Heat-assisted Magnetic Recording Technology Research Articles

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.

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.

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.

Influence of Recorded Pattern on Background Interference impact in Magnetic Recording

This paper discusses the impact of background interference on a recorded pattern for heat-assisted magnetic recording technology (HAMR). Several patterns of the background track were examined, with the log bit error rate and signal to noise ratio measured via a spin-stand tester using HAMR head and media. It was found that the low frequency pattern gave the highest BER and SNR loss due to the strong magnetic field from the adjacent tracks. Similar to its practical use, the PRBS pattern also showed high interference. These observations may be used to support HDD areal density growth.

Engineered Ceria Slurries for Electronic Glass Substrate CMP Application

Glass substrates have gradually gained more attention as the media platters in hard disk drives (HDD) used in laptops, data centers, and cloud storage due to its advantageous properties. Particularly, advanced technologies like Heat Assisted Magnetic Recording (HAMR) in HDD and Through-Glass-Via (TGV) in 3D packaging applications all require the use of glass substrates with precisely controlled surface properties. Glass substrate’s advantages include lower weight, improved flatness, less flutter at high RPM’s and lower thermal expansion as compared to aluminum / NiP substrates. This lower expansion when exposed to heat makes it ideal for HAMR technology, which will be the key to creating hard disk drives with growing storage capacity. Chemical mechanical polishing (CMP) is a method to eliminate surface defects induced by grinding & lapping to meet final requirements of HAMR or TGV for glass substrates with surface finishes down to a few angstrom level or less. Key performance targets include removal rate, surface roughness, TTV, FSW, ceria residue / particle content (cleanability) on glass disks as well as recyclability. In this study, slurries for CMP are investigated utilizing engineered ultra-pure ceria particles which are known to have high material removal rate (MRR) on glass substrates. Various particle sizes, morphology, and chemical additives were evaluated. Results of systematic CMP tests including MRR, surface roughness, recyclability, and most importantly, ease of ceria residue removal from the glass substrate will be reported. Morphology of ceria particles was found to play a critical role in MRR. An optimized combination of ceria morphology and additive chemistries enables high MRR together with improved Ceria residue / cleanability performance. Figure 1

Experimental Study on Smear Characteristics Upon Laser Heating in Air and Helium in Heat-Assisted Magnetic Recording

In heat-assisted magnetic recording (HAMR) technology, it is necessary to heat the magnetic layer of disks at a temperature that exceeds the Curie temperature (Tc). When the disk is laser irradiated, the lubricating film and diamond-like carbon (DLC) protective film on the disk surface are heated at high temperatures (300 °C-500 °C). If the lubricant film, DLC thin film, or organic contamination on the disk surface evaporates or thermally decomposes upon laser heating, it adheres as a smear on the slider surface. In view of the negative effect exerted thereof, the solution to this problem is, therefore, critical for the successful application of HAMR. Furthermore, recently developed helium-enclosed hard disk drives are currently utilized in industries. It is, thus, expected that the aforementioned novel HAMR technology will be employed in helium-enclosed drives. In this study, the effects of DLC thin films on the smear characteristics upon the laser heating of disks in air and He are experimentally investigated, and the manner with which these films on the disk surface affects the smear characteristics is clarified. Moreover, the technical countermeasures necessary to reduce the smear with respect to the hydrocarbons from the DLC thin films in HAMR are elucidated and proposed in this article. Simply put, the technical countermeasures depend on the heating temperature, and the use of a mixture of He and air or oxygen gases is found to be effective. This method, however, may not be significantly useful when the heating temperatures exceed 500 °C.

Investigation of magnetic anisotropy and heat dissipation in thin films of compensated antiferromagnet CuMnAs by pump–probe experiment

We recently reported on a method to determine the easy axis position in a 10 nm thick film of the fully compensated antiferromagnet CuMnAs. The film had a uniaxial magnetic anisotropy and the technique utilized a magneto-optical pump and probe experiment [Saidl et al. Nat. Photonics 11, 91 (2017)]. In this contribution, we discuss the applicability of this method for the investigation of a broader set of epitaxial CuMnAs films having different thicknesses. This work reveals that the equilibrium magnetic anisotropy can be studied only in samples, where this anisotropy is rather strong. However, in the majority of CuMnAs films, the impact of a strong pump pulse induces nano-fragmentation of the magnetic domains and, therefore, the magnetic anisotropy measured by the pump–probe technique differs substantially from that in the equilibrium conditions. We also demonstrate that the optical pump–probe experiment can be used very efficiently to study the local heating and heat dissipation in CuMnAs epitaxial layers. In particular, we determined the electron–phonon relaxation time in CuMnAs. We also observed that, for a local film heating by a focused laser, the thinner films are heated more, but the heat is dissipated considerably faster than in the case of thicker films. This illustrates that the optical pump–probe experiment is a valuable characterization tool for the heat management optimization in the CuMnAs memory devices and can be applied in a similar way to those used during the heat-assisted magnetic recording technology development for the latest generation of hard drive disks.

Thermopower of double-barrier magnetic tunnel junctions with a middle La0.7Sr0.3MnO3 layer

Magnetic tunnel junctions (MTJs) are promising devices for heat-assisted magnetic recording technology. Quantum transport of nanoscale double-barrier MTJs with a middle La0.7Sr0.3MnO3 (LSMO) layer (DBMTJs-LSMO) is investigated by the non-equilibrium Green’s function formalism. The thermopower is described in the linear response regime. The results of the DBMTJs-LSMO show an enhancement of the thermopower compared to DBMTJs-CoFeB and conventional single-barrier MTJs (SBMTJs) due to the LSMO physical properties and also the existence of the quantum-well (QW) states. The parallel (P) and anti-parallel (AP) thermopower of and are achieved at room temperature for DBMTJs-LSMO. The tunnel magneto-thermopower ratio is found to be at room temperature along with its maximum value of at for DBMTJs-LSMO.

Thermal Diffusion Phenomena of the Functional Polymer Lubricants for the Heat-Assisted Magnetic Recording

Materials in a newly designed head–disk interface in the heat-assisted magnetic recording (HAMR) have to satisfy harsh operating conditions including cyclic thermal stresses generating large thermal gradient where the heated lubricants evaporate as well as thermally flow-out. Therefore, the stumbling block in the HAMR technology is to fully understand the system behavior under large thermal effects on the lubricant diffusion related to the depletion in the heated zone. The previous studies have investigated local thermal diffusion, depletion, and recovery phenomena of nonfunctional perfluoropolyether lubricant by studying the lubricant behavior at the center and the formation of a rim at the edge of heated zone, while the diffusion mechanism in the heated area has not been fully understood. In this paper, we examined the thermal effects on the lubricants for the application of HAMR in two aspects: 1) thermal diffusion for the replenishment and 2) lubricant evaporation for the loss of the lubricant by using the molecular dynamics.

High field enhancement between transducer and resonant antenna for application in bit patterned heat-assisted magnetic recording.

Heat-Assisted Magnetic Recording (HAMR) is a well-known technology that uses the concept of a plasmonic antenna to shrink the optical focused spot to sizes of tens of nanometers, in order to overcome the limits of conventional magnetic recording and to reach areal density >1 Tb/in2. In this paper, we propose a novel concept that allows to increase a power transfer to the recording media deposited on the patterned substrate that takes a full advantage of the resonant antenna conditions to achieve further intensity enhancement in the gap and consequently high-power transfer to the recording media. Apart from the high field enhancement, the proposed concept ensures uniform field distribution in the gap that translates into uniform and higher temperature in the recording media, which is highly desired in the HAMR technology.

A Partially Pole-Embedded Transducer for Heat-Assisted Magnetic Recording

Although significant progress has been made toward achieving 10 Tbpsi and beyond, the continued progression of the hard-disk drive development remains a challenging endeavor. A panoptic view indicates several factors that contribute to the challenge, including media as well as recording head design challenges. The heat-assisted magnetic recording (HAMR) head, specifically, has the broad responsibility of producing a magnetic field, delivering light to the optical transducer, and transforming optical energy into localized thermal energy within the media. Resulting thermal gradients in the recording layer exceeding 10 K/nm can often yield a satisfactory recording performance; however, this can be at the expense of elevated temperatures in the system. Here, we discuss an analysis of a write pole-embedded near-field transducer and demonstrate that it can not only provide improved optical efficiency and cooling in the head but also has the potential to achieve competitive thermal gradients in the recording layer, exceeding 20 K/nm, with peak temperatures 750 K T < 850 K. Moreover, the best case among the parameter space explored here demonstrates the HAMR recording signal-to-noise ratio levels exceeding 22.5 dB at 1000 kFCI. The results of the analysis strongly suggest that a magnetic write pole-embedded transducer potentially may provide an attractive path toward higher linear densities with the HAMR technology.

Reduction of recording noise caused by nano-scale variations in Curie temperature

Heat-Assisted Magnetic Recording (HAMR) is expected to be the next-generation magnetic recording technology: it is designed to meet increasing higher areal density demands from cloud storage. Until now, there are no feasible solutions to address nm-scale Curie temperature variance in FePt-based single layer media. This variance is the leading noise source, and thus limits further development of HAMR technology. Here, we describe a superparamagnetic write process that can greatly mitigate the impact of Curie temperature variance by recording information at the blocking temperature of the magnetic material. To demonstrate the benefits of this write process, we employ a bilayer structure with a superparamagnetic write layer and FePt as a long-term storage layer. By developing the theory behind the recording mechanism, we are able to introduce design rules addressing the importance of elevated Curie temperature in the write layer and optimization of quantum mechanical coupling between the two functional layers.

Microstructure and magnetic properties of ultrathin FePt granular films

FePt granular films with grain size smaller than 10 nm are promising candidates for storage media used in the next generation heat-assisted magnetic recording technology. However, FePt films show degraded magnetic properties when the grain size is reduced to this scale, which cannot be explained solely by the finite size theory. In this study, we explored the structural cause of property degradation by employing advanced electron microscopy and atomistic modeling. Structural features unique to the nanostructured FePt granular films at significantly reduced grain sizes of 2∼8 nm were studied by high-resolution scanning transmission electron microscopy with geometric aberrations corrected up to the third order. Two critical structural parameters, the threshold grain size corresponding to the upper size limit of the FePt grains with zero chemical ordering and the sub-nanometer thin interfacial impurity at grain boundaries, were identified. A new atomistic model was developed to correlate these structural characteristics with key magnetic properties such as Curie temperature, saturation magnetization, magnetocrystalline anisotropy, and their grain-to-grain variation. The model shows good agreement with the experimental magnetic data and explains the gap in magnetic properties between the bulk and nanostructured FePt.

Efficient excitation and amplification of the surface plasmons

One dimensional (1D) grating has been fabricated (using focused ion beam) on 50 nm gold (Au) film deposited on higher refractive index Gallium phosphate (GaP) substrate. The sub-wavelength periodic metal nano structuring enable to couple photon to couple with the surface plasmons (SPs) excited by them. These grating devices provide the efficient control on the SPs which propagate on the interface of noble metal and dielectric whose frequency is dependent on the bulk electron plasma frequency of the metal. For a fixed periodicity (Λ = 700 nm) and slit width (w = 100 nm) in the grating device, the efficiency of SPP excitation is about 40% compared to the transmission in the near-field. Efficient coupling of SPs with photon in dielectric provide field localisation on sub-wavelength scale which is needed in Heat Assisted Magnetic recording (HAMR) systems. The GaP is also used to emulate Vertical Cavity Surface emitting laser (VCSEL) in order to provide cheaper alternative of light source being used in HAMR technology. In order to understand the underlying physics, far-and near-field results has been compared with the modelling results which are obtained using COMSOL RF module.Apart from this, grating devices of smaller periodicity (Λ = 280 nm) and slit width (w = 22 nm) has been fabricated on GaP substrate which is photoluminescence material to observe amplified spontaneous emission of the SPs at wavelength of 805 nm when the grating device was excited with 532 nm laser light. This observation is unique and can have direct application in light emitting diodes (LEDs).

Electromagnetic and Thermomechanical Analysis of Near-Field Heat Transfer in Heat-Assisted Magnetic Recording Heads

Heat-assisted magnetic recording (HAMR) is a relatively new data storage technology that promises to satisfy the increasing demands for higher storage densities. Heat transfer across the nanometer-size head-disk gap does not follow classical heat transfer principles during the operation of an HAMR hard-disk drive. The HAMR technology relies on laser heating of a sub-diffraction-limit region of the magnetic medium to lower its coercivity and allow the magnetic head to write data in the form of tightly packed bits. Near-field heat transfer strongly affects the thermal protrusion produced by laser heating. A typical HAMR head model with all key components was used in a coupled electromagnetic-thermomechanical analysis to investigate the effect of near-field heat transfer on the thermal characteristics of the magnetic head. The effects of various heat sources and read/write conditions on the development of the thermal protrusion and the surface temperature distribution at the trailing edge of the slider body are examined in the context of simulation results.

Nanostructure, structural stability, and diffusion characteristics of layered coatings for heat-assisted magnetic recording head media

The intense laser heating in heat-assisted magnetic recording (HAMR) has been a major hindrance to HAMR technology from becoming commercially viable. Thermal damage of the near-field transducer (NFT) and write pole (WP) embedded in the trailing edge of the magnetic head due to failure of the protective carbon overcoat after prolonged heating at an elevated temperature are major obstacles. Therefore, the main objective of this study was to develop an effective coating method for HAMR heads. This was accomplished by introducing a new class of layered coatings consisting of ultrathin amorphous carbon (a-C) overcoat, adhesion (SiN) layer, and buffer (NiCr or TaOx) layer sequentially deposited onto Au and FeCo base layers to mimic the layer stacking of NFT and WP elements, respectively. The structural stability of the a-C overcoats and diffusion characteristics of each comprising layer under conditions of heating at 350 °C for 30 min in an Ar atmosphere were investigated by high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). For most stacking configurations the HRTEM/STEM and EELS results generally revealed some layer intermixing and minute carbon atom rehybridization in the heated a-C overcoats. The findings of this investigation suggest that further optimization of the developed layered coatings can provide a viable solution to thermal damage of HAMR heads.

Effect of Viscoelasticity on Lubricant Behavior Under Heat-Assisted Magnetic Recording Conditions

In Heat-Assisted Magnetic Recording (HAMR) technology, the lubricant layer coating on the disk is exposed to severe thermal conditions, leading to evaporation, depletion, and chemical degradation. In general, those studying the effects of laser exposure on lubricant depletion and recovery have assumed the lubricant to be a viscous fluid and have modeled its behavior using lubrication theory. However, PFPE lubricant depletion and recovery behavior at the timescale of HAMR conditions (microsecond to millisecond) is known to be that of a viscoelastic fluid. In this paper, we introduce a modification to the traditional lubrication equation that accommodates viscoelastic effects. The results suggest that this method is numerically unstable for small laser spot sizes, close to the target of HAMR. Accordingly, we developed a novel approach to model the viscoelastic depletion and recovery behavior of PFPE ultra-thin films using a Finite Element Analysis. We show that this new method is able to model the entire range of material viscoelasticity, from purely viscous to purely elastic extremes. The results show that the viscoelastic effects become remarkably pronounced with a decrease in laser spot size. For the micron-size laser spots, close to typical experimental conditions, the lubricant behaves like a viscous fluid. For the laser spot size of 20 nm, close to the target of HAMR, it behaves like an elastic material. In exposing the consequences of this viscoelastic behavior, this study predicts that lubricant flow due to thermo-capillary effects will not be a significant issue in the development of the HAMR technology. Rather, future efforts should concentrate on the thermal degradation and evaporation effects upon HAMR lubricants.

Simulation of magnetic footprints for heat assisted magnetic recording

The heat assisted magnetic recording (HAMR) technology has been the promising candidate to overcome the thermal stability limitation at higher capacities of the hard disk drive. In this work, the characteristics of magnetic footprint of the medium written by HAMR were investigated through the micromagnetic simulations. The Voronoi granular media was firstly modeled, then the magnetic footprint technique was performed to observe the media behaviors at various linear densities. The results indicated that the pattern of magnetic footprint can be perverted at higher densities, which essentially causes a reduction of readback signal power. Also, the dependence of media grain size on the signal power shows that the larger grain size media can provide higher signal power at low linear density, while the smaller grain size media gives higher signal at high density. Thus, the magnetic footprint regarding the HAMR technology needs to be optimized to achieve the efficient recording system.

Achievement and expected issues in heat assisted magnetic recording

The main goals in magnetic recording are the continuous increase of the areal recording density and the highest possible signal-to-noise ratio (SNR). The conventional magnetic recording has approached its physical limits. Further growth of the areal density is limited by the superparamagnetic effect and by the limited possibilities to further improve write heads design and pole materials in order to enhance the writing field. The perpendicular magnetic recording (PMR) with pole head and perpendicular media is the best alternative, especially to defer the superparamagnetic limit. PMR is also better situated to face the challenging design trilemma of magnetic recording: To increase the areal recording density, smaller grain volumes are needed, but to ensure the thermal stability of recorded information, the anisotropy should be increased accordingly; or, the increased anisotropy asks for higher writing fields, which are unavailable with the saturation magnetization of the magnetic materials of the current heads. Obviously, an alternative technology is needed to overcome the physical limit of conventional perpendicular recording (CPR). The most promising successor of CPR is the heat–assisted magnetic recording (HAMR), which is a multidisciplinary technology, a combination of magnetic and optical recording technology that proved experimentally areal densities of more than 1 Tb/in2 . HAMR allows the use of very smallgrain media, required for recording at ultra-high densities, with a larger magnetic anisotropy at room temperature, thus assuring a good thermal stability, while the local heating leads to a temporary magnetic softening of the medium. The realization of HAMR involved some challenges from both optical design and media properties. In the optical design the use of near-field transducers (NFT) represents an important advance. Regarding the media, a thermal design was used to produce the heating and cooling of the media within a very short time, about 1 ns, in order to achieve the desired data rate and generate a large thermal gradient for sharp bit edge definition. The structure of a HAMR system is briefly discussed, as well as the processes characterizing the writing process. A special attention is paid to the requirements for the materials needed for this type of recording: the granular L10-ordered FePt:X alloy sputtered on glass disk, the FePt-C and FePtC-Ag granular films and the bitpatterned media. Some important challenges of the HAMR technology are also summarized.