Heat-assisted Magnetic Recording Process Research Articles

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.

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.

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.

Micromagnetic modeling of the heat-assisted switching process in high anisotropy FePt granular thin films

The dynamic process of assisted magnetic switchings has been simulated to investigate the associated physics. The model uses a Voronoi construction to determine the physical structure of the nanogranular thin film recording media, and the Landau–Lifshitz–Bloch equation is solved to evolve the magnetic system in time. The reduction of the magnetization is determined over a range of peak system temperatures and for a number of anisotropy values. The results show that the heat-assisted magnetic recording process is not simply magnetization reversal over a thermally reduced energy barrier. To achieve full magnetization reversal (for all anisotropies investigated), an applied field strength of at least 6 kOe is required and the peak system temperature must reach at least the Curie point (Tc). When heated to Tc, the magnetization associated with each grain is destroyed, which invokes the non-precessional linear reversal mode. Reversing the magnetization through this linear reversal mode is favorable, as the reversal time is two orders of magnitude smaller than that associated with precession. Under these conditions, as the temperature decreases to ambient, the magnetization recovers in the direction of the applied field, completing the reversal process. Also, the model produces results that are consistent with the concept of thermal writability; when heating the media to Tc, the smaller grains require a larger field strength to reverse the magnetization.

Analytical Estimation of Transition Jitter for the Heat-Assisted Magnetic Recording Process

In the heat-assisted magnetic recording (HAMR) process, the transition jitter is known to depend on the grain size as well as the heat spot thermal gradient. We find that the jitter also depends on the Voronoi grain size distribution, as well as exhibiting a surprising nonlinear dependence on reader width. By combining the noise due to these dependencies we used an analytical formulation to estimate the jitter in the HAMR process. The accuracy of the analytical formulation is verified by comparing the predicted jitter to micromagnetic simulations. This simple formulation provides both physical insight and conserves computational time relative to lengthy (and complex) recording simulations.

L10 FePt films with high T capping layer for Heat Assisted Magnetic Recording (HAMR)

Abstract The effect of a 1.5 nm thick CoX layer (CAP) of high Curie temperature TC, deposited on top of an L10 Fe50Pt50 layer (MAG) on the properties of the MAG/CAP bilayer is investigated. Two series of samples are studied: “w/o CAP” are single MAG layers of thickness in the 3.8–10.5 nm range and “w/CAP” are MAG/CAP bilayers of MAG thickness in the 3.8–10.5 nm range and 1.5 nm CAP. For both series, magnetization reversal is investigated at TRT = 300 K and is shown to be coherent rotations up to a MAG thickness threshold of tcr ∼ 7.5 nm and incoherent, domain-wall mediated, above tcr. The dependence of d H RC / d T T = T C , where HRC is the remanence coercivity, on the MAG thickness indicates tcr is poorly dependent on temperature from TRT up to the MAG TC. The MAG TC distribution, of average 〈TC〉 and standard deviation σTC, is evaluated for both series. The results indicate 〈TC〉 is higher and σTC/〈TC〉 is lower w/CAP than w/o, particularly at low MAG thicknesses. This behavior is interpreted as a consequence of spin exchange hardening in the MAG layer in the proximity of the MAG/CAP interface. For the w/CAP case, the saturation field of the Thermoremanent magnetization exhibits a non-motononic dependence on the MAG thickness, displaying a maximum. This behavior indicates the CAP assists the applied field to set the MAG layer magnetization during a HAMR process, the assist effect becoming more efficient the lower is the MAG thickness.

Design for cooling near-field transducer using surface plasmon polariton waveguide for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is studied to realize a high recording density of over 4 Tbit/in.2. The HAMR process requires light to be concentrated into a small area, which becomes heated on a recording medium. We studied a plasmonic waveguide (PWG) for surface plasmon polariton (SPP) propagation, which can easily cool down the near-field transducer (NFT) at the end of the PWG. It is important for the PWG to efficiently concentrate SPP to the tip of a magnetic head. We analyzed a PWG structure with protection layers, such as diamond-like carbon (DLC) and lubricant layers, to concentrate SPP onto the media and to effectively heat the recording area while keeping the NFT temperature below 100 °C.

Probing the role of carbon microstructure on the thermal stability and performance of ultrathin (

An understanding of the factors influencing the thermal stability of ultrathin carbon overcoats (COCs) is crucial for their application in heat-assisted magnetic recording (HAMR) at densities ≥ 1 Tb/in(2). Two types of non-hydrogenated ultrathin (∼1.5 nm) COCs were investigated after being subjected to laser-induced localized heating (at temperatures > 700 K) as envisaged in HAMR. Filtered cathodic vacuum arc (FCVA)-processed carbon with tuned C(+) ion energies of 350 eV followed by 90 eV provides significantly higher sp(3) C-C hybridization than magnetron sputter deposition even at very low thicknesses of ∼1.5 nm. As a result, the FCVA-deposited ultrathin carbon overcoats displayed excellent thermal stability along with improved wear and corrosion resistance. On the other hand, the sputtered carbon exhibited carbon loss and topographical and structural changes after laser irradiation owing to lower sp(3) hybridization. Therefore, this study highlights the pivotal role of carbon microstructure, primarily sp(3) hybridization, in non-hydrogenated carbon overcoats to maintain excellent thermal stability during the recurring high-temperature cycles in a HAMR process.

Experimental set-up for simultaneously measuring changes in LD temperature and flying height of HAMR-HGA

Heat-assisted magnetic recording (HAMR) is a promising technology for overcoming the performance limit of the conventional magnetic head in a hard disk drive. The HAMR-HGA consists of a HAMR-head slider, a suspension, and a laser diode (LD) mounted on the slider. An optical near-field transducer (NFT) and a waveguide are near the write-pole in the head slider. During the HAMR process, current is applied to an LD, and the laser beam is coupled into the waveguide and delivered to the NFT. The NFT further concentrate the focused optical spot and the optical spot locally heats the recording medium, thereby reducing the media coercivity. The temperature of the LD and the slider, however, increases. The slider is, moreover, locally deformed, and the flying-height (FH) changes. Therefore, an experimental set-up to simultaneously evaluate the FH change and LD temperature of the HAMR-HGA was required to develop the HAMR technology. We developed a novel experimental set-up for simultaneously evaluating laser characteristics (power, voltage, and wavelength), the increase in LD temperature, and the FH change of a HAMR-HGA. By using this set-up to measure these characteristics of our prototype, the HAMR-HGA showed that the FH decreased as the LD temperature increased. The LD temperature is directly related to the laser characteristics. The change in laser characteristics affects the read-write performance of HAMR. The FH change also affects the performance. Therefore, the developed experimental set-up should be useful in improving HAMR-HGA.

Heat assisted magnetic recording: Grain size dependency, enhanced damping, and a simulation/experiment comparison

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.