Marine acoustic field prediction (MAFP) is essential for a variety of application scenarios. Given the rapid temporal changes in the ocean environment, MAFP requires time-dependent acoustic parameters. Ocean models are generally employed to provide dynamic acoustic parameters for the water column; however, few models extend this capability to sediments. Nonetheless, both observations and simulations have revealed that sediment temperature variations in shallow seas significantly affect the sediment acoustic properties. This paper proposes conducting MAFP using an ocean-sediment coupled model, which integrates the water column and sediment through the ocean bottom heat flux process, enabling simultaneous modeling of both the sediment and water temperatures. The MAFP results from an ocean-only model are compared with those of the ocean-sediment coupled model. The findings indicate that the coupled model enhances the MAFP in two significant ways. First, it provides a time-varying sediment temperature field, allowing the use of temperature-sensitive sediment acoustic parameters that evolve over time. Second, it yields a more accurate water temperature profile. These enhancements could significantly reduce MAFP errors, underscoring the critical role of the coupled model, particularly in shallow-water environments.
Ensemble waveform analysis is used to calculate signal to noise ratio (SNR) and other recording characteristics from micromagnetically modeled heat assisted magnetic recording waveforms and waveforms measured at both drive and spin-stand level. Using windowing functions provides the breakdown between transition and remanence SNRs. In addition, channel bit density (CBD) can be extracted from the ensemble waveforms using the di-bit extraction method. Trends in both transition SNR, remanence SNR, and CBD as a function of ambient temperature at constant track width showed good agreement between model and measurement. Both model and drive-level measurement show degradation in SNR at higher ambient temperatures, which may be due to changes in the down-track profile at the track edges compared with track center. CBD as a function of cross-track position is also calculated for both modeling and spin-stand measurements. The CBD widening at high cross-track offset, which is observed at both measurement and model, was directly related to the radius of curvature of the written transitions observed in the model and the thermal profiles used.
Microwave Assisted Magnetic Recording (MAMR) is a type of energy assisted recording technology that uses microwaves to assist in the recording process. This allows for the use of high anisotropy media to maintain good thermal stability at high areal densities [1]. In MAMR, a spin torque oscillator (STO) is placed between the main pole and trailing shield to generate a magnetic field at microwave frequencies. The magnetic recording trilemma [2] is overcome by reducing the coercive field of the media via this microwave field, thereby aiding in the switching of media grains. Appropriate media stack configurations for MAMR and STO optimization have been investigated by both analytical theory and micromagnetic simulation [3], [4], [5]. In this paper, MAMR performance is evaluated on a reference design using our MAMR micro-magnetic model and the results are reported using EWSNR metrics [6]. In this study, we compare performance metrics for MAMR with other recording technologies such as Perpendicular Magnetic Recording (PMR) and Heat Assisted Magnetic Recording (HAMR). Recording media dynamics are modeled using the Landau-Lifshitz-Gilbert (LLG) equation for PMR and MAMR, and utilizing the renormalized LLG method for HAMR [7]. The DC magnetic write field is a current generation PMR writer design with 55 nm physical pole width and 1.0 T peak field strength. The STO stack has an optimized geometry that applies a 3D circularly polarized AC field to the media. STO field strength (∼0.1Hk) and oscillating frequency (∼35GHz) are determined by the magnetic properties of the field generation layer (FGL) and the magnitude of the injected spin current. The magnetic head to media spacing (HMS) is fixed at 6.0 nm and head velocity is modeled to be 20 m/s. For MAMR, single layer media with varying anisotropy are considered. The PMR ECC multilayer media model, which is calibrated based on current PMR products, contains multiple magnetic layers and non-magnetic break layers that provide optimal exchange coupling. The HAMR media is a single layer L1 0 FePt media with a Curie temperature distribution of 3% and anisotropy field distribution of 10%. The down track thermal gradient used in the HAMR model is around 8K/nm, which is consistent with common near field transducer designs. The average media grain size is around 8.0 nm with 17% grain size distribution. A magnetoresistive reader with 30 nm width is used for obtaining the play back signal. Magnetic information is encoded using pseudo random bit sequences (PRBS), which mimic real user data. Figure 1 shows a comparison between PMR, HAMR and MAMR with thick free layer designs in terms of common performance metrics. Ensemble waveform analysis is used to calculate the total spatial SNR, the breakdown between transition and remanence SNR contributions, and the channel bit density (CBD) [6]. Bit error rate (BER) is calculated using a pattern dependent Viterbi detector [8]. The use of an NFT in HAMR allows the recording of much narrower tracks than PMR, with similar CBD and reasonable SNR and BER. In general, MAMR exhibits a large CBD, which may be related to intersymbol interference and track edge erasure caused by the demagnetization field. This high CBD results in much worse BER than both HAMR and PMR at all track width and linear density combinations considered. For the same ADC (shown in the last two table items in figure 1), SNR and BER are much better when the linear density is higher and the track width is larger. Since track edge demagnetization effects seem to have such a large effect in MAMR, this data suggests MAMR should aim to increase ADC via higher linear density and lower track density. For thick free layers, multi domains and multi-scattering contributions to electron's propagation can greatly deteriorate STO performance. Therefore, SNR versus recording media Hk for MAMR with thin free layer designs below 15 nm and STO width of 40 nm is shown in Figure 2. The figure suggests an optimal for MAMR media of around 25 kOe. MAMR SNR degradation at 20 kOe is due to erasure effects for low Hk material [9]. For high anisotropy media ∼30 kOe, the transition SNR is reasonable (around 12 dB), but remanence noise is high, causing a 2 dB loss in total SNR.
Recording simulations on heat-assisted magnetic recording (HAMR) are implemented to study the dependences of grain size and Gilbert damping on the recording performance. Modified damping and temperature shift for renormalized media cells are needed to address the non-equilibrium nature of HAMR process. It is found that using smaller grains helps reduce transition jitter but increases track width. Larger damping is also found effective in reducing the jitter. Finally, the applicability of this simulation method is corroborated by comparing with experimental results.
A new scheme for the simulation of heat-assisted magnetic recording (HAMR) that systematically includes fluctuating material properties above a predefined length scale, while retaining magnetostatic interactions, is introduced. Renormalized media parameters Ms , Ku , Aex , and α damp , suitable for useful length scales, are found numerically. These renormalized parameters are then used to model the Voronoi-cell-composed medium in the HAMR simulation. Transition jitters are obtained under various conditions. The results show that moderate maximum temperature of the heat spot, intergranular exchange coupling, media thickness of at least 10 nm, nonzero canting angle of the head field, relatively low head velocity, and large head-field strength are helpful for a successful recording.
Magnetotransport properties of granular oxide-segregated CoPtCr films were studied on both macroscopic and microscopic length scales by performing bulk and point-contact magnetoresistance measurements, respectively. Such a perpendicular magnetic medium is used in state-of-the-art hard disc drives and if combined with magnetoresistive phenomena (for read/write operations) may lead to a novel concept for magnetic recording with high areal density. While the bulk measurements on the films showed only small variations in dc resistance as a function of applied magnetic field (magnetoresistance of less than 0.02 %), the point-contact measurements revealed giant-magnetoresistance-like changes in resistance with up to 50,000 % ratios. The observed magnetorestive effect could be attributed to a tunnel magnetoresistance between CoPtCr grains with different coercivity. The tunneling picture of electronic transport in our granular medium was confirmed by the observation of tunneling-like current-voltage characteristics and bias dependence of magnetoresistance; both the point-contact resistance and magnetoresistance were found to decrease with the applied dc bias.
How far can we miniaturize magnetic recording media? Reducing grain size and layer thickness nearly to the atomic level requires finer handling than in typical micromagnetic calculations. The authors develop atomistic simulation of spin dynamics to study magnetization reversal in complex granular media, to optimize performance and push areal density above 1 Tbit/in${}^{2}$. They demonstrate the importance of magnetostatic interactions and inter- and intralayer exchange couplings, and visualize switching at the atomic level to reveal the reversal mechanism. This study transfers knowledge from a university setting directly to industry and real-world problems.
Magnetic hard disk drives (HDDs) store over 90% of the world's digital data enabling the internet and economical access to data to power everything from social media to self-driving cars. Heat assisted magnetic recording (HAMR) is being developed as the next recording system for HDDs. HAMR will bring profound changes to the HDD components and architecture, incorporating laser diodes in an innovative plasmonic light delivery system into the recording heads, and novel nano-magnetic materials and layer architectures in the recording media [1]. Seagate demonstrated the promise of HAMR with a 1 -2 Tb/in 2 areal density demonstration [2, 3] and the subsequent demonstration of a fully functional drive with more than 1000 write power-on hours [4, 5]. The significant progresses have been enabled by breakthroughs in media and head technology together with drive integration. With continuing development efforts in reliability and data density HAMR will serve the demand for economical hard disk drive storage solutions for the world's ever growing data. As announced in a recent blog, Seagate is now shipping HAMR units to select customers for integration tests, and will start shipping commercial HAMR products to key customers by the end of 2018 [6]. This paper will cover the key enablers for HAMR technology to support both high areal density and linear density, which will be critical for bringing a new "S" curve for the magnetic recording industry. It will also be discussed about the challenges and breakthroughs in fundamental magnetic properties for the recording layer and near field optical transducers, to support the extension of HAMR to around 4-5 Tbit/in 2 .
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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