Areal Storage Density Research Articles

Graphene overcoats for ultra-high storage density magnetic media

Hard disk drives (HDDs) are used as secondary storage in digital electronic devices owing to low cost and large data storage capacity. Due to the exponentially increasing amount of data, there is a need to increase areal storage densities beyond ~1 Tb/in2. This requires the thickness of carbon overcoats (COCs) to be <2 nm. However, friction, wear, corrosion, and thermal stability are critical concerns below 2 nm, limiting current technology, and restricting COC integration with heat assisted magnetic recording technology (HAMR). Here we show that graphene-based overcoats can overcome all these limitations, and achieve two-fold reduction in friction and provide better corrosion and wear resistance than state-of-the-art COCs, while withstanding HAMR conditions. Thus, we expect that graphene overcoats may enable the development of 4–10 Tb/in2 areal density HDDs when employing suitable recording technologies, such as HAMR and HAMR+bit patterned media

Maximum Likelihood Detection for 3-D-MAMR

3-D magnetic recording is a candidate next-generation magnetic recording where multiple recording layers are used, each layer storing its own independent data, which has the potential to extend the storage areal density while having several challenges to overcome. On the writing side, one proposal is to use the microwave-assisted magnetic recording that energizes a particular layer in the stack with a field from a spin-torque oscillator to select a specific layer for writing. Readback is also a challenge as the standard reader picks up a superposition of signals from the multiple layers. The data from each layer have to be accurately recovered despite the high levels of interference between the layers. In this article, we assess the impact of such interference and look for ways to mitigate it to realize the gains of multilayer recording. We perform an initial study on the capacity of the summation of a pair of binary channels, followed by a model for a two-layer recording system with the lower layer having worse dispersion. Our study indicates the potential for some net areal-density gains from the introduction of the second layer that could lead to the increased longevity of the magnetic recording industry.

Transfer Behaviors of PFPE Lubricant from Disk Surface to Slider

The rapid growth in storage areal density of hard disk drive (HDD) forces the flying height of slider has been currently reduced to less than 2 nm. In such a narrow spacing, perfluoropolyether (PFPE) lubricant is easier to transfer from disk surface to slider under fluid force field and heat transfer field, which is found to have a great effect on date storage reliability. Firstly, the lubricant transfer volume expression with respect to lubricant molecular weight is established. Secondly, the finite element models (FEM) of both Tri-pad positive pressure slider and flat slider are developed, and based on this, the PFPE lubricant transfer behaviors in HDD are investigated. Finally, four type unite cells composed of lubricant molecules and air molecules are established by using Materials Studio software, then both dynamic temperature and diffusion coefficients are researched based on molecular dynamics theory. Results show that the PFPE lubricant amount decreases with increasing pitch angles, rotation angles, slider flying height and disk rotation velocity, while it increases when heat source power increases. The diffusion coefficients of ZTMD lubricant is the maximum in the four lubricants we researched, while the temperature change ratio of Z-15 lubricant is the maximum.

Design of Single-Molecule Multiferroics for Efficient Ultrahigh-Density Nonvolatile Memories.

It is known that an isolated single‐molecule magnet tends to become super‐paramagnetic even at an ultralow temperature of a few Kelvin due to the low spin switching barrier. Herein, single‐molecule ferroelectrics/multiferroics is proposed, as the ultimate size limit of memory, such that every molecule can store 1 bit data. The primary strategy is to identify polar molecules that possess bistable states, moderate switching barriers, and polarizations fixed along the vertical direction for high‐density perpendicular recording. First‐principles computation shows that several selected magnetic metal porphyrin molecules possess buckled structures with switchable vertical polarizations that are robust at ambient conditions. When intercalated within a bilayer of 2D materials such as bilayer MoS2 or CrI3, the magnetization can alter the spin distribution or can be even switched by 180° upon ferroelectric switching, rendering efficient electric writing and magnetic reading. It is found that the upper limit of areal storage density can be enhanced by four orders of magnitude, from the previous super‐paramagnetic limit of ≈40 to ≈106 GB in.−2, on the basis of the design of cross‐point multiferroic tunneling junction array and multiferroic hard drive.

Microstructure Analysis on Size Distribution During Film Growth in HAMR Media

Grain size and size distribution reduction in L10 FePt granular thin film is becoming critical to increase the areal storage density and realize the full potential of heat-assisted magnetic recording media. FePt and amorphous carbon or mixture with other segregants are often deposited at elevated temperature in order to promote the L10 ordering in FePt. Due to the materials system and high temperature involved in the fabrication process, the bimodal distribution of the grains is often observed with a significant amount of small grains with diameter <3 nm. The bimodal distribution of the grains could potentially lead to a significant microstructure nonuniformity and thus grain-to-grain properties variation, such as Curie temperature (Tc), texture, ordering, and magnetic properties. In this paper, we performed a detailed analysis on microstructure evolution of FePt-C thin film during the growth process on different initial layer conditions to understand the origin of bimodal grain size distribution in the FePt-C media. Experimental modeling study of depositing FePt on continuous carbon layer at elevated temperature was conducted to understand the nucleation and growth process.

Setting Write Spacing in Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) has the potential to deliver higher storage areal density beyond 1 Tb/in2. A laser is integrated in the flying read/write head to heat up the magnetic media using a near field transducer (NFT). A large portion of the laser energy is absorbed inside the head, resulting in elevated temperature around the NFT. Thermal protrusions of different magnitudes and time scales occur after turning on the laser. These laser heating-induced protrusions must be characterized for each head and compensated in HAMR drives. In this paper, a magnetic recording-based method is proposed to perform in situ protrusion measurements and assist in setting the write spacing. This method has been successfully demonstrated on the spinstand and implemented into HAMR drives.

A Soft-5/6 Modulation Code With Iterative ITI Subtraction Scheme in Multireader TDMR Systems

The huge growth of the consumer electronic industry is driving the need for achieving ultrahigh areal storage densities. 2-D magnetic recording is a promising high-density storage technology where the shingled writing technique is employed for increasing the track per inch (TPI) and achieving higher areal densities. Nevertheless, high TPI causes a reader to cover higher number of tracks, which leads to the intertrack interference (ITI); hence the system performance is severely deteriorated. Therefore, we first propose the soft-5/6 modulation code together with iterative ITI subtraction scheme. Here, the soft information encoding and decoding schemes operate based on Boolean logic circuit for using in the iterative system. Furthermore, we also propose to move the upper and lower readers closer to the center reader to avoid the ITI effect from their outer tracks, which will then be performed together with ITI subtraction scheme. Moreover, this proposed reading technique is also applied together with the soft-4/5 modulation code. The simulation results show that the proposed systems are superior to the conventional system, especially at high user density.

Thermal Response Time of Media in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording product. A laser is introduced to the HAMR system to heat magnetic media to reduce the media coercivity. The thermal response of the media becomes very critical for the success of the magnetic writing process. The study of thermal response time in HAMR relies on the setup configurations, such as laser spot sizes, the way that laser energy is delivered to media and the media structures. In this paper, the thermal response time of HAMR media under three different heating methods is systematically investigated through experiments and numerical analysis. A lumped model is built to simplify the heat conduction problem to understand the difference in thermal responses under various experimental conditions. Dominant layers are identified under those experimental conditions. The transient thermal response is mainly determined by the dominant layers. Engineering the dominant layers helps the most in optimizing the thermal performance of the media. Our study clearly suggests that for HAMR systems, optimizing the thermal properties of the heat sink layer is the key to reducing variations in the transient thermal process resulting from changes in the linear speed.

Material Transfer Inside Head Disk Interface for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording products. Laser heating is implemented in HAMR to achieve magnetic writing of the very high coercivity media. However, the high temperature environment creates several reliability challenges for the head disk interface (HDI). In this paper, material transfer within the HDI under HAMR recording conditions is studied. The mechanisms of material transfer are explored via experiments and modeling. This study revealed that temperature difference and mechanical interaction between the head and media are the main mechanisms for material transfer inside the HDI. Possible methods to remove the material are also discussed in this paper.

Study of the areal density in the read heads with spin valves with nano-oxide-layer insertion

Read heads performances have been analyzed by simulation (HFSS) in the case of the devices designed with a category of spin valve structures with nano-oxide-layer insertion, based on Co alloys. Different structures of the stack of layers in the spin valve were considered in order to increase the magnetic resistance due to contribution of the insertion layer. A parametrical evolution of the resistance-area product variation Δ RA , and of the areal storage density have been obtained using the simulation and theoretical data. Results indicate areal densities up to 780 Gbit/in 2 for the system manufactured in practice, when the variation of the resistance-area product was greater than 7.4…9 mΩ•μm 2 , strongly dependent on the nanooxide insertion layer. These performances can be overlapped by parameters correlation, indicated by simulation. Consequently, our simulations offer solutions for improving the recording performances by controlling the spin valve structure.

Basic noise mechanisms of heat-assisted-magnetic recording

Heat-assisted magnetic recording (HAMR) is expected to be a key technology to significantly increase the areal storage density of magnetic recording devices. At high temperatures, thermally induced noise becomes a major problem, which must be overcome in order to reliably write magnetic bits with narrow transitions. We propose an elementary model based on the effective recording time window (ERTW) to compute the switching probability of bits during HAMR. With few assumptions, this analytical model allows to gain deeper insights into basic noise mechanisms, like AC and DC noise. Finally, we discuss strategies to reduce noise and to increase the areal storage density of both bit-patterned and granular media.

Areal density optimizations for heat-assisted magnetic recording of high-density media

Heat-assisted magnetic recording (HAMR) is hoped to be the future recording technique for high-density storage devices. Nevertheless, there exist several realization strategies. With a coarse-grained Landau-Lifshitz-Bloch model, we investigate in detail the benefits and disadvantages of a continuous and pulsed laser spot recording of shingled and conventional bit-patterned media. Additionally, we compare single-phase grains and bits having a bilayer structure with graded Curie temperature, consisting of a hard magnetic layer with high TC and a soft magnetic one with low TC, respectively. To describe the whole write process as realistically as possible, a distribution of the grain sizes and Curie temperatures, a displacement jitter of the head, and the bit positions are considered. For all these cases, we calculate bit error rates of various grain patterns, temperatures, and write head positions to optimize the achievable areal storage density. Within our analysis, shingled HAMR with a continuous laser pulse moving over the medium reaches the best results and thus has the highest potential to become the next-generation storage device.

Heat-assisted magnetic recording of bit-patterned media beyond 10 Tb/in2

The limits of areal storage density that is achievable with heat-assisted magnetic recording are unknown. We addressed this central question and investigated the areal density of bit-patterned media. We analyzed the detailed switching behavior of a recording bit under various external conditions, allowing us to compute the bit error rate of a write process (shingled and conventional) for various grain spacings, write head positions, and write temperatures. Hence, we were able to optimize the areal density yielding values beyond 10 Tb/in2. Our model is based on the Landau-Lifshitz-Bloch equation and uses hard magnetic recording grains with a 5-nm diameter and 10-nm height. It assumes a realistic distribution of the Curie temperature of the underlying material, grain size, as well as grain and head position.

The Importance of Depth-Varying Fields for MAMR Switching-Field Reduction

Microwave assisted magnetic recording (MAMR) was proposed to increase the data storage areal density in hard disk drives [1]. With a localized AC field generated by a spin torque oscillator (STO), MAMR significantly reduces the switching field when the AC field frequency matches the ferromagnetic resonance frequency of the media elements [2]. In this study we present micromagnetic modeling of media switching-field reduction due to AC fields at GHz frequencies. We employ a 4-layer media model based on current exchange-coupled media with the top layer describing a large lateral exchange “cap” with three low lateral exchange “oxide” layers below. We examine hysteresis loops in two scenarios: depth invariant applied fields (external and AC), and depth invariant external field with depth varying AC field. We then do a DC-band write study with realistic writer field and macrospin-based STO field to gauge switching-field reduction in a scenario more similar to actual recording.

Stretchable, weavable coiled carbon nanotube/MnO2/polymer fiber solid-state supercapacitors.

Fiber and yarn supercapacitors that are elastomerically deformable without performance loss are sought for such applications as power sources for wearable electronics, micro-devices, and implantable medical devices. Previously reported yarn and fiber supercapacitors are expensive to fabricate, difficult to upscale, or non-stretchable, which limits possible use. The elastomeric electrodes of the present solid-state supercapacitors are made by using giant inserted twist to coil a nylon sewing thread that is helically wrapped with a carbon nanotube sheet, and then electrochemically depositing pseudocapacitive MnO2 nanofibers. These solid-state supercapacitors decrease capacitance by less than 15% when reversibly stretched by 150% in the fiber direction, and largely retain capacitance while being cyclically stretched during charge and discharge. The maximum linear and areal capacitances (based on active materials) and areal energy storage and power densities (based on overall supercapacitor dimensions) are high (5.4 mF/cm, 40.9 mF/cm2, 2.6 μWh/cm2 and 66.9 μW/cm2, respectively), despite the engineered superelasticity of the fiber supercapacitor. Retention of supercapacitor performance during large strain (50%) elastic deformation is demonstrated for supercapacitors incorporated into the wristband of a glove.

Modification of the structural and magnetic properties of granular FePt films by seed layer conditioning

The steadily increasing amount of digital information necessitates the availability of reliable high capacity magnetic data storage. Here, future hard disk drives with extended areal storage densities beyond 1.0 Tb/in2 are envisioned by using high anisotropy granular and chemically L10-ordered FePt (002) perpendicular media within a heat-assisted magnetic recording scheme. Perpendicular texturing of the [001] easy axes of the individual grains can be achieved by using MgO seed layers. It is therefore investigated, if and how an Ar+ ion irradiation of the MgO seed layer prior to the deposition of the magnetic material influences the MgO surface properties and hereby the FePt [001] texture. Structural investigations reveal a flattening of the seed layer surface accompanied by a change in the morphology of the FePt grains. Moreover, the fraction of small second layer particles and the degree of coalescence of the primarily deposited FePt grains strongly increases. As for the magnetic performance, this results in a reduced coercivity along the magnetic easy axis (out of plane) and in enhanced hard axis (in-plane) remanence values. The irradiation induced changes in the magnetic properties of the granular FePt-C films are traced back to the accordingly modified atomic structure of the FePt-MgO interface region.

Controlled synthesis and assembly into anisotropic arrays of magnetic cobalt-substituted magnetite nanocubes.

Cubic cobalt-substituted magnetite CoxFe3-xO4 nanocubes (NCs) with uniform composition distributions of Co, Fe and O in the NCs, obtained via solution synthesis, are reported in this paper. Through the control of the reaction conditions, the size of the cubic NCs could be tuned from 35 to 110 nm. It was found that the cubic shape could easily induce the (400) orientation of the NCs on a Si substrate, and applying an external magnetic field in the out-of-plane direction could further enhance the (400) orientation of these NCs on the Si substrate. The highest coercivity of 2.07 kOe could be obtained by assembling the NCs in the external magnetic field. The reported magnetic cobalt-substituted magnetite NCs provide an ideal class of building blocks for studying ferrimagnetic nanoparticle (NP) assemblies with easily controlled magnetic alignment for magnetic tape recording with ever increased areal storage density.

Multiferroic composites for magnetic data storage beyond the super-paramagnetic limit

Ultra high-density magnetic data storage requires magnetic grains of &amp;lt;5 nm diameters. Thermal stability of such small magnetic grain demands materials with very large magneto-crystalline anisotropy, which makes data write process almost impossible, even when Heat Assisted Magnetic Recording (HAMR) technology is deployed. Here, we propose an alternative method of strengthening the thermal stability of the magnetic grains via elasto-mechanical coupling between the magnetic data storage layer and a piezo-ferroelectric substrate. Using Stoner-Wohlfarth single domain model, we show that the correct tuning of this coupling can increase the effective magneto-crystalline anisotropy of the magnetic grains making them stable beyond the super-paramagnetic limit. However, the effective magnetic anisotropy can also be lowered or even switched off during the write process by simply altering the applied voltage to the substrate. Based on these effects, we propose two magnetic data storage protocols, one of which could potentially replace HAMR technology, with both schemes promising unprecedented increases in the data storage areal density beyond the super-paramagnetic size limit.

Investigation of the Local Temperature Increase for Heat Assisted Magnetic Recording (HAMR)

Heat assisted magnetic recording (HAMR) has been proposed for the next generation hard disk drive due to its potentially higher storage areal density. In HAMR systems, a laser is focused onto the disk to heat the magnetic storage layer to the Curie temperature. In this paper, a novel method is proposed to study the temperature increase in a perpendicular magnetic recording media. The disk was prerecorded with a uniform single tone magnetic pattern by flying a magnetic head above it and using a magnetic read/write device. The decay of the magnetic pattern resulting from laser heating under different laser power levels was evaluated quantitatively using a magnetic force microscopy. The temperature increase was estimated by use of an Arrhenius-Neel model of thermally activated magnetization. In addition, a 3-D numerical model was created to obtain the disk's thermal response under the laser heating. The thermally erased area was calculated from the temperature distribution based on the numerical results. The calculated erased area was compared with the experimental results from the magnetic images. A good agreement was observed between the experimental and the simulation results. The method should be reliable when used with HAMR media also whenever it becomes available.

Tribological Degradation of Head–Disk Interface in Hard Disk Drives Under Accelerated Wear Condition

To further increase the areal storage density of hard disk drive, one solution is to reduce the spacing between the magnetic head and disk to sub-1-nm regime. Such ultra-low spacing introduces great challenges to tribological reliability in the head-disk interface (HDI). Therefore, it is necessary to understand the characteristics and mechanisms of HDI degradation in the tribological condition. This paper investigates the degradation behaviors of HDI based on an accelerated wear experiment. Acoustic emission and thermal asperity sensors were used simultaneously to monitor the touch down dynamics before and after each cycle of the overdrive test to characterize the HDI degradation behaviors. Through analyzing the evolutions of HDI dynamic characteristics, the degradation mechanisms, including wear mechanism shift and HDI instability propagation were figured out.