idea is to use multiple islands in two dimensions to represent one bit. The redundancy in such a multi-island representation reduces the impact of the write in errors and decreases the effect of the media noise. We show that these multi-island representations offer improved bit error rate (BER), although only one readback sample per bit is used for equalization and detection. Here, a micromagnetic writing model and a read channel including media noise are used for the simulations, where optimized writer and reader architectures with one, two, and three read elements are investigated. Simulations at a 1 ${\rm Tb}/{\rm in}^{2}$ channel bit density, for the target BER of $10^{-2}$ and readback with two readers, indicate that the readback of BPM recording (BPMR) based on 1 ${\rm Tdot}/{\rm in}^{2}$ (1 dot/bit), 2 ${\rm Tdots}/{\rm in}^{2}$ (2 dots/bit), and 4 ${\rm Tdots}/{\rm in}^{2}$ (4 dots/bit) island density can tolerate 1, 3.1, and 3.3 dB more reader noise compared with the granular media. For readback with two readers, at $10^{-2}$ target BER, writing with 4 ${\rm Tdots}/{\rm in}^{2}$ and 2 ${\rm Tdots}/{\rm in}^{2}$ island density can tolerate the media noise (modeled by island position jitter and island size fluctuation) levels of 11% and 8.5% compared with the 1 ${\rm Tdot}/{\rm in}^{2}$ with the media noise level of 5%. When the channel bit density is increased to 1.5 ${\rm Tb}/{\rm in}^{2}$ , such a multi-island approach can still provide improved reader noise and media noise tolerances compared with the standard BPMR with one bit per island and to granular media at the same density.

This paper proposes a novel write precompensation scheme that adjusts the current magnitude, not the timing, to mitigate the serious 2-D nonlinear transition shift (NLTS) during the shingled writing process in a two-dimensional magnetic recording (TDMR) system. This scheme calculates the needed optimum write field for the current transition being written by subtracting the demagnetizing field produced by the previous written bits from a fixed write field and determines the optimum writing current based on the minimum mean-squared-error criterion between the needed field and the available fields within an established field library. Such a write precompensation scheme is tested with a micromagnetic writer and exchange coupled composite media, and readback with a rotated reader array combined with a read channel. Simulation indicates that the proposed write precompensation technique can decrease bit error rate compared with the case without write precompensation and approach the performance of a system with negligible NLTS. Correspondingly, the maximum user areal density is increased from 3.2 to 4.1 Tbits/in $^{\mathrm {\mathbf {2}}}$ for a TDMR system with 8 nm Voronoi grains due to the proposed write precompensation.

A shingled recording head design is proposed that makes use of a saturation magnetization gradient to focus flux into the corner of the writing pole. This design yields field gradients of 800 Oe/nm and can write more than 2.5 Tbits/in2 of user data onto a 10 nm thick ECC recording layer with 6 nm Voronoi grains. Maximum user densities are predicted to be over 4.66 Tbits/in2 with 8 nm grains and 12 Tbits/in2 with 5 nm grains using a perfect read and write scheme at a 1:1 bit aspect ratio. For read back, three contact dual differential MR head designs that have two independent free layers with magnetization pointing oppositely are examined for the purpose of extending magnetoresistive technology. The differential head shows good cross track resolution that will be advantageous at the low bit aspect ratios likely for two-dimensional magnetic recording. Results show that an array of three dual differential heads can successfully resolve an 8 nm track surrounded by adjacent tracks that have pseudo random bit sequences written to them, even though each head is wider than the track.

Two-dimensional magnetic recording (TDMR) differs from traditional track recording in that the bit-size-to-grain-size ratio is drastically reduced so that the channel bits are roughly of the same sizes as those of the magnetic media grains. This is envisioned to be achieved by shingled writing, a write-process in which the corner-writer partially overwrites the previously written adjacent track, effectively writing very closely spaced narrow tracks with no guard bands. Since the tracks are very narrow, they can be read by an array of read-elements whose spacing is equal to the narrow track pitch, creating a two-dimensional (2-D) array of readback signals (the two dimensions being the cross-track and the down-track dimensions). In the absence of an array of read elements, the same 2-D readback can be obtained by progressive scans of a single read element that reads one narrow track at a time and stores the readback signal in a 2-D array. In this paper, we present simple magnetic grain media models and shingled-write process models that capture the essence of TDMR. We assume that the granular recording medium consists of randomly shaped tiles (each tile represents a grain), randomly covering the medium plane. We then derive a suitable 2-D read/write process model. Using proper information-theoretic inequalities and bounding techniques, we derive methods to bound the capacities of TDMR channels. The results reveal that information capacities in excess of 0.6 user bits per grain are possible to attain over TDMR channels.

Two-dimensional magnetic recording (TDMR) is a novel storage architecture that, in theory, can achieve a density of up to 10 Tb/in2. It uniquely differs from other proposed next-generation architectures because of its reliance on sophisticated 2-D signal-processing algorithms. Recently, a number of contributions have been made in the development of read-channel models and detectors for TDMR systems. In this paper, we provide a detailed review on all important read-channel models under consideration. Our discussion focuses on the purpose of each model, placing a special emphasis on the suitability of the Voronoi model for the purpose of designing detectors. We also propose several detection schemes for TDMR based on the Voronoi model and present some numerical results.

We report a study of bit-patterned media recording channels containing written-in errors. From a signal processing point of view, we consider an abstract model for the overall recording channel, incorporating errors caused by timing and write ability. Based on the resulting channel model, we develop various trellis structures, and propose several detection algorithms that can be used to recover the recorded data. Furthermore, we investigate achievable information rates for patterned media recording systems with different levels of written-in error probability, providing an information theoretical performance assessment for such systems.

When conventional longitudinal and perpendicular magnetic recording architectures reach their well-known superparamagnetic limits, one candidate to break through the limit is heat-assisted magnetic recording (HAMR). In this paper, we investigate the effect of cross-track transition location and transition parameter (a-parameter) profiles in an HAMR system. Here, we apply microtrack modeling and the thermal Williams-Comstock model to longitudinal recording to determine the transition location and a-parameter profiles along the cross-track direction in the presence of a temperature profile. We then explore the effect of temperature on the isolated transition response of the system.

During the defect scanning process on the production line, when asperities are detected on the surface of a magnetic storage medium, that location is marked, and no data are stored to marked locations. However, some asperities go undetected, and cause problems during data recovery. In this paper, we introduce a new method to detect and cancel the thermal asperity effects for perpendicular magnetic recording systems. We compare the new method to the existing methods in terms of the bit error rate after Viterbi detection.

The response of a heat-assisted magnetic recording (HAMR) system is very sensitive to the laser spot position. The response is determined by transition characteristics like the center, curvature, and length. In this paper, by using the thermal Williams-Comstock model and the microtrack model, the effects of laser spot position on transition characteristics are investigated for both longitudinal and perpendicular recording from a read channel systems perspective. The general trend in their variation is determined and used to explain the resultant change in response. By simulation, we determine the post-Viterbi dominant error events for longitudinal HAMR system. Finally, we apply the well-known MTR codes to mitigate such errors and present their bit-error performance

Baseline wander (BLW) effects generated by the high-pass pole of the preamplifier in a perpendicular recording system are low-frequency distortions that cause degradation in system performance. Since perpendicular systems also have a considerable amount of signal content at low frequencies, BLW can cause considerable performance loss. We propose and evaluate a decision feedback-based algorithm which compensates for the undesired BLW effects and improves the overall performance.

Unlike longitudinal recording, perpendicular recording (PR) read-back signals have a significant amount of low-frequency content. This constitutes a problem in using the available channel chip architectures designed for longitudinal recording systems. In this paper, we explore the idea of transforming perpendicular read-back signals to longitudinal looking ones. The differentiator and the Hilbert transform are the natural choices for such applications. Here, we generalize this concept of transform filtering and introduce general transform filters. We then evaluate the bit-error-rate performances of those filters in conjunction with various PR targets, preamplifiers and noise characteristics. Although this method allows the use of existing longitudinal channels, it results in performance loss.