High-density Magnetic Recording Media Research Articles

FePt Grain Size Limit and Required Switching Field in the Presence of Surface Anisotropy

Surface anisotropy that opposes crystalline anisotropy can become significant for highly packed grains. In this paper, we present an estimation of FePt grain size limit in the presence of surface anisotropy. The estimated grain size is optimized for rectangular, circular and hexagonal grain, respectively. In this manner, the thermally stable granular media can be attained for high areal density magnetic recording system. Micromagnetic simulations are performed to predict the required switching field for FePt granular media in the presence of surface anisotropy. The results reveal possible engineering challenges faced in designing high density magnetic recording medium.

High Coercivity FePtSiN Films With L1$_{0}$–FePt Nanoparticles Embedded in a Si-Rich Matrix

FePtSiN films consisting of FePt nanoparticles embedded in Si-rich matrix were fabricated on silicon substrates by direct current (dc) reactive magnetron sputtering followed by vacuum annealing. The effects of Si-N additions and annealing temperature on the structure and magnetic properties were investigated. The as-deposited films had face-centered cubic (fcc) structure, which transforms into the face-centered tetragonal (fct) structure after thermal annealing at 600°C. The grain size of FePt increased with the annealing temperature but decreased with increasing Si-N content. Increasing Si content led to the formation of Si-N-rich amorphous phase distributed between the FePt nanograins, which reduced the lattice distortion and increased the coercivity. The fct-FePt films annealed at 700°C exhibited very high coercivity, up to 13.6 kOe at room temperature and about 17.5 kOe at 100 K. These FePtSiN films have shown promise for high-density magnetic recording medium.

Effect of various single metallic seed-layers on the magnetic properties of Co/Pd multilayers

We studied the effects of Pd, Ru, and Ta seed-layers on the perpendicular magnetic properties of Co/Pd multilayers, promising materials for high-density magnetic recording media. Among the various seeds, the Ta seed-layer demonstrated a large coercivity and most highly promoted fcc (111) preferred orientation of the Co/Pd multilayer. We also patterned films into island arrays with sizes of 2 × 2 μm 2, 5 × 5 μm 2, and 10 × 10 μm 2 to investigate the effect of size on the magnetic properties. The coercivity increased as the island size decreased. The 2 × 2 μm 2 patterned Co/Pd multilayer with the Ta seed-layer showed a coercivity about six times greater than that of the un-patterned film. This increase can be explained by a change in the magnetization reversal mechanism from domain wall propagation to domain rotation.

Antiferromagnetically coupled capped bit patterned media for high-density magnetic recording

We report micromagnetic modeling of a bit patterned media where a two-dimensional array of patterned composite islands is antiferromagnetically coupled to a continuous capping layer. This media allows optimization of writability, switching field distributions, and readback response. Lateral and vertical exchange introduced through the coupling with the capping layer compensates the dipolar interactions between islands and antiferromagnetic coupling is employed to modulate the high-density readback response.

Morphological Controls of Goethite Particle via Wet Synthesis from Ferrous Solution Applied for High-density Magnetic Recording Media

Morphological control of goethite particles by various operation conditions in a bench scale was devised in an attempt to improve magnetic properties of the end products. Reaction parameters were systematically varied and the detailed crystallographical analyses based on X-ray diffractometry and electron micrographs were employed as principal analytical tools.When ferrous sulfate and sodium hydroxide were used, precursors immediately before goethite were Fe(OH)2 or Green rust (II), at excess or deficiency of alkali, respectively. The superiority of goethite particles is mainly attributed to adequate lateral growth along (110) as compared to the growth rate in (010). Preferential lateral growth, in turn, was predominated by the concentration of FeOH+ complex ions. Green rust (II) was stabilized by the buffering function of Fe2+ in the solution. Coexistence of green rust (II) with goethite enables steady growth of goethite preferentially in the axial direction, resulting in an increase in the aspect ratio.Since morphology of those magnetic materials is sustained from those of goethite, it is safe to conclude that proper choice of the starting ferrous source, extent of alkali excess and concentration of dissolved oxygen are the key factors for better goethite particles and, hence, better materials for magnetic recording.

ChemInform Abstract: Development of High Density Magnetic Recording Media for Hard Disk Drives: Materials Science Issues and Challenges

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

High coercivity magnetic multi-wall carbon nanotubes for low-dimensional high-density magnetic recording media

Fe-embedded multi-wall carbon nanotubes (MWCNTs) were fabricated using Fe-catalyst by the chemical deposition method. Microscopic characterizations showed that the well-aligned MWCNTs were ∼ 80 mm in length, with outer diameter of 20–50 nm and inner diameter of 10–20 nm. Magnetic properties were characterized in temperatures of 5 K and 305 K, which revealed that the MWCNTs exhibited high coercivity of 2600 Oe at 5 K and 732 Oe at 305 K. These values are much higher than that of bulk iron (∼ 0.9 Oe) and Fe/Co/Ni nanoparticles or nano-wire arrays (∼ 200–500 Oe) at the room temperature. This high coercivity and the structure of single-domain Fe nanoparticles isolated by anti-ferromagnetic MWCNTs make it a promising candidate for low-dimensional high-density magnetic recording media.

Tailoring Exchange Coupling between Magnetic Nano-Grains of High Density Magnetic Recording Media

Intergranular exchange coupling between magnetic nano-grains is one of important parameters to develop magnetic recording media towards the areal density of 1000 Gbits/in2. In this presentation, various processings, involving oxide co-sputtering for perpendicular media, spontaneous phase separation for longitudinal media, and grain boundary (GB) diffusion derived from underlayer or overlayer to induce tunable exchange coupling, have been reviewed to tailor the exchange coupling, including their main principles, present achievements and engineering challenges. Much attention is paid to the physical origin of magnetically induced phase separation of Co-Cr-based alloy, which governs media noise and coercivity, and its applications to the longitudinal media. A proposal of GB diffusion is also highlighted in detail to show a feasibility to tailor the exchange coupling, including calculation results and experimental evidences.

Conversion of Worm-Shaped Antiferromagnetic Hematite to Ferrimagnetic Spherical Barium-Ferrite Nanoparticles for Particulate Recording Media

We used a modified, two-step hydrothermal process to produce worm-shaped antiferromagnetic hematite-barium iron oxide particles and converted them to 25-45 nm spherical ferrimagnetic barium-ferrite (S-BaFe) nanoparticles for high-density magnetic recording media application. Saturation magnetization and coercivity of the S-BaFe nanoparticles were 50.7 emu/g and 4311 Oe, respectively. The thermal stability of KuV/k B T ≈ 81 was estimated for the S-BaFe nanoparticles from time-dependent remanent coercivity measurements.

Perpendicular Magnetization of FePt Alloy Films Epitaxially Grown on Si(100)

This study examined the structure and perpendicular magnetization of FePt films grown on Pt/Fe/MgO(100) buffered Si(100) substrates by molecular beam epitaxy. The [Fe(0.17nm)/Pt(0.2nm)]<TEX>$_N$</TEX> multilayers were prepared at room temperature to form a <TEX>$L1_0$</TEX>-FePt phase after vacuum annealing. Perpendicular magnetic anisotropy (PMA) was observed in the films after at least 15 repetitions (N = 15) of Fe/Pt deposition and annealing at <TEX>$300{^{\circ}C}$</TEX> for 1 hour. Careful structural analysis of the films was carried out by x-ray diffraction and high-resolution transmission electron microscopy. These results will assist in the development of the low temperature <TEX>$L1_0$</TEX>- FePt deposition process, which will be essential for future extremely high density magnetic recording media.

Magnetic Properties of Nanocrystalline Barium Hexaferrite Thin Films Prepared by Sol-Gel Method

Single-phase nanocrystalline BaFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> thin films have been prepared on Si (110) substrates by sol-gel method. The efforts have been done to reduce the crystallite size of the single-phase barium hexaferrite thin films in order to develop the thin films for high-density magnetic recording media. Precursor solutions were primed with various Fe/Ba ratios and two kinds of basic agents. Then the coated films were heat treated at different temperatures. The effects of calcination temperature, molar ratio of Fe/Ba, and basic agent on phase composition, crystallites size, morphology, and magnetic properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and vibrating sample magnetometery (VSM) techniques. The results showed that the lowest calcination temperature for production of the single-phase barium hexaferrite thin film by sol-gel method was 700degC. This product exhibited a nanostructure with high in-plane coercivity.

Electrochemical Fabrication of CoPt Nanodot Arrays on Glass Disks by UV Nanoimprint Lithography

A fabrication process for CoPt nanodot arrays using an electrochemical approach was proposed. The nano-patterned substrates were formed using electron beam lithography (EBL) and UV nanoimprint lithography (UV-NIL). CoPt was electrodeposited into nano-patterned Si substrates. CoPt nandot arrays with 35 nm diameter and 50 nm pitch using EBL and 50 nm diameter and 100 nm pitch using UV-NIL were successfully fabricated. In addition, we attempted to apply 1 inch glass disk substrates with CoZrNb underlayer as soft magnetic underlayer (SUL). We focused on the surface condition of the underlayers to deposit CoPt with uniformity and high magnetic properties. As a result, CoPt nanodot arrays were successfully fabricated on a 1 inch glass disk substrate with SUL using thin Cu intermediate layers. From these results, it has been found that the combination of the UV-NIL and electrochemical processes is a candidate for the fabrication of ultra high-density magnetic recording media.

Development of high density magnetic recording media for hard disk drives: materials science issues and challenges

<title/>The tremendous increase in areal density of hard disk drives is mainly ascribed to harmonic development between magnetic recording media and heads in their scaling, especially allowing a commercial transition from the longitudinal to perpendicular recording system. This paper reviews the main features, recent breakthroughs and future potentials of both the longitudinal and the perpendicular media from a viewpoint of materials science. Special attention is firstly paid to the ‘trilemma’ problem for the media, i.e. the compromise among writability, thermal stability and signal to noise ratio (SNR). The evolution of media materials these years are then addressed with emphasis on the thermodynamic origin of magnetically induced phase separation of Co-Cr based alloys, which governs media noise and coercivity, and its applications to the current longitudinal media. The materials challenges for media to achieve 500 Gb in.−2 and above are further predicted from the viewpoints of thermal stability improvement and microstructure control of media materials, and their engineering issues have been discussed for the current Co-Cr based alloys, potential FePt and CoPt ordered, phase separated Co-W based alloys and magnetic rare earth compounds. Finally, the future media approaching 1 Tb in.−2 and beyond are addressed with respect to the principles, progress, engineering challenges and future directions.

Correlation between crystallographic texture and the degree of L10-ordering in post-annealed Ag/CoPt bilayers and comparison with Ag/CoPt nanocomposites

CoPt and FePt compound films with L10 ordered structure have been intensively studied, due to their extremely high uniaxial magnetocrystalline anisotropy which makes them suitable for application in ultra high density magnetic recording media. A basic requirement in these type of media is the development of strong perpendicular magnetic anisotropy. The role of Ag underlayers in promoting strong (001) crystallographic texture and perpendicular magnetic anisotropy in post-annealed Ag/CoPt and Ag/FePt bilayers (BLs) has already been reported, along with a possible correlation between L10 formation and development of (001) crystallographic texture. In this work we present new data, which provide further evidence that there is indeed such a correlation during the annealing process of Ag/CoPt BLs. The most obvious manifestation of this correlation is the fact that the X-ray intensity ratios I001/I002 (used as a measure of the degree of L10-ordering) and I002/I111 (used as a measure of the crystallographic texture) and the coercivity Hc and relative remnant magnetization mr (for field H normal to the surface of the films) exhibit the same kind of dependence from the thickness of the Ag underlayer. Comparison with respective crystallographic data from post-annealed Ag/CoPt nanocomposites (NCs) shows that in the case of NCs the (001) texture starts to degrade for lower total film thickness, compared to the case of BLs. This difference can be attributed to the structural incoherence in the growth of the CoPt grains imposed by the presence of Ag inside the Ag/CoPt NCs, while in BLs Ag is only used as an underlayer. Based on the above data and on detailed Heavy Ion Elastic Rutherford Back Scattering (HIRBS) measurements, performed on post-annealed Ag/CoPt NCs, we propose a possible mechanism for the interpretation of the observed correlation. The proposed mechanism is based on the reduction of total strain (residual strain of as-deposited film and transformation strain due to deformation of the unit cell as L10-CoPt is formed) throughout the annealing process.

Quantification of L1 phase volume fraction in annealed [Fe/Pt]n multilayer films

The heat treatment of [Fe/Pt]n multilayer films is a promising approach to form the L10 FePt phase at a reduced temperature, which is of interest for applications as high-density magnetic recording media and high-energy permanent magnets. The volume fraction of the L10 FePt phase in the annealed films strongly influences their magnetic properties. This paper introduces a novel method based on hollow cone dark field transmission electron microscopy imaging to quantify the L10 FePt phase volume fraction. This method is used to characterize two sets of [Fe/Pt]n multilayer films with varying deposition temperature and periodicity. It was found that both the deposition temperature and bilayer periodicity are significant to the structure and magnetic properties of annealed films and to the extent to which the L10 phase is formed. A correlation between L10 phase volume fraction, grain size, and magnetic properties was also observed.

CTAB-Assisted Hydrothermal Synthesis and Magnetic Characterization of Ni_xCo_{1-x}Fe_2O_4 Nanoparticles (x = 0.0, 0.6, 1.0)

Nickel ferrite, Ni_xCo_{1-x}Fe_2O_4, NPs (where x = 0.0, 0.6 and 1.0) were successfully synthesized by a rapid and reproducible CTAB-assisted hydrothermal route. The influence of different hydrolyzing agents on the particle size and magnetic behavior of Ni_xCo_{1-x}Fe_2O_4 NPs was investigated. Particles showed very high phase purity and crystallinity in powder XRD analysis. Compositions of Co, Fe, and Ni in fabricated powders were also determined by AAS and results are in very good agreement with the targeted compositions. Samples hydrolyzed using NH_3 showed no significant changes in the particle size and morphology. NH_3 hydrolyzed samples were much smaller than their NaOH hydrolyzed counterparts, which was attributed to the strength and concentration of the hydrolyzing agents, NH_3 being about 6 times more concentrated than NaOH. This in turn influenced the nucleation rate thus the size of each nucleus formed. Strong temperature and Ni concentration dependence of magnetic parameters was observed. These samples are considered as promising materials for high density magnetic recording media.

The microstructure and magnetic properties of electrodeposited Co–Pt thin films on Ru buffer layer

For high-density magnetic recording media, this study examined the crystal structure and the texture of electrodeposited cobalt–platinum (Co–Pt) films on Ru buffer layer. A 15-nm-thick Co–Pt film exhibited very high out-of-plane coercivity and squareness, which were 6248 Oe and 0.89, respectively. The coercivity, H c, of Co–Pt films grown on Ru buffer layer decreased significantly with increasing thickness, possibly due to the lattice misfit of 5.4% between Co–Pt and Ru, leading to the decrease of perpendicular magnetic anisotropy (PMA) of Co–Pt films as indicated by the observed hexagonal-closed-packed (HCP) (1 1¯ 0 1) plane of Co–Pt films. According to nano beam diffraction pattern (NBDP), however, Co–Pt film grown on Ru layer of HCP exhibited mixed HCP and FCC phases. Also, cross-sectional TEM image suggests that the high PMA may result from the columnar structure of physically isolated Co–Pt grains with the c-axis perpendicular to the film plane.

Fe–Pt–MgO stacked films for perpendicular magnetic recording

Fe–Pt–MgO stacked storage layer constructed by [Fe–Pt/Fe–Pt–MgO/Fe–Pt] trilayered structure was proposed for a next-generation high-density perpendicular magnetic recording medium. The Fe–Pt–MgO composite middle layer was prepared by sputtering the Fe–Pt–MgO composite-type target including relatively large MgO content of 50 vol%. The Fe–Pt(0 0 1) seed layer deposited on MgO underlayer was effective in forming the ordered fct(0 0 1) phase for the Fe–Pt–MgO composite film. The reduction of transition jitter noise and the suppression of signal overlap were observed in the stacked-type medium with the Fe–Pt–MgO middle layer of 1 nm thickness. The improvement of recording properties is attributed to the pinning effect of magnetic domain wall by the Fe–Pt–MgO composite layer inserted into the middle of pure Fe–Pt storage layer.

Prospects for bit patterned media for high-density magnetic recording

Prospects for bit patterned media (BPM) of more than 1 Tb/in 2 are discussed. Improvement in the pattern drawing for small feature size and high precision is necessary for fabrication process. Deviation in the magnetic properties should be estimated and reduced. The etching damage seems not to be large. Design of the substructure of the magnetic dot is necessary for reducing the deviation. BPM is also a good template for technologies to increase the recording density. Combination of BPM with heat-assisted recording or exchange-coupled layers is advantageous for high-density recording.

In Vitro Cellular Uptake and Cytotoxic Effect of Functionalized Nickel Nanoparticles on Leukemia Cancer Cells

Nickel nanoparticles (Ni NPs) have been applied in a wide range of areas because of their unique structure and properties such as catalysts, high-density magnetic recording media and others. However, little effort has been paid to their biological application and the concrete effect of Ni NPs on biological systems is still unknown. In this study, the possibility of the utilization of the magnetic Ni NPs in cancer cell studies was explored and the effects of the Ni NPs capped with positively charged tetraheptylammonium on leukemia K562 cells in vitro were investigated. Our observations of optical microscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies indicate that the morphological changes of cancer cells induced by Ni NPs could be apparently observed. The results of 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl-tetrazolium bromide (MTT) assay, DNA fragmentation and flow cytometry studies demonstrate that the Ni NPs could exert cytotoxicity to leukemia K562 cells at high concentration, and subsequently induce both apoptosis and necrosis of target cancer cells, whilst it had little impact on target cells when at low concentration. Meanwhile, functionalized Ni NPs with positively charged groups could enhance the permeability of cell membrane and facilitate the cellular uptake of outer target molecules into cancer cells. These findings reveal the potential mechanism of Ni NPs to target cancer cells which could induce the cytotoxicity to leukemia cancer cells and suggest the possibility for applications of the Ni NPs in related clinical and biomedical areas.