Heat-assisted Magnetic Recording Research Articles

The Scaling of Signal-to-Noise in Heat-Assisted Magnetic Recording

The Scaling of Signal-to-Noise in Heat-Assisted Magnetic Recording

Write Head Design for Control of Transition Curvature in Heat-Assisted Magnetic Recording

Write Head Design for Control of Transition Curvature in Heat-Assisted Magnetic Recording

Dual-layer FePt-C granular media for multi-level heat-assisted magnetic recording

Multi-level magnetic recording is a new concept for increasing the data storage capacity of hard disk drives. However, its implementation has been limited by a lack of suitable media capable of storing information at multiple levels. Herein, we overcome this problem by developing dual FePt-C nanogranular films separated by a Ru-C breaking layer with a cubic crystal structure. The FePt grains in the bottom and top layers of the developed media exhibited different effective magnetocrystalline anisotropies and Curie temperatures. The former is realized by different degrees of ordering in the L10-FePt grains, whereas the latter was attributed to the diffusion of Ru, thereby enabling separate magnetic recording on each layer under different magnetic fields and temperatures. Furthermore, magnetic measurements and heat-assisted magnetic recording simulations showed that these media enabled 3-level recording and could potentially be extended to 4-level recording, as the ↑↓ and ↓↑ states exhibited non-zero magnetization.

Study of grain-patterned and highly ordered L10-FePt HAMR media using reactive molecular dynamics method

Embedded Mask Patterning (EMP) has been proposed as a cost-effective fabrication method to be capable of patterning sub-5-nm grain sizes for highly ordered L10-FePt media for Heat-Assisted Magnetic Recording (HAMR). Understanding the etching mechanism of FePt is critical to maintaining the highly ordered L10 structure and low damage to magnetic grains. In this research, a reactive Molecular Dynamics (MD) model is developed to study methanol (MeOH) plasma etching on highly ordered continuous L10-FePt media film. The model describes the reactive interaction mechanism between the plasma products CO/H2 molecules and Fe/Pt atoms. It shows the dominant Fe-C interaction upon the dissociation of CO ligands leads to formation of large and volatile Fen-C clusters contributing to high chemical etch yield.

Writing temperature and thickness dependence of perpendicular magnetic-dot magnetization reversal probability

Investigation is necessary to figure out magnetization dynamics at high temperatures due to the low power requirement for heat-assisted magnetic recording technology. The Landau-Lifshitz-Gilbert equation was employed to assess the dynamic of magnetization in perpendicular magnetic-dots at elevated temperatures, considering the thermal contribution to the effective field. The simulated perpendicular magnetic-dot has a 50×50 nm2 surface area with thicknesses of 10 nm, 50 nm, and 100 nm. This study reveals that the probability of magnetization reversal and the threshold field in the thermally-assisted magnetic reversal process of a magnetic dot is influenced by the thickness and writing temperature. Below the Curie point, as sample thickness increases, demagnetization energy increases, requiring a larger external magnetic field for magnetization reversal. For the thicker sample, the magnitude of the writing field decreases as the temperature increases, which corresponds to temperature-dependent magnetization. Meanwhile, the optimum writing field is obtained below the Curie point. The magnetic-dot with 100 nm of thickness has an effective writing temperature of 739.7 K with a corresponding writing field is 250 Oe while its reversal probability at zero magnetic field is up to 0.2. Magnetic-dot thickness affects magnetization reversal probability due to domain wall propagation mechanisms during magnetic switching that result from the demagnetization factor. Moreover, as the temperature approaches the Curie temperature, the probability of zero-field magnetization reversal in the absence of an external magnetic field is observed to escalate which has a direct impact on the writing field magnitude. It was found that the effective writing temperature is below the Curie temperature, which allows several hundred Oersted of the writing field.

Effect of Curie Temperature Distributions on the User Areal Density of Heat-Assisted Magnetic Recording

Effect of Curie Temperature Distributions on the User Areal Density of Heat-Assisted Magnetic Recording

Granular micromagnetic HAMR model: investigation of damping dependence and parametric optimization for high performance

Heat assisted magnetic recording (HAMR) is a novel high-density magnetic recording technology that relies on thermal assist from a laser during the writing process. To achieve high writing performance, it is important to study and optimize the crucial factors affecting the magnetization reversal mechanism at elevated temperature. In this work, we use a multiscale approach that combines atomistic and micromagnetic models to study the magnetization reversal behavior in the recording medium. The atomistic model allows to parameterize accurately the macroscopic approach, which is utilized to model the system and its dynamics. We perform a parametric investigation of the switching properties as a function of the HAMR setup characteristics as well as the material properties, such as magnetic damping. The results show that high damping and moderate external fields can achieve high-performance HAMR media characterized by high switching probability, short switching time and low peak temperature. We demonstrate that switching occurs via the linear reversal mechanism. By systematic variation of the longitudinal susceptibility we force a transition to coherent reversal and demonstrate that this reduces the switching probability, showing linear reversal to be an important component within the HAMR process.

Effect of Humidity on Head Smear Generation in Head-Assisted Magnetic Recording

The causes of head smears deposited on the magnetic head surface due to laser heating in heat-assisted magnetic recording should be investigated to achieve high recording densities of hard disk drives. These factors, such as outgassing from components and humidity, also affect smear formation in hard disk drives. In this study, we focused on the effect of relative humidity on smears generated via laser heating. Three types of lubricant film (dipped, ultraviolet-treated, and rinsed) were prepared, and the amount of smear generated on a glass pin surface was evaluated using a pin-on-disk tester with a laser heating system. It was confirmed that the amount of smear increased as the humidity increased, and lubricant films with higher chemical bonding fractions showed less smear generation and lower dependence on humidity. In addition, time-of-flight secondary ion mass spectroscopy confirmed that the migration of cobalt from the magnetic layer was accelerated by laser heating and humidity.

Control of grain density by varying lattice mismatch in FePt-C film for heat assisted magnetic recording

Controlling the number density of grains in magnetic recording media is critical for improving the areal density of the recording. Herein, we report the dependence of grain density and microstructure of FePt-based granular films for heat-assisted magnetic recording media on the lattice mismatch between the FePt films and substrates SrTiO3, MgAl2O4, MgO and TiO2. The grain density and grain size of the FePt films increased and decreased, respectively, with the increasing lattice mismatch between the films and substrates. The optimum lattice mismatch for the FePt-based granular films was found to be 8%, which corresponds to the MgO substrate. FePt films grown on TiO2 substrate, which had the largest lattice mismatch of 16%, showed poor morphology and low grain density. The results imply that an underlayer with which a film develops a suitable mismatch plays a key role in controlling the microstructure and magnetic properties of the film.

Thermal Effect of the Back Radiation from Disk to Head after Laser Heating in HAMR

In a heat-assisted magnetic recording, the thermal effect of the head/disk interface has an important influence on the stability of the recording data. In this paper, we will discuss the thermal radiation from the disk, more specifically, the magnetic recording layer, which is at high temperature after laser heating, to the magnetic head, which is at room temperature. The radiative heat flux can be represented by the Poynting vector. In the near-field band, an effective way to obtain the electromagnetic fields is to solve the Maxwell’s equations combined with fluctuational electrodynamics. The near-field back radiation between specific head and disk material is calculated by the fluctuation-volume-current method. The radiative heat energy will induce the thermal deformation of the magnetic head, which will be discussed by the simulation, laying the foundation for adjusting and controlling the flying status.

Monolayer Capping Provides Close to Optimal Resistance to Laser Dewetting of Au Films.

Next-generation heat-assisted magnetic recording (HAMR) relies on fast, localized heating of the magnetic medium during the write process. Au plasmonic near-field transducers are an attractive solution to this challenge, but increased thermal stability of Au films is required to improve long-term reliability. This work compares the effect of nanoscale Al, AlOx, and Ta capping films on Au thin films with Ti or Ta adhesion layers for use in HAMR and other high-temperature plasmonic applications. Thermal stability is investigated using a bespoke laser dewetting system, and SEM and AFM are extensively used to interrogate the resulting dewet areas. The most effective capping layers are found to be 0.5-1 nm of Al or AlOx, which can eliminate dewetting under certain conditions. Even one monolayer of AlOx is shown to be highly effective in reducing dewetting. In the case of thicker capping layers of Ta and AlOx, the Au film can easily dewet underneath, leaving an intact capping layer. It is concluded that thinner capping layers are most effective against dewetting as the Au cannot dewet without breaking them and pulling them apart during the dewetting process. A simple model based on energetics considerations is developed, which explains how thinner capping layers can more effectively protect the metal from pore or fissure creation. The model provides some convenient guidelines for choosing both the substrate and capping layer, for a given metal, to maximize the resistance to laser-induced damage.

Effectiveness of a pulsed laser in heat-assisted magnetic recording

Recently, much effort has been directed towards increasing the areal density of heat-assisted magnetic recording (HAMR). Here, we use our HAMR recording simulation that employs renormalized media parameters to examine the potential use of a pulsed laser instead of a continuous laser. Proper tuning of the synchronization between magnetic and laser pulses yields improved thermal gradients and comparable (or improved) recording performance relative to a continuous laser. Importantly, it also produces less average heat in the media, which is expected to improve near field transducer lifetime. Results also show that the optimized pulsed laser reduces adjacent track erasure relative to a continuous laser, which is important for non-shingled recording.

Effect of water on mechano-chemical reactions of perfluoropolyether lubricant films in heat-assisted magnetic recording: A reactive molecular dynamics study

A comprehensive understanding of the decomposition of perfluoropolyether (PFPE) films is crucial for establishing effective lubrication of heat-assisted magnetic recording (HAMR) systems. In this study, to examine the combined influence of heat, mechanical stress, and water, we conducted nonequilibrium reactive molecular dynamics simulations using our ReaxFF force field for the PFPE D-4OH films mixed with water. We thoroughly analyzed the reaction rates, types, pathways, and products, and identified vulnerable bonds. The results demonstrate that water significantly accelerates the reactions of D-4OH, particularly at lower normal pressures. We found that water molecules form clusters and are surrounded by the D-4OH end groups. Consequently, water induces hydrolysis of D-4OH or catalyzes reactions between D-4OH by dissociating C–O bonds in the end groups.

Design Considerations Including Rotating Head Technique for Shingled Heat-Assisted Magnetic Recording

Predictions for Signal-to-Noise Ratio (SNR) and User Density (UD) are presented for a large variety of shingled Heat Assisted Magnetic Recording (HAMR) systems. Calculations are performed using a micromagnetic simulation that treats the heated media cells using renormalized spin blocks. Varied parameters include grain pitch, heat spot Full Width Half Maximum, reader width, and shield to shield spacing. Results are optimized with respect to bit length and grain pitch. Unlike previous work, the base condition feature systems already available experimentally (to allow for comparison), extending to media and heads that allow greatly extended capacities. We also discuss the benefits of a rotated read head for these geometries.

Microstructure evolution in FePt-Cr2O3 granular thin films

Increase in the FePt grain density is necessary to achieve an ultra-high recording density of the heat-assisted magnetic recording. Herein, the highest grain density in FePt-Cr2O3 granular thin films was obtained by utilizing a densified FePt nucleation layer. The reduced magnetic properties were attributed to the grain shape and effect of elemental diffusion at the FePt/Cr2O3 interface. The equilibrium shape of the FePt grains indicated that the solid–liquid interfacial energy in the FePt granular system is a key factor in controlling the microstructures.

Data-driven optimization of FePt heat-assisted magnetic recording media accelerated by deep learning TEM image segmentation

The main bottleneck for heat-assisted magnetic recording (HAMR) to achieve a potential areal density of 4 Tb/in2 is the difficulty in obtaining FePt-X nanogranular media with an ideal stacking structure of perfectly isolated L10-FePt columnar nanograins. Here, we present a fully automated routine that combines a convolutional neural network and machine vision to enable data mining from transmission electron microscopy images of FePt-C nanogranular media. This allowed us to generate a dataset and implement a machine learning optimization model that guides process parameters to achieve the desired nanostructure, i.e., small grain size with unimodal distribution and a large coercivity, which was successfully validated experimentally. This work demonstrates the promise of data-driven design of high-density HAMR media.

Optical forces in heat-assisted magnetic recording head-disk interface

A main challenge in Heat-Assisted Magnetic Recording technology is the build-up of contaminants called smear on the near field transducer. In this paper, we investigate the role of optical forces originating from the electric field gradient in the formation of smear. First, based on suitable theoretical approximations, we compare this force with air drag and the thermophoretic force in the head-disk interface for two smear nanoparticle shapes. Then, we evaluate the force field’s sensitivity to the relevant parameter space. We find that the smear nanoparticle’s refractive index, shape, and volume significantly impact the optical force. Further, our simulations reveal that the interface conditions, such as spacing and the presence of other contaminants, also influence the magnitude of the force.

Signal-to-Noise Ratio in Heat-Assisted-Recording Media: A Comparison between Simulations and Experiments

We develop a code to extract the signal-to-noise ratio (SNR) arising from the magnetic film in a recording medium. The approach allows us to separate the remanence and transition contributions from the global spatial noise. The results are in excellent agreement with the analysis performed on the same data sets by means of Seagate proprietary software based on ensemble wave-form analysis. We then apply this analytical approach to the results of heat-assisted magnetic recording (HAMR) dynamics simulations by means of the open-source multi-time-scale micromagnetic code mars and compare these with experimental spin-stand measurements of analogous systems. The proposed model could be used as the standard tool to understand the underlying physics of the noise components affecting HAMR operations and how to decrease the noise arising from the medium to improve the writing performance of HAMR.

Simulation on double-ring-resonator-type device considering gain for heat-assisted magnetic recording

Simulation on double-ring-resonator-type device considering gain for heat-assisted magnetic recording

Two Strategies to Mitigate Thermally-Induced Material Buildup in Heat-Assisted Magnetic Recording

Two Strategies to Mitigate Thermally-Induced Material Buildup in Heat-Assisted Magnetic Recording