High-density Magnetic Recording Research Articles

Generation of a super-oscillation magnetization hotspot by reversing the electric dipole array radiation

We propose an all-optical approach to realize a super-oscillatory longitudinal magnetization hotspot with about 0.1λ in a 4π focusing system using time-reversed dipole radiation and vector diffraction theory calculations. Such a magnetization behavior is induced by focal fields of opposite polarities in the central and peripheral regions based on the inverse Faraday effect. By regulating the destructive interference in the central and peripheral regions of the focal field, a super-oscillating magnetization field is generated. Moreover, two important parameters affect the size of the magnetization field are also explored. Superoscillating magnetization hotspots in the focal region can be achieved by either increasing δ (the intensity of electric field) or reducing dn (the distance of electric field). The achieved ultrasmall magnetization spot shows great prospects in high-density magnetic recording, magnetic resonance imaging, and magnetic sensors.

Room-temperature ferromagnetism, half-metallicity and spin transport in monolayer CrSc2Te4-based magnetic tunnel junction devices.

The discovery of novel two-dimensional (2D) half-metallic materials with a robust ferromagnetic (FM) order and a high Curie temperature (Tc) is attractive for the advancement of next-generation spintronic devices. Here, we propose a monolayer with stable 2D intrinsic FM half-metallicity, i.e., the CrSc2Te4 monolayer, which was constructed by intercalating a monolayer of 1T-CrTe2-type sandwiched between two layers of 2H-ScTe2 monolayers. Our calculations reveal that it exhibits exceptional dynamical, thermal, and mechanical stabilities accompanied by a robust half-metallicity characterized by a wide bandgap of 1.02 eV and FM ordering with a high Tc of 326 K. Notably, these properties remain intact in almost the entire range of the biaxial strain from -5% to 5%. Furthermore, our investigations demonstrate excellent spin transport capabilities, including an outstanding spin-filtering effect, and a remarkably high tunneling magnetoresistance ratio peaking at 6087.07%. The remarkable magnetic features of the 2D CrSc2Te4 monolayer with room temperature FM, intrinsic half-metallicity, and 100% spin-polarization make it a promising candidate for the next-generation high-performance spintronic nanodevices as well as high-density magnetic recording and sensors.

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.

Pattern constraints limiting nonlinear transition shift in high density magnetic recording

Nonlinear transition shift (NLTS) produced by the interaction between transitions is a major source of distortions in magnetic recording. NLTS must be reduced to realize higher density and maintain signal quality. However, there is some confusion about the production and behavior of NLTS in previous reports. In this paper, combined with micromagnetic simulation, we have developed a complete theoretical model to predict NLTS on perpendicular magnetic media. The results show that one-level NLTS (caused by one previous transition) will monotonically decrease as the present transition becomes further away from the previous one, and multilevel NLTS (caused by consecutive previous transitions) will firstly increase and quickly approach a constant value as there are more consecutive transitions. Particularly, it is demonstrated that NLTS and its impact on the media signal-to-noise ratio can be significantly limited by pattern constraints.

Magnetization properties of radially polarized Bessel-Gaussian vortex beams with radial phase modulation in a 4π focusing system.

This paper explored the optically induced magnetization properties of radially polarized Bessel-Gaussian vortex beams with radial phase modulation in a 4π high numerical aperture (NA) focusing system, which is based on the vector diffraction theory and the inverse Faraday effect. The results show that in the case of radial modulation parameter L=0, one longitudinal magnetization chain with adjustable length can be obtained by modulating the truncation parameter β. When the radial modulation parameter L=1.3, two magnetization chains can be obtained by modulating the truncation parameter β. By modulating the radial modulation parameter L, two magnetization chains along the optical axis can be generated, each with four dark magnetic traps; meanwhile, the spacing between two magnetization chains can be adjusted. These results may be helpful in high-density all-optical magnetic recording, atom capture, and magnetic resonance microscopy.

Leveraging Journaling File System for Prompt Secure Deletion on Interlaced Recording Drives

With the growing awareness of secure computation, more and more users want to make their digital footprints securely deleted and irrecoverable after updating or removing files on storage devices. To achieve the effect of secure deletion, overwritten-based secure deletion techniques have been proposed to overwrite invalidated storage space with scrambled data content. Nevertheless, overwritten-based secure deletion requires explicit write requests for rewriting invalidated storage space with random data, thus incurring additional internal write traffic on storage devices. Recently, this inefficient aspect of secure deletion is exacerbated by the emergence of high-density interlaced magnetic recording (IMR) technology. IMR technology enhances the storage density of hard disk drives by interlacing narrower top tracks on wider bottom tracks. Owing to the interlaced track layout, securely erasing invalidated data on bottom tracks interferes with adjacent top tracks and track rewrites are required to preserve valid data on top tracks; thus, overwritten-based secure deletion on IMR drives significantly enlarges the internal write traffic and impairs the secure deletion efficiency. In this paper, instead of directly alleviating the constraint induced by the interlaced track layout, we propose leveraging the interlaced track layout of IMR drives in combination with the journaling data stream of journaling file systems to enable prompt secure data deletion while minimizing the write traffic of secure deletion operations. Experimental results suggest that the performance improvement achieve by the proposed prompt secure deletion (P-SD) strategy is 51.84% on average, when compared with the previous TrackLace approach on IMR drives.

A Universal SMR-aware Cache Framework with Deep Optimization for DM-SMR and HM-SMR Disks

To satisfy the enormous storage capacities required for big data, data centers have been adopting high-density shingled magnetic recording (SMR) disks. However, the weak fine-grained random write performance of SMR disks caused by their inherent write amplification and unbalanced read–write performance poses a severe challenge. Many studies have proposed solid-state drive (SSD) cache systems to address this issue. However, existing cache algorithms, such as the least recently used (LRU) algorithm, which is used to optimize cache popularity, and the MOST algorithm, which is used to optimize the write amplification factor, cannot exploit the full performance of the proposed cache systems because of their inappropriate optimization objectives. This article proposes a new SMR-aware cache framework called SAC+ to improve SMR-based hybrid storage. SAC+ integrates the two dominant types of SMR drives—namely, drive-managed and host-managed SMR drives—and provides a universal framework implementation. In addition, SAC+ integrally combines the drive characteristics to optimize I/O performance. The results of evaluations conducted using real-world traces indicate that SAC+ reduces the I/O time by 36–93% compared with state-of-the-art algorithms.

Thermal Modulation of Magnetization Dynamics in Nanometer-Thick L10-FePt Nanogranular and Continuous Films for High-Density Magnetic Recording Media

Thermal modulation of magnetism in magnetic recording media is a novel technique to increase the recording density. L10-FePt is a promising material for the media; however, experimental observation of its magnetization dynamics characterized by the Gilbert damping constant α is technically difficult because the dynamics is in the terahertz (THz) frequency range. A cutting edge technique in THz spintronics, an all-optical pump–probe method, leads us to achieve the detection of THz magnetization dynamics in nanometer-scale thin films with thermal heating. Here, thermal modulation of the magnetization dynamics for L10-FePt nanogranular and continuous films is discussed using the all-optical method. The effective damping constant for the nanogranular film decreased from 0.18 to 0.064 with increasing the temperature, and the thermal modulation decreased the extrinsic contribution, while this increased α above 530 K. This observation regarding the thermal modulation of the magnetic damping is valuable to design future high-density magnetic recording media.

A Unified, SNR-Aware SC-LDPC Code Design With Applications to Magnetic Recording

Spatially coupled (SC)-low-density parity-check (LDPC) codes are known to have outstanding error-correction performance and low decoding latency, which make them an excellent choice for high-density magnetic recording (MR) technologies. Whereas previous works on LDPC and SC-LDPC codes mostly take either an asymptotic or a finite-length design approach, we propose a unified framework for jointly optimizing the codes’ thresholds and cycle counts to address both regimes. We focus on circulant-based (CB) SC-LDPC code family as a representative, high-performance exemplar of structured SC-LDPC codes. The framework is based on efficient traversal and pruning of the code search space, building on the fact that the performance of a CB SC-LDPC code depends on some characteristics of the code’s partitioning matrix, which by itself is much smaller than the code’s full parity-check matrix. We then propose an algorithm that traverses all non-equivalent partitioning matrices and outputs a list of codes, each offering an attractive point on the trade-off between asymptotic and finite-length performance. Our simulations show that our framework results in SC-LDPC codes that outperform the state-of-the-art constructions, over both additive white Gaussian noise (AWGN) and partial response (PR) channel models, and that it offers the flexibility to choose low-signal-to-noise ratio (SNR), high-SNR, or in- between SNR region considering system requirements, e.g., that of the MR device.

Effect of Zn2+-Sn4+ co-substitution on structural and magnetic properties of SrFe12-2xZnxSnxO19 (x = 0–2) M-type strontium ferrite

We studied the structure and magnetic properties of Zn2+-Sn4+ co-substituted M-type strontium ferrite by X-ray diffraction (XRD), Fourier transformer infrared (FT-IR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), and superconducting quantum interference device magnetometer (SQUID-VSM). The lattice constants a and c increase monotonically with increasing substitution x, which is primarily attributed to the fact that the ionic radii of Zn2+ and Sn4+ are larger than those of Fe3+. The room temperature magnetic measurement results indicate that the saturation magnetization (Ms) can reach the maximum value Ms = 77 emu/g when x = 0.25, originating from the occupation of Zn2+ and Sn4+ in the spin-down 4f1 and 4f2 sites, respectively. Next, when x ≥ 0.5, the nonmagnetic Zn2+ and Sn4+ weaken the superexchange interaction, resulting in a monotonic decrease in saturation magnetization and coercivity. The sample with x = 0.25 has the optimum magnetic properties of Ms = 77 emu/g, Mr = 35 emu/g and Hc = 2203 Oe, and it is a promising candidate for high-density magnetic recording materials.

Structural characterization of ultrathin diamond-like carbon overcoats for high areal density magnetic recording

This paper reports a comprehensive study on chemical structures of ultrathin diamond-like carbon (DLC) overcoats for high areal density magnetic recording. The DLC overcoats were fabricated in a range of thicknesses between 0.5 and 5.0 nm with Si interlayer on NiFe substrates using a pulsed filtered cathodic vacuum arc (PFCVA) technique. The overcoat thicknesses were accurately determined by high–resolution transmission electron microscopy and wavelength dispersive X-ray fluorescence techniques. The surface morphology, chemical composition, microstructure, and tribological properties were systematically investigated by atomic force microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, Raman spectroscopy techniques, and a reciprocating wear test. The results showed that the surface morphologies of DLC overcoats were atomically smooth with a root mean square surface roughness below 0.23 nm. The DLC overcoats had high sp3 bonds fraction. However, as the overcoat thickness was thinner than 1 nm, the sp2 bonds fraction suddenly increased resulting in a significant increase in the coefficient of friction. The obtained results demonstrated the limitation of the thickness of the DLC overcoat fabricated by the PFCVA technique as a protective coating for high–density magnetic storage devices.

High repetition granular Co/Pt multilayers with improved perpendicular remanent magnetization for high-density magnetic recording

Thanks to the strong perpendicular magnetic anisotropy (PMA), excellent processing compatibility as well as novel spintronic phenomenon, Co/Pt multilayers have been attracting massive attention and widely used in magnetic storage. However, reversed magnetic domains come into being with the increasing layer repetition ‘N’ to reduce magneto-static energy, resulting in the remarkable diminishment of the remanent magnetization (M r). As a result, the product of M r and thickness (i.e., the remanent moment-thickness product, M r t), a key parameter in magnetic recording for reliable data storing and reading, also decreases dramatically. To overcome this issue, we deposit an ultra-thick granular [Co/Pt]80 multilayer with a total thickness of 68 nm on granular SiN x buffer layer. The M r t value, M r to saturation magnetization (M s) ratio as well as out of plane (OOP) coercivity (H coop) are high up to 2.97 memu/cm2, 67%, and 1940 Oe (1 Oe = 79.5775 A⋅m−1), respectively, which is remarkably improved compared with that of continuous [Co/Pt]80 multilayers. That is because large amounts of grain boundaries in the granular multilayers can efficiently impede the propagation and expansion of reversed magnetic domains, which is verified by experimental investigations and micromagnetic simulation results. The simulation results also indicate that the value of M r t, M r/M s ratio, and H coop can be further improved through optimizing the granule size, which can be experimentally realized by manipulating the process parameter of SiN x buffer layer. This work provides an alternative solution for achieving high M r t value in ultra-thick Co/Pt multilayers, which is of unneglectable potential in applications of high-density magnetic recording.

Enhanced Magnetic Properties of Electrodeposited Co2FeSn Film with High Structural Order

Near stoichiometric Co2FeSn Heusler alloy films with a highly ordered L2 1 structure and mean grain size of 23 ± 1 nm have been prepared on a polycrystalline copper substrate for the first time by electrodeposition route. Deposition potential −4.0 V yielded the stoichiometric alloy composition in the potentiostatic mode. The highly disordered as-deposited alloy film crystallized into the ordered stable Heusler alloy structure upon heat treatment under vacuum. The heat-treated film exhibited the highest magnetic moment (5.18 ± 0.04 μ B/f.u. at 5 K) and Curie temperature (1123 K) ever achieved in this alloy in any form. The very high value of K eff (∼106 erg c.c.−1) obtained for this alloy makes this alloy promising for high-density magnetic recording application. Ab initio studies using GGA + U approach provide information on the minority gap near the Fermi level and its tunability as a function of the electron correlation factor represented by the Hubbard parameter U.

Aluminum-titanium-cobalt substituted epsilon iron oxide nanosize hard magnetic ferrite for magnetic recording and millimeter wave absorption

We synthesized aluminum-titanium-cobalt substituted epsilon iron oxide (ATC-ε-Fe2O3), ε-Al0.11Ti0.04Co0.05Fe1.80O3 (1), ε-Al0.14Ti0.04Co0.05Fe1.77O3 (2), and ε-Al0.26Ti0.04Co0.05Fe1.65O3 (3). The magnetic coercive field (Hc) values are 7.1 kOe (1), 6.5 kOe (2), and 4.4 kOe (3). Zero-field ferromagnetic resonance frequencies (fr) were observed at 112 GHz (1), 109 GHz (2), and 100 GHz (3) using terahertz time-domain spectroscopy. These Hc and fr values are suitable for high-density magnetic recording and high-frequency millimeter wave absorbers. Additionally, the present series is non-toxic, low-cost, and could be used as practical magnetic fillers for magnetic recording and millimeter wave absorber applications.

Influence of Bi3+ ions substitution on structural, magnetic, and electrical properties of lead hexaferrite

The Microwave induced sol-gel auto-combustion process was used to make lead hexaferrite, PbFe12-xBixO19 (x=0.2 to 0.8), with Bi3+ as a replacement. X-ray powder diffractometer (XRD), scanning electron microscopy (SEM), vibrating sample Magnetometry (VSM), Fourier transform infrared spectroscopy (FT-IR), and Impedance analysis was used to test the impact of added Bi3+ ions on the structure, morphology, magnetic, and dielectric properties of lead hexaferrite. Single-phase hexagonal ferrite nanoparticles with average crystalline sizes of 43 to 59 nm were achieved with the space group of P63/mmc. The magnetic analysis revealed that when the Bi3+ ion is supplemented in the Pb ferrite,the coercive and saturation magnetization of the material was amplified. The substance's dielectric constant, electric conductivity, and loss tangent were calculated for frequencies of 20 Hz to 10 MHz, indicating that it is appropriate for wide applications like permanent magnets, high-density magnetic recording media.

Influence of Chemical Substitution and Light Element Insertion on the Magnetic Properties of Nanocrystalline Pr2Co7 Compound

It is well recognized that intermetallics based on rare-earth (R) and transition metals (T) display numerous interesting magnetic properties, leading to potential applications in different fields. The latest progress regarding magnetic properties and the magnetocaloric effect (MCE) in the nanostructured Pr2Co7 compound, as well as its carbides and hydrides, is reviewed in this paper. Some of this progress reveals remarkable MCE performance, which makes it attractive in the field of magnetic refrigeration at high temperatures. With the purpose of understanding the magnetic and magnetocaloric characteristics of these compounds, the crystal structure, microstructure, and magnetism are also brought into focus. The Pr2Co7 compound has interesting magnetic properties, such as a high Curie temperature TC and uniaxial magnetocrystalline anisotropy. It crystallizes in a hexagonal structure (2:7 H) of the Ce2Ni7 type and is stable at relatively low temperatures (Ta ≤ 1023 K), or it has a rhombohedral structure (2:7 R) of the Gd2Co7 type and is stable at high temperatures (Ta ≥ 1223 K). Studies of the magnetocaloric properties of the nanocrystalline Pr2Co7 compound have shown the existence of a large reversible magnetic entropy change (ΔSM) with a second-order magnetic transition. After its substitution, we showed that nanocrystalline Pr2Co7−xFex compounds that were annealed at Ta = 973 K crystallized in the 2:7 H structure similarly to the parent compound. The extended X-ray absorption fine-structure (EXAFS) spectra adjustments showed that Fe atoms preferably occupy the 12k site for x ≤ 1. The study of the magnetic properties of nanocrystalline Pr2Co7−xFex compounds revealed an increase in TC of about 26% for x = 0.5, as well as an improvement in the coercivity, Hc (12 kOe), and the (BH)max (9 MGOe) product. On the other hand, the insertion of C atoms into the Pr2Co7 cell led to a marked improvement in the TC value of 21.6%. The best magnetic properties were found for the Pr2Co7C0.25 compound annealed at 973 K, Hc = 10.3 kOe, and (BH)max = 11.5 MGOe. We studied the microstructure, hydrogenation, and magnetic properties of nanocrystalline Pr2Co7Hx hydrides. The crystal structure of the Pr2Co7 compound transformed from a hexagonal (P63/mmc) into an orthorhombic (Pbcn) and monoclinic (C2/c) structure during hydrogenation. The absorption of H by the Pr2Co7 compound led to an increase in the TC value from 600 K at x = 0 to 691 K at x = 3.75. The Pr2Co7H0.25 hydride had optimal magnetic properties: Hc = 6.1 KOe, (BH)max = 5.8 MGOe, and TC = 607 K. We tailored the mean field theory (MFT) and random magnetic anisotropy (RMA) methods to investigate the magnetic moments, exchange interactions, and magnetic anisotropy properties. The relationship between the microstructure and magnetic properties is discussed. The obtained results provide a fundamental reference for adapting the magnetic properties of the Pr2Co7, Pr2Co6.5Fe0.5, Pr2Co7C0.25, and Pr2Co7H0.25 compounds for potential permanent nanomagnets, high-density magnetic recording, and magnetic refrigeration applications.

Analysis of structural, electric and magnetic features of Bi3+ substituted nanocrystalline calcium hexaferrite

Calcium hexaferrite (CaFe12−xBixO19, x=0.2–0.8) was successfully brewed through a microwave-induced sol–gel auto-combustion technique with Bi3+ as a substitute. The impact of added Bi3+ ions on the structure, morphology, magnetic, and dielectric properties of calcium hexaferrite was analyzed using an X-ray powder diffractometer (XRD), Field emission scanning electron microscopy (FE-SEM), vibrating sample Magnetometry (VSM), Fourier transforms infrared spectroscopy (FT-IR), and impedance study. The peak position in the XRD pattern confirms the development of single-phase hexagonal ferrite nanoparticles with a space group of P63/mmc and an average crystalline size of 46–56 nm. Magnetic analysis revealed that when the Bi3+ ion is added to Ca ferrite, the material's coercive force improves, but the saturation magnetization decreases. The dielectric constant, electric conductivity, and loss tangent of the substance were determined for frequencies ranging from 20 Hz to 1 MHz, indicating appropriate for various applications, including permanent magnets and high-density magnetic recording media.

Effect of Yb doping on the structural and magnetic properties of cobalt ferrite nanoparticles

The low coercivity of spinel ferrites is a significant barrier limiting their use in high-density magnetic recording applications. The large magnitude of the magnetocrystalline anisotropy of the rare earth group elements can lead to a high coercivity field by doping the magnetic ferrites with these elements. In this research, using the hydrothermal method, Fe+3 in cobalt ferrite was substituted with Yb+3 to make CoFe2-xYbxO4 with x = 0, 0.025, 0.05, 0.075, and 0.1. By analyzing the X-ray diffraction analysis and confirming the spinel structure, the crystallite sizes were obtained in the range of 31 nm to 45 nm. The lattice parameter was estimated to be in the range of 8.30 nm to 8.36 nm. The FTIR analysis of the nanoparticles showed the ν1 absorption band in the range 584 Cm−1 to 594 Cm−1. The Raman spectroscopy analysis showed that CoFe2-xYbxO4 nanoparticles have a mixed spinel structure. Hysteresis loops at room temperature and 10 K obtained by the vibrating sample magnetometer (VSM) indicate that the coercivity field (HC) increases significantly from 0.55 to 1.05 KOe and 9.48 to 12.00 KOe, respectively, by increasing Yb doping from x = 0 to x = 0.1.

High-resolution alternating magnetic force microscopy using an amorphous FeB-based tip driven by an inverse magnetostrictive effect: Imaging of the high-density magnetic recording media

The improved spatial resolution of the alternating magnetic force microscopy (A-MFM) technique developed in this work is demonstrated using a tip coated with a very thin magnetostrictive Fe-B-based amorphous film. Its magnetization can be periodically reversed using an applied alternating current magnetic field, which leads to alternating mechanical stress at the tip end, thus affecting localized magnetization reversal via an inverse magnetostrictive effect. Spatial resolution improvement is achieved via effective reversible magnetization volume at the sharpened tip end, which promotes a decrease in the total magnetic volume and maintains sufficient sensitivity at a lower coating thickness. A-MFM uses frequency modulation induced by an off-resonant alternating magnetic force between the tip and the sample. Therefore, a magnetic force can be selectively detected near the sample surface, independent of non-magnetic interactions. Due to their smooth surfaces without crystalline grain boundaries, large magnetization, and large magnetostriction compared with those of other magnetic materials, Fe-based amorphous films are suitable for tip coating applications. A 500-kfci perpendicular magnetic recording medium (PMR) was imaged using a 6-nm thick amorphous Fe-B coated tip to produce highly resolved images with minimal detection of the wavelength of the magnetic field of less than 10 nm. Also, 1600-kfci PMR imaging was successfully achieved, with the images showing individual ∼ 15-nm bits.

Double-Exchange Bias Modulation under Horizontal and Perpendicular Field Directions by 3D Nanocomposite Design.

Exchange bias (EB) presents the interfacial coupling between ferromagnetic (FM) and antiferromagnetic (AFM) phases, which could be applied for high-density data storage and magnetic recording. In thin films, the EB effect could be realized in either a FM/AFM multilayer structure or a FM/AFM vertically aligned nanocomposite (VAN) form, which allows the interfacial coupling tuning along the horizontal or perpendicular directions, respectively. Here, to combine the schemes of multilayer and VAN structures, a new 3D nanocomposite has been designed, which is La0.7Sr0.3MnO3 (LSMO)/NiO VAN layers with inserted LSMO or NiO layers. Such a 3D nanocomposite structure provides a great platform to tailor the EB effect along both horizontal and perpendicular directions. Specifically, the sample with a NiO interlayer exhibits the highest EB field (HEB) of 350 Oe and 475 Oe under in-plane and out-of-plane field, respectively. Furthermore, the HEB value and Curie temperature (Tc) can be tuned by different 3D nanostructures. This work demonstrates the double EB modulation with the designed 3D nanostructures as a new route toward advanced magnetic data storage and spintronic devices.