Heat-assisted Magnetic Recording Media Research Articles

Three-Layer Exchange Coupled Composite Media for Heat-Assisted Magnetic Recording

Three-Layer Exchange Coupled Composite Media for Heat-Assisted Magnetic Recording

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.

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.

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.

Zero-State Insert Dual-Layer HAMR Media Recording

Heat-assisted magnetic recording (HAMR) can support extremely high areal densities based on the thermal stability argument. However, significant challenges remain to reduce switching probability distribution (SPD) to improve media SNR. Exchange coupled composite (ECC)-type architectures, such as the designs employed in perpendicular media, are being explored for HAMR media and show potential promise at reducing SPD. In this article, we propose a new approach for the media design that consisting of an exchange decoupled dual-layer HAMR media architecture. In this design, a few net zero magnetization state grains form near the transition resulting in a transition noise reduction when a small difference in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\text {c}}$ </tex-math></inline-formula> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 5$ </tex-math></inline-formula> % was applied between the two layers. A geometric 2-D recording model was used to explore this dual-layer media design space and transition noise response. The significant transition noise reduction under the grain size limit condition is attributed to a negative correlation between noise from the top and bottom layers. This simple model was supplemented with micromagnetic recording simulations that verify stable zero-state grains are established near the transition during the recording process. The simulations confirm the significant transition noise reduction, but an increase in the dc noise was observed. Once the dc noise was addressed by increasing the applied field magnitude, a significant SNR gain of 1.4 dB was achieved compared with the equivalent single-layer media.

FePt–BN granular HAMR media with high grain aspect ratio and high L1 ordering on corning LotusTM NXT glass

Fabricating highly ordered tall L10–FePt with a small grain pitch distance on a commercially available glass substrate is crucial to realize heat-assisted magnetic recording (HAMR) media for industrial manufacture. We have realized tall FePt grains surrounded by crystalized h–BN on Corning NXT™ glass deposited at elevated temperatures in the presence of radio frequency (RF) bias. In this paper, we discuss the effect of deposition temperature on the order parameter of L10–FePt–BN granular media. Well-isolated L10–FePt–BN granular media with a grain diameter of 6.5 nm and height of 11 nm is achieved. These films exhibit a high order parameter of 0.85 with a perpendicular coercivity of 35 kOe.

Spin-Stand Measurements to Extract the Switching Distributions of Heat-Assisted Magnetic-Recording Media

A new measurement method to characterize the switching temperature distributions of heat-assisted magnetic recording (HAMR) media at recording time scales is presented. The method uses HAMR components in conventional recording systems, such as spin-stand testers and hard disk drives. Using dc noise measurements, a 2-D map of the probability of grains switching is extracted as a function of the medium temperature and of the applied field, which are controlled by the HAMR head laser power and writer current, respectively. These measurements give access to details of the HAMR media temperature and field distributions, as well as provide useful information related to the operation of the HAMR system.

Characterization of the Thermal Time Constants of HAMR Media

Here we describe experimental spin stand measurements as well as simulations of the time constants associated with thermal transients in heat-assisted magnetic recording (HAMR) media. The thermal transients associated with both increasing and decreasing steps in laser power contain at least two time constants each, one on the order of 1 ns and another on the order of 10 ns. These time constants are associated with the central thermal peak in the media and the diffuse thermal background, respectively. In addition, the measured thermal fall time is longer than the thermal rise time due to the disk’s motion relative to the head. Whereas in conventional HAMR a constant laser power is applied during writes, a number of proposed advanced recording schemes use pulsed laser power to improve either recording performance or reliability. The thermal time constants reported here suggest care must be taken when implementing such schemes so that transition locations are not shifted to the detriment of areal density capability.

Heat-assisted magnetic recording — Micromagnetic modeling of recording media and areal density: A review

Hard disk drives have become the most commonly used mass storage devices because of their low cost and high storage capacity. However, the current perpendicular magnetic recording technology is reaching the physical limit for its areal density capability (ADC) due to superparamagnetism. To accommodate the rapid increase in the volume of digital data produced globally, heat-assisted magnetic recording (HAMR) has been introduced as a next-generation recording technology. In HAMR, a laser locally heats the media to reduce the coercivity, with the information recorded during the cooling process. To investigate the high-temperature magnetization dynamics that occur during this form of recording, computational micromagnetics is a useful tool. This article provides a brief review of micromagnetic modeling for HAMR and summarizes simulation-based studies on HAMR media. A review of these simulation results can serve as a guideline for the prediction of the ADC limit of HAMR.

Machine Learning Approach for Evaluation of Nanodefects and Magnetic Anisotropy in FePt Granular Films

This paper reports a machine learning approach for evaluating micromagnetic and microstructural parameters from demagnetization curves of FePt granular films for heat-assisted magnetic recording (HAMR) media. We developed a neural network to predict parameters of magnetic anisotropy and volume fractions of defects such as [200] misoriented grains, {111} twined variants, and disordered grains. The neural network was trained on a synthetic dataset of out-of-plane demagnetization curves that were simulated using the micromagnetic model constructed from actual nanostructure of a FePt-X HAMR medium. Predicted nanodefects agreed well with those estimated by synchrotron X-ray diffraction, and the demagnetization curve simulated with the predicted parameters accurately reproduced the experimental one. This work paves the way for a high-throughput magnetometry-based characterization of FePt granular media for its structural optimization toward higher areal density of HAMR.

Understanding the growth of high-aspect-ratio grains in granular L1-FePt thin-film magnetic media

A systematic investigation has been performed to optimize the microstructure of L10-FePt–SiOx granular thin films as recording media for heat-assisted magnetic recording. The FePt–boron nitride (BN) nucleation layer, which is stable even at 700 °C, is used to control the grain sizes and microstructure during the high-temperature processing. The study finds that films of high-aspect-ratio FePt grains with well-formed silicon oxide (SiOx) grain boundaries require the grading of the deposition temperature during film growth as well as the grading of the silicon oxide concentration. Well-isolated columnar grains of L10-FePt with an average height greater than 11 nm and diameters less than 7 nm have been achieved. Transmission electron microscopy analysis of the microstructures of samples produced under a variety of non-optimal conditions is presented to show how the microstructure of the films depends on each of the sputtering parameters.

Interplay of the Thermal and Magnetic Fields in HAMR

The writing process in heat-assisted magnetic recording (HAMR) is an interplay of two effects: thermal and magnetic. These factors together affect the writing contour that defines the transitions. Here, we explore the effects of this interplay on the switching behavior of the HAMR media from which we can derive the writing temperature. We use these results to explore the applicability of a recently proposed analytical formulation that predicts transition jitter. We show that dynamic effects modify the shape of the writing contour caused by changes in the writing temperature across the written track.

Transmission electron microscopy image based micromagnetic simulations for optimizing nanostructure of FePt-X heat-assisted magnetic recording media

The areal density of the heat-assisted magnetic recording (HAMR) technology has a potential to reach 4 Tb/in2. However, the bimodal grain size distribution and structural defects that are unavoidable in actual FePt-X nanogranular media must be minimized for further reduction of a jitter noise. Here we report transmission electron microscopy (TEM) image based micromagnetic simulations of the FePt-X nanogranular media in order to elucidate the role of structural defects such as [200] misalignment and {111} twins on the magnetic properties. Micromagnetic approximation of experimental out-of-plane hysteresis loops allows to evaluate micromagnetic parameters and volume fractions of the defects in MgO(6 nm)/FePt-BN(1 nm)/FePt-(BN,C,SiO2)(7 nm) granular thin films. Synchrotron XRD of the films validated that the structural defects can be accurately evaluated by the proposed approach. The developed TEM image based micromagnetic simulations can directly link nanostructure of the FePt-X media with magnetic properties, boosting not only the optimization of the HAMR media but also a design of magnetic nanomaterials in general.

High temperature nanomechanical properties of sub-5 nm nitrogen doped diamond-like carbon using nanoindentation and finite element analysis

Nitrogen doped diamond-like carbon (NDLC) is a candidate protective coating in state-of-the-art heat assisted magnetic recording (HAMR) media. To ensure the robustness of the media particularly at higher temperature applications, mechanical properties of ultra-thin sub 5-nm NDLC coatings are of great interest. Due to instrument limitations and very shallow films, it is very challenging to accurately measure sub-5 nm NDLC films and other HAMR components from experiments without substrate effects. In this study, very shallow nanoindentations were performed, and the results were fitted with finite element analysis using a modified indenter geometry to predict the elastic modulus and yield strength of NDLC films of two different thicknesses (3.5 and 4.5 nm) and other components without any substrate effect. Results showed that higher NDLC film thickness led to better elastic modulus and yield strength at 25 °C before and after heating and at 300 °C. Hardness to yield strength ratio (H/Y) for NDLC films was also determined and found within the range of 2.2–2.8, which is higher than the H/Y ratio of DLC films from earlier studies. This implied the dependence of H/Y ratio on the thickness, temperature conditions, and chemical structure of NDLC films. Results also showed that the yield strength of FeCo metal layer and glass substrate in HAMR media decreased at 300 °C, but almost fully recovered to their initial properties after removal of heat.

Analysis of Adjacent Track Erasure in the HAMR Media

Writing closely spaced tracks in the cross-track direction helps increase the storage density but leads to erasure of the adjacent tracks. This phenomenon is called adjacent track erasure (ATE). This article aims to establish the presence and numerical extent of ATE in different heat-assisted magnetic recording (HAMR) media. We find that thermal exchange coupled composite (ECC) media can be more susceptible to ATE compared with single layer FePt media of equivalent thickness. We implement optimization techniques to reduce the ATE in different media and increase the recording signal to noise ratio (SNR) in this process. While the ATE in single layer FePt media seems to be relatively impervious to changes in the applied field angle, small field angles appear to reduce the ATE for the high $T_{c}$ thermal ECC media. However, small angle also reduces the SNR, leading to the conclusion that 30° actually yields the best SNR even in the presence of overwrite. Introducing a finite intergranular exchange coupling improves the writing performance of the single layer FePt and reduces the ATE. We rigorously calculate the thermal decay constant and use it to evaluate the performance of pulsed laser recording. Finally, we establish a hypothesis that helps explain the presence of ATE in different HAMR media. We believe the ATE is proportional to the thermal stability factor at a temperature just below the write temperature. The proposed hypothesis correctly predicts the ATE in the single layer FePt media assuming coherent rotation. Our results can help increase the HAMR storage density and make HAMR media resistant to the ATE.

Magnetization dynamics of granular heat-assisted magnetic recording media by means of a multiscale model

Heat assisted magnetic recording (HAMR) technology represents the most promising candidate to replace the current perpendicular recording paradigm to achieve higher storage densities. To better understand HAMR dynamics in granular media we need to describe accurately the magnetisation dynamics up to temperatures close to the Curie point. To this end we propose a multiscale approach based on the micromagnetic Landau-Lifshitz-Bloch (LLB) equation of motion parametrised using atomistic calculations. The LLB formalism describes the magnetisation dynamics at finite temperature and allows to efficiently simulate large system sizes and long time scales. Atomistic simulations provide the required temperature dependent input quantities for the LLB equation, such as the equilibrium magnetisation and the anisotropy, and can be used to capture the detailed magnetisation dynamics. The multiscale approach makes possible to overcome the computational limitations of atomistic models in dealing with large systems, such as a recording track, while incorporating the basic physics of the HAMR process. We investigate the magnetisation dynamics of a single FePt grain as function of the properties of the temperature profile and applied field and test the micromagnetic results against atomistic calculations. Our results prove the appropriateness and potential of the approach proposed here where the granular model is able to reproduce the atomistic simulations and capture the main properties of a HAMR medium.

Experimental study of media heat sink thickness impact on signal-to-noise ratio in heat-assisted magnetic recording

Recently, heat-assisted magnetic recording (HAMR) has been demonstrated to extend the areal density growth over the superparamagnetic limited. One key component of this technology is a heat sink layer in HAMR media, which benefits the thermal gradient and transition noise. However, the disadvantage of the heat sink layer (HS) has not been fully explored. In this paper, we investigate the background interference (BGI) impact as a result of the heat sink layer via the spin-stand tester. HAMR heads included a light delivery system that have measured a signal-to-noise ratio and down-track thermal gradient on a variety of HS thickness. Subsequently, we found that a thicker HS is a trade-off between the BGI and thermal gradient. Thus, it remains challenging to achieve an ultra-high areal density using the thermal media design.

Magnetic in-plane components of FePt nanogranular film on polycrystalline MgO underlayer for heat-assisted magnetic recording media

A systematic study is conducted on the effect of MgO substrate/underlayer in terms of the surface roughness, grain boundary, and crystallographic texture on the magnetic in-plane components (medium noise) of the FePt-based heat-assisted magnetic recording (HAMR) media. Cross-sectional transmission electron microscope observations revealed that good (001) texture can be developed in FePt nanogranular films even when they are grow on a rough surface of a single crystal-MgO (001) substrate or across the grain boundaries of a (001)-textured polycrystalline-MgO underlayer. The misoriented FePt grains with magnetic easy axes (c-axes) tilted away from the film normal direction mainly originate from their epitaxial growth on the initially misoriented MgO underlayer grains. Micromagnetic simulation results revealed that the FePt grains with misorientation angle larger than ∼20° are attributed to the detected magnetic in-plane components. Based on these experimental results, we discuss how to improve the (001) texture of the polycrystalline underlayer as well as to develop FePt-based HAMR media with enhanced signal-to-noise ratio.

Composite media with reduced write temperature for heat assisted magnetic recording

A major issue plaguing Heat Assisted Magnetic Recording (HAMR) is the high writing (Twrite) and peak heat spot temperatures (Tpeak). To counter this, a low temperature thermal exchange coupled composite (ECC) media with reduced write temperature relative to previous high temperature thermal ECC media is introduced. The FePt Tc is reduced from 700 K to 500 K, and the write layer Tc is reduced from 900 K to 600 K. Optimizations for Ms and Ku of the write layer generate values of Ms = 700 emu/cm3 and Ku = 1.0 × 107 erg/cm3 at 300 K. Switching Probability Distribution (SPD) calculations with lowered Tpeak = 650 K indicate Twrite = 487 K. This indicates a Twrite reduction by 34% as compared to the previous write temperature of 738 K for the high temperature thermal ECC media. Examining the FWHM of the SPD under 0% and 3% Tc variation indicates a noise reduction of ∼20% relative to single layer high Tc FePt media, and a much larger reduction relative to low Tc FePt media. The jitter values at lowered Tc approach the ideal value set by the grain size in the absence of any Tc and Ku variation. Simulations with uncorrelated and correlated Tc and Ku variations in the write and FePt layer generate similar results. Reduction of FePt Tc may reduce the anisotropy relative to its undoped value: reduction of anisotropy by 33% is found to adversely affect the SPD by only 3–4%, but has a larger impact on jitter. These results help establish the usefulness of a low temperature thermal exchange coupled composite media for HAMR.

Resolving Anomalies in the Critical Exponents of FePt Using Finite-Size Scaling in Magnetic Fields

FePt is the primary material being considered for the development of information storage technologies based on heat-assisted magnetic recording (HAMR). A practical realization of HAMR requires understanding the high-temperature phase transition behavior of FePt, including critical exponents and Curie temperature distributions as the fundamental HAMR media design characteristics. The studies so far found a significant degree of variability in the values of critical exponents of FePt and remain controversial. Here we show that at the heart of this variability is the phase transition crossover phenomenon induced by two-ion anisotropy of FePt. Through Monte-Carlo simulations based on a realistic FePt effective Hamiltonian we demonstrate that in order to identify the critical exponents accurately, it is necessary to base the analysis on field-dependent magnetization data. We have developed a two-variable finite size scaling method that accounts for the field effect. Through the use of this method, we show unambiguously that true critical exponents of FePt are fully consistent with the three-dimensional Heisenberg universality class.