Heat-assisted Magnetic Recording Research Articles

The role of interfacial intermixing on HAMR dynamics in bilayer media

We use an atomistic spin model to simulate FePt-based bilayers for heat assisted magnetic recording (HAMR) devices and investigate the effect of various degrees intermixing that might arise throughout the fabrication, growth and annealing processes, as well as different interlayer exchange couplings, on HAMR magnetisation dynamics. Intermixing can impact the device functionality, but interestingly does not deteriorate the properties of the system. Our results suggest that modest intermixing can prove beneficial and yield an improvement in the magnetisation dynamics for HAMR processes, also relaxing the requirement for weak exchange coupling between the layers. Therefore, we propose that a certain intermixing across the interface could be engineered in the fabrication process to improve HAMR technology further.

Micromagnetic Simulation of L10-FePt-Based Transition Jitter of Heat-Assisted Magnetic Recording at Ultrahigh Areal Density.

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.

Magnetic Thin Films used for Memory Devices: A Scientometric Analysis

Ultra-thin ferromagnetic films coupled with nonmagnetic layers are extensively studied for their potential applications in data storage. The conventional memory industry is currently in the spotlight, with researchers worldwide contributing toward better data storage technologies that can tackle the data storage problems posed by this era of big data. Various technologies such as perpendicular magnetic recording (PMR) and heat-assisted magnetic recording (HAMR) have been introduced since Poulson’s idea of storing signals was conceptualized. With the introduction of new technologies, like PMR and HAMR, various materials have to be restudied for potential recording media implementation. It is vital to carry out scientometric/bibliometric studies before extensive research. It gives us an insight into the significance of the topic and the gap in the research work. This paper presents a scientometric study from 2010 to 2022 of all the magnetic thin films used for various applications. It provides an overview of the technologies and literature study of their development in data storage applications. The study discusses the analytic results from different comprehensive high indexed databases such as Scopus, Web of Science (WoS), Google Scholar, and the Institute of Electrical and Electronics Engineers (IEEE). To contribute meaningful insights and the perfect representation of the extracted data, various tools like iMapbuilder, Gephi and RAWgraphs are used in this article.

Influence of finite-size effects on the Curie temperature ofL10−FePt

We employ an atomistic model using a nearest-neighbor Heisenberg Hamiltonian exchange to study computationally the dependence of the Curie temperature of $L{1}_{0}$-FePt on finite-size and surface effects in heat-assisted magnetic recording (HAMR) media. We demonstrate the existence of a size threshold at 3.5 nm below which the impact of finite-size effects starts to permeate into the center of the grains and contributes to the reduction of the Curie temperature. We find a correlation between the Curie temperature and the percentage of atomistic bonds lost on the surface as a function of grain size, which can be extended to apply to not only $L{1}_{0}$-FePt but also generic magnetic systems of any crystal structure. In a recording medium, the inevitable grain size dispersion leads to an irreducible contribution to the dispersion of the Curie temperature. Therefore, our paper gives insight into finite-size effects, which have been predicted to be a serious limitation of HAMR.

Topology optimization of a heat-assisted magnetic recording write head to reduce transition curvature using a binary optimization algorithm utilizing the adjoint method

In this work, the possibility to reduce transition curvature in heat-assisted magnetic recording, using a conventional write head design, by shaping the recording field to counteract the circular profile of the heat pulse is investigated. Topology optimization of the head tip is performed in order to create the desired cross-track field profile for increasing distances from the write head tip. For the topology optimization, the adjoint method is utilized to calculate the necessary gradients and a binary optimization scheme is proposed. The optimizations are performed considering linearized material parameters reducing the computational complexity and the results are compared to optimizations incorporating the full non-linear material behavior. The optimized field profiles are evaluated for their influence on the read-back process. To do so, switching probability phase diagrams are calculated and the curvature parameter, the signal to noise ratio and the channel bit density are extracted. The presented results show that while transition curvature can be reduced by shaping the cross-track profile of the write field, this alone does not consequently lead to an improvement of the read back process. Therefore, completely new head designs, considering additional parameters have to be investigated.

Optimizing the Thickness and Diameter of Dual Structure Patterned Media Dots for Heat-Assisted Magnetic Recording

Dual structure bit patterned media were optimized for heat-assisted magnetic recording (HAMR). Each dot contained two discrete recording structures and was therefore able to store two bits of information. The optimized parameters were the dot diameter and the thicknesses of the recording structures. Analysis of the thermal stability of the magnetization at elevated temperatures suggested a design with a thicker recording structure on the bottom, where the head field was weaker, and a thinner structure on the top. Recording simulations showed that the optimal design could lead to user areal recording densities in excess of 9 Tbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Dependence of Bit Error Rates on HAMR Transition Curvatures

Transition curvature limits the performance of heat-assisted magnetic recording (HAMR), especially when bit length (BL) is reduced below 10 nm. In this article, in order to quantify this issue, the dependence of signal-to-noise ratios (SNRs) and bit error rates (BERs) on HAMR transition curvatures is investigated utilizing previously obtained curvature values. An artificial recording method is used and a Viterbi algorithm is applied to decode the readback signals and calculate BERs. A pseudorandom binary sequence (PRBS) is used. Two BLs are tested: 7.5 and 8.5 nm. Different reader widths (RWs), shield-to-shield distances, and magnetic fly heights are studied in order to seek optimized reader configurations. Generally, transition curvatures have a negative effect on SNR and BER performance, but this effect can be mitigated by the reduction of certain reader dimensions.

Topological extraordinary optical transmission

The incumbent technology for bringing light to the nanoscale, the near-field scanning optical microscope, has notoriously small throughput efficiencies - of the order of 10^(-4) - 10^(-5), or less. We report on a broadband, topological, unidirectionally-guiding structure, not requiring adiabatic tapering and in principle enabling near-perfect (ideally, ~100%) optical transmission through an unstructured single (POTUS) arbitrarily-subdiffraction slit at its end. Specifically, for a slit width of just lambda_eff / 72 (lambda_0 / 138) the attained normalized transmission coefficient reaches a value of 1.52, while for a unidirectional-only (non-topological) device the normalized transmission through a lambda_eff / 21 (~lambda_0 / 107) slit reaches 1.14 - both, limited only by inherent material losses, and with zero reflection from the slit. The associated, under ideal conditions, near-perfect optical extraordinary transmission (POET) has implications, among diverse areas in wave physics and engineering, for high-efficiency, maximum-throughput nanoscopes and heat-assisted magnetic recording devices.

Models of advanced recording systems: A multi-timescale micromagnetic code for granular thin film magnetic recording systems

Micromagnetic modelling provides the ability to simulate large magnetic systems reliably without the computational cost limitation imposed by atomistic modelling. Through micromagnetic modelling it is possible to simulate systems consisting of thousands of grains over a time range of nanoseconds to years, depending upon the solver used. Here we present the creation and release of an open-source multi-timescale micromagnetic code combining three key solvers: Landau-Lifshitz-Gilbert; Landau-Lifshitz-Bloch; Kinetic Monte Carlo. This code, called MARS (Models of Advanced Recording Systems), is capable of accurately simulating the magnetisation dynamics in large and structurally complex single- and multi-layered granular systems as is shown by comparison to established atomistic simulation results. The short timescale simulations are achieved for systems far from and close to the Curie point via the implemented Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers respectively. This enables read/write simulations for general perpendicular magnetic recording and also state of the art heat assisted magnetic recording (HAMR). The long timescale behaviour is simulated via the Kinetic Monte Carlo solver, enabling investigations into signal-to-noise ratio and data longevity. The combination of these solvers opens up the possibility of multi-timescale simulations within a single software package. For example the entire HAMR process from initial data writing and data read back to long term data storage is possible via a single simulation using MARS. The use of atomistic parameterisation for the material input of MARS enables highly accurate material descriptions which provide a bridge between atomistic simulation and real world experimentation. Thus MARS is capable of performing simulations for all aspects of recording media research and development. This ranges from material characterisation and optimisation to system design and implementation. The object orientated nature of MARS is structured to facilitate quick and simple development and easy implementation of user defined custom simulation types which can utilise either timescale or a combination of both timescales. Program summaryProgram title: MARSCPC Library link to program files:https://doi.org/10.17632/8mx7cndcdx.1Developer's repository link:https://bitbucket.org/EwanRannala/mars/Code Ocean capsule:https://codeocean.com/capsule/2549929Licensing provisions: MITProgramming language: C++Supplementary material: MARS testing methodology (PDF), HAMR simulation example video.Nature of problem: A combined model that enables the complete modelling of magnetic recording processes at elevated temperatures covering all time scales from writing (nanoseconds) up to long term data storage (years). The model must also accurately describe the granular nature of the recording media as grain sizes are reduced to a few nanometres.Solution method: Short timescale behaviours are captured via the Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers for low and high temperature systems respectively. The long time scale behaviours are captured via a kinetic Monte Carlo solver. To enable complex models which account for mixed timescale behaviours the solvers are implemented as a single class structure which allows for dynamic solver selection. The granular structure is generated via a Laguerre-Voronoi tessellation with a custom implemented packing algorithm to produce highly realistic grain size distributions. Complex thermal dependencies of materials can be incorporated via atomistic parameterisation forming a multi-timescale model of the material.

Elliptical plasmonic near-field transducer and v-shape waveguide designs for heat assisted magnetic recording

We proposed a new elliptical near-field transducer (NFT) and V-shaped waveguide design, which provides multiple pathways to control and optimise the thermal performance of the optical delivery paths for heat-assisted magnetic recording (HAMR). The principle of operation of the design utilises the properties of the localised surface plasmon resonance (LSPR) of the metallic elliptical NFT. We demonstrated a peak temperature in the recording media normalised to the incident laser power (T Media /P In ) ranging from ∼ 128 K/mW to more than 181 K/mW. We also achieved tunable thermal gradients as high as 23.0 K/nm and 18.7 K/nm in the recording media’s down-track and cross-track directions, respectively. Moreover, we exemplified that the inevitable temperature rise in the plasmonic NFT could be tuned to remain under 400 K for maximum heat in the recording media &gt; 700 K. The best thermal performance obtained corresponded to the aspect ratio of the antenna a/b ∼ 0.89 with the minor axis of the antenna a = 85 nm.

Simulation on a non-axisymmetric ring resonator with a nano-antenna for heat-assisted magnetic recording

Heat-assisted magnetic recording is a technology to improve recording density of hard disk drives. The authors’ group previously proposed a device, in which a Au nano-antenna as a near-field transducer was attached to a GaAs ring resonator as a light source. The ring resonator should stably oscillate with a single mode to generate near-field light at the tip of the nano-antenna with sufficient energy density. A non-axisymmetric ring resonator structure was proposed this time to improve the energy density and laser oscillation stability. When the decenter between the inner and outer circles of the ring resonator increased, the resonance frequency increased and eigenmodes with higher radial mode order disappeared, which leads to stable laser oscillation. Moreover, when the decenter was 200 or 250 nm, the normalized energy density at the tip of nano-antenna increased 1.2–2.4 times compared with an axisymmetric ring resonator structure while keeping the quality factor to more than 5000.

Track width measurement in heat-assisted magnetic recording systems using the cross-track DC noise method

This paper uses a cross-track DC noise measurement technique to determine the written track width in heat-assisted magnetic recording (HAMR). The method is approached as a simple, no-extra-hardware-required technique on a spin-stand to determine the written track width as well as the DC noise amplitude at the center track in a HAMR system. The track width data show the expected trends with changing laser operating current () and write current (), and they are strongly correlated with track density capabilities and physical near-field transducer (NFT) peg width. The written track width is essentially defined by the which increases as increases. The method is useful for comparing the magnetic responses of different writer pole or NFT peg geometries, as well as media design comparison to provide insight into the head/component necessary to improve HAMR system performance, which is useful HAMR recording parameters for head/media designers and test engineers.

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.

Adjacent Track Interference in Heat-Assisted Magnetic Recording

We examine here the adjacent track interference (ATI) problem that arises when writing in an adjacent track for heat-assisted magnetic recording combined with shingled magnetic recording (shingled HAMR). For ATI, the bit error rate is a function of readout track width. We calculate the bit error rate and the minimum normalized readout field for various readout track widths. The factors affecting ATI are grain temperature and grain volume. We compare ATI and writing sensitivity for all combinations of 2 and 4 Tbpsi, and 6 and 9 grains/bit under the conditions of constant grain volume and constant grain height. For constant grain volume, 4 Tbpsi shingled HAMR is less advantageous than 2 Tbpsi shingled HAMR in terms of ATI due to the higher grain temperature caused by the smaller grain pitch. The grain number per bit must be lower and the grain height must be greater to be able to improve ATI for 4 Tbpsi shingled HAMR. We show there to be a loss of writing sensitivity for 4 Tbpsi shingled HAMR due to a statistical factor caused by the lower grain number per bit and the higher thermal stability factor resulting from the larger grain volume.

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.

Improved Recording Performance With Writer Design and Joint Learning-Based Neural Network for Heat-Assisted Interlaced Magnetic Recording

Heat-assisted interlaced magnetic recording (HIMR) further increases the areal density with the interlaced track layout architecture compared to conventional heat-assisted magnetic recording. However, besides the 2-D inter-symbol interference (ISI) and thermal jitters, the transition curvatures due to the circular thermal profile cause severe erasures after write and obvious nonlinear distortions of readback signals. This deteriorates the recording performance severely. Especially the degrees of transition curvatures for the low temperature written track (LT) and high temperature written track (HT) are distinct, which makes the correction of transition curvature with only the writer design impossible. Correspondingly, a comprehensive method consisting of writer design and multitrack detection algorithm with joint learning-based neural network (JLNN) is proposed to mitigate the effects of transition curvature and 2-D ISI for both LT and HT of HIMR. First, a designed writer with splitting main poles produces the spatially nonuniform write field along the crosstrack direction to correct the transition curvature of LT. Then the multitrack detection with JLNN and improved Bahl–Cocke–Jelinek–Raviv (BCJR) detector (JLNN-IB) algorithm is studied to further mitigate the effects of residual transition curvatures, thermal jitters, and 2-D ISI for both HT and LT. Specifically, the readback signals of read head array and decoded log-likelihood ratio (LLR) outputted by the low-density parity check (LDPC) decoder are fed into JLNN to obtain the equalized signal and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$a$ </tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">priori</i> probability (AP) of three tracks. Then the equalized signal as well as AP of current track and corresponding soft bit estimations of adjacent tracks are embedded into the branch metrics of improved BCJR detector for data detection, and the LDPC decoder is cascaded for the error correction. The simulation indicates that at the recording density of 2.11 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the proposed comprehensive writer design and JLNN-IB algorithm provide 9.2 and 9.7 dB signal to noise ratio (SNR) gains compared to the conventional 2-D neural network equalizer and 2-D linear equalizer under the uniform write field and circular thermal profile, respectively.

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.

Simulation of a Thermally Efficient Heat-Assisted Magnetic Recording Ridge Waveguide NFT on an AlN Heat Sink

A thermally efficient ridge waveguide near-field transducer (NFT) design on an AlN heat sink is presented. Through simulation, we found our design can achieve thermal efficiency (TE), defined as the ratio of peak temperature rise in the medium to that in the NFT as high as 2.93. The ridge waveguide NFT is placed on a heat sink of high thermal conductivity material AlN, while otherwise surrounded by SiO2 as the top cladding and cavity filling material. The relatively large metallic waveguide structure can dissipate heat effectively even in the presence of finite interface thermal conductance between the NFT and the AlN. Different NFT excitation methods are examined, with an impinging dielectric waveguide mode leading to the highest TE. Some challenges are identified in using excitation by resonant disk structures. We note that surface plasmon is confined inside the NFT cavity, while heat escapes through the outer surface of the NFT, separating the dielectric interfaces responsible for optical propagation and heat transfer. This allows for further reduction in thermal interface resistance by adding an interface adhesion layer like Cr at the NFT/AlN interface without sacrificing optical properties.

1-D Simulation of Ultrafast-Pulsed Laser into Nano-Sized Multilayered Structure (Ni81Fe19/Cu/YIG/GGG) for Memory Device Applications

Spintronic offers a solution by exploiting spin instead of electron charge since spin current propagation can occur in principle without dissipation. One of the applications involve within this project for storage media is heat-assisted magnetic recording (HAMR). The objective of this study is to simulate the behavior of thermal gradient to generate a pure spin current using an ultrafast femtosecond (fs) laser in a nano-sized multilayered structure of (Al2O3/Ni81Fe19 (Py)/Cu/Y3Fe5O12 (YIG)/Gd3Ga5O12 (GGG)) at room temperature. A ferromagnetic/spacer/magnetic insulator nano-sized multilayered is the proposed structure for this study. Electron spin, directed by the external applied magnetic field, is transferred via the spacer to the magnetic insulator, leading to the generation of a spin-wave within the last layer. The ultrafast laser generates a spark of spin diffusion to get spin current. The thermal behavior within the trilayer simulated using COMSOL Multiphysics (v 5.5) is presented and supported by the theoretical model. Simulation results showed the effect of thickness and time on the generated spin current. Moreover, the thickness of the permalloy layer plays an essential role in generating a high-temperature gradient within a magnetic insulator to generate a spin current.

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.