Recording Layer Research Articles

Fabrication of ultrathin FePt(Cu) ordered alloy thin films directly on glass disks from Pt(Cu)∕Fe bilayers

Addition of Cu into Fe∕Pt bilayered structure causes high coercivity and large magnetic anisotropy in the perpendicular direction when the thickness of the bilayer is less than 6nm. In the thickness range less than 4nm, Pt∕Fe bilayered films with Cu added annealed at 600°C in hydrogen atmosphere exhibits larger coercivity than the bilayer without Cu added. Pure Pt∕Fe bilayered film exhibits relatively small coercivity, less than 4kOe, and no perpendicular magnetic anisotropy in this range. On the other hand, Pt(Cu)∕Fe bilayered film exhibits large coercivity of 5kOe and relatively higher perpendicular magnetic anisotropy in the thickness range more than 5nm. Pt(Cu)∕Fe bilayered film can be expected to be applied to thin magnetic recording layer below 4nm or less.

Effect of nonuniform microstructure on magnetic switching behavior in CoCrPt–SiO2 perpendicular magnetic recording media

The effect of nonuniform microstructure on magnetic switching behavior for CoCrPt–SiO2 single layer media with oxygen contents (OCs) from 4% to 10% and recording layer thickness (tmag) from 2to27nm is investigated. Plan-view transmission electron microscopy images clearly show nonuniform microstructure of CoCrPt–SiO2 with OC=10% along the film thickness direction: poor grain isolation at a thin layer, excellent grain isolation at a medium layer, and formation of subgrains at a thick layer. The highest coercivity (Hc) is not observed on the medium with the best grain isolation. An initial CoPt layer with less grain isolation forms on top of Ru interlayer with increasing OC in films. It exhibits domain wall switching, resulting in significant incoherent switching. This is consistent with large magnetic activation volume at tmag⩽6nm. Other plausible incoherent switching mechanism for thicker layer media showing low Hc is likely due to formation of subgrains with less grain isolation.

Microstructure and magnetic properties of CoCrPt–SiO2 perpendicular recording media with synthetic nucleation layers

Thermally stable, highly textured CoCrPt–SiO2 perpendicular magnetic recording media with ∼6nm size exchange decoupled grains and tight grain size distribution were prepared on synthetic nucleation layers. The media structure consisted of 14nm thick CoCrPt–SiO2 recording layer on one Ru intermediate layer. For these films, insertion of a synthetic nucleation layer directly under the recording layer was seen to reduce the grain sizes from 6.7to6.2nm and grain size distribution from 17% to 14%. Significant reduction in the intergranular exchange coupling interactions could be achieved while improving magnetic properties. The crystallographic texture of the recording layer remained strongly (00⋅2) oriented with the dispersion Δθ50 below 3.7°. Isothermal remanence magnetization and thermal stability behavior were studied. A model that describes the role of the synthetic nucleation layer in affecting the grain growth and magnetic properties of the recording layer is presented to understand these observations.

Microstructure and recording mechanism of Bi–Fe–(N) layer for high-density write-once optical disk

Bi–Fe–(N) thin film as the recording layer of high density digital versatile disk recordable optical disk was developed. For the disk sample with optimized layer structure, maximum partial response signal to noise ratio of 21.1 and minimum simulated bit error rate of 5×10−7 were achieved at the write power=5.7mW. Transmission electron microscopy characterization illustrated that the separation of Bi and Fe elements to form the coarse granular structure in the mark regime during laser irradiation comprised of the recording mechanism.

Numerical simulation of probe induced surface plasmon resonance coupling nanolithography

A new nanolithographic technology, the probe induced surface plasmon resonance coupling nanolithography (PSPRN), is presented and analyzed numerically by using finite difference time domain method for the loss and dispersive materials. The PSPRN uses a fundamental mode Gaussian beam with wavelength of 514.5nm to excite the Kretschmann surface plasmon resonance, and utilizes the metal probe local-field enhancement effect to realize nanolithography. The influences of different distances between probe and recording layer and different sizes of tip on local field enhancement and the distribution of electric field intensity amplitude on the surface of recording layer were investigated. Results show that the local field enhancement effect is most significant and the electric field intensity amplitude contrast ratio is maximal when the probe is in contact with the recording layer. When the distance between tip and recording layer is 5nm, the distribution width of the relative electric field intensity amplitude above the critical value for near-field nanolithography on the surface of recording layer is close to the size of the tip.

Control of Interfacial Structure for Granular Type Perpendicular Magnetic Recording Media

For realizing perpendicular magnetic recording media (PRM) for hard disk drive (HDD), precisely controlled fabrication technology with angstrom scale according to the required properties is essential from material, processes, and physical view point. PRM presently consist of three functional parts; a soft magnetic underlayer (SUL), a nonmagnetic intermediate layer, and a recording layer (RL). The control of domain structure through interlayer coupling induced by RKKY-like interaction in SUL and narrower size distribution of the magnetic particles with magnetically decoupling in recording layer are indispensable for advanced PRM with high signal to noise ratio and thermal stability. In this paper, structure and magnetic properties for SUL and RL in a real fabricated PRM is discussed in connection with sputtering process parameters and also target materials.

Fabrication of 1 Tdots/in2 Patterned Media with Anodic Alumina Technique

We investigated the potential of an anodic alumina template for high-density patterned media. Anodic alumina hole density of 1.86 Tdots/in2 was achieved by suppressing surface roughness by alloying aluminum with Ti and Hf. However, it was difficult to fabricate the embedded type patterned media with a recording layer of thinner than 25nm in an anodic alumina template of 1.03 Tdots/in2. This necessitated a switch from an embedding process to a coating process, so we developed a new anodizing process for patterned media of 1 Tdots/in2. The process formed the niobium oxide pillar structure successfully with high density in an anodic alumina template. 1.03 Tdots/in2 patterned media fabrication was achieved by depositing the Co70Pt30 recording layer on the pillars.

Pinning Effect Induced by Underlayer in TbFeCo Magnetic Recording Media

Pinning effect induced by a grainlike structure on top of a TbFeCo recording layer with an Al or Ag underlayer is investigated. We confirm that the pinning effect depends on the surface structure of the recording layer as well as the underlayer. The Al-underlayered TbFeCo medium shows higher recording density during thermally assisted magnetic recording and higher coercivity than those of the Ag-underlayered one. We can observe that only the Al-underlayered TbFeCo medium has the grainlike structure on top of the TbFeCo recording layer, which reflects well a grainlike structure on the Al underlayer's surface. It indicates that the surface profile of not only the underlayer but also the TbFeCo recording layer laminated onto the underlayer is necessary for inducing the strong pinning effect in the TbFeCo magnetic recording media.

光記録媒体用色素

A lot of optical recording media have been proposed, but only CD-R and DVD-R media are widely accepted. They have many advanced features, such as high compatibility to ROM player because of their high reflectivity, as well as easy and low cost production by applying a simple design comprising a dye recording layer and a reflective layer, and by applying a spin-coating method to form the dye recording layer. Most important factor to realize such advantage is development of organic dyestuffs. In this review, I will introduce what it takes to put the dyestuffs into practical use and the dyestuffs that survive the tough race.

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The feasibility of magnetic contact duplication for perpendicular magnetic recording (PMR) media at 1 Tbit/inch2 was investigated by using micromagnetic simulation. The exchange constant dependence of the duplication performance has a maximum value. Two factors improve the duplication performance: decreasing the c-axis distribution of the recording layer and decreasing the distance between master and slave disks. However, the anisotropy field dispersion of the recording layer has almost no influence on the duplication performance. When the c-axis distribution is decreased, the exchange constant is optimized and the spacing is less than 5 nm, the magnetic contact duplication for PMR media at 1 Tbit/inch2 is feasible.

New Design for Obtaining Intermediate Layer of 2 nm in Granular-type Perpendicular Magnetic Recording Media

A new granular-type perpendicular magnetic recording media was designed to reduce the intermediate layer thickness. A medium with Ru intermediate layer of only 2 nm was prepared successfully on a highly oriented high-Bs FeCo soft underlayer (SUL). The CoPt-TiO2 recording layer had a high crystal orientation, a high Hc of 6.5 kOe, and a high squareness ratio of 0.99. The magnetic property of the SUL was also good. The granular structure of the CoPt-TiO2 recording layer in the new-designed medium is similar to that in the old-designed one.

Pseudo-hcp nonmagnetic intermediate layer for granular media with high perpendicular magnetic anisotropy

Materials with the hexagonal close-packed structure (hcp) and the face-centred-cubic structure with stacking faults (pseudo-hcp) are examined for the nonmagnetic intermediate layer (NMIL) in order to suppress variant growth of magnetic grains for granular-type perpendicular recording media. Judging from the analysis of the epitaxial growth of Co-based magnetic grains, it has been found that a lattice misfit between NMIL and magnetic grain of less than 6% and a spreading coefficient of wettability of magnetic grain on NMIL of greater than 0.3 J m−3 are required for the recording layer to have high perpendicular magnetic anisotropy energy.

Feasibility study of micro‐undulator for T‐ray sources by using FEM

AbstractPossibility of development of micro‐undulator utilizing perpendicular magnetic recording was explored by 2‐D FEM simulation. In the simulation FePt and CoCr layer were compared as an appropriate recording layer of the undulator. Also variation of maximum field and THD with aspect ratio and gap height were simulated so that they should be optimized in a given thickness of recording layer. From both the simulation results and Bitter patterns recorded on FePt/Fe micro‐undulator, the possibility of development of micro‐undulator by utilizing perpendicular recording technique could be confirmed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Grain size reduction of CoCrPt–SiO2 media by oxidation of RuCr intermediate layer grain boundary

AbstractThe grain size reduction mechanism of CoCrPt‐SiO2 perpendicular magnetic recording media by oxygen reactive sputtering of RuCr intermediate layer has been investigated as a function of oxygen content in the Ar sputtering gas. The media stack was Ta/Ru/RuCr/CoCrPt‐SiO2. It was found that, by the oxygen addition, Cr and Ru oxides were formed at grain boundaries of the RuCr layer and this was the reason of the intermediate layer grain size reduction, which induced the reduction of the recording layer grain size. C‐axis alignment of RuCr (0002) plane, however, was degraded as the oxygen gas ratio increased. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Novel actuator with flat movable part for optical pickup head

AbstractIn an optical pick‐up head, the objective lens actuator is a key component used to move the objective lens to the exact position for focusing the laser spot on the disc recording layer and keeping the laser spot following certain track on the disc. In order to be used in a compact pick‐up head for high density drives, the objective lens actuator must be compact and controllable in focusing, tracking, and radial tilt directions. In this paper, an electromagnetic three‐axis‐controllable actuator with a novel structure was designed and verified. Magnetic circuit analysis and structural modal analysis were also done in this work. Results of the frequency response tests done with the prototypes corresponded with the simulation results, and the reading tests done by using a pick‐up head with the actuator designed in this work proved the feasibility of this design. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Origin of incoherent magnetic switching behavior in CoCrPt–SiO2 perpendicular magnetic recording media

Magnetic switching behavior for CoCrPt–SiO2 single layer media with oxygen contents (OCs) from 4% to 10% and recording layer thickness (tmag) from 2to27nm is investigated. Significant incoherent switching, caused by domain wall motion, and large activation volume are observed at tmag⩽6nm regardless of OC, consistent with nonuniform microstructure with less grain isolation at tmag=4nm. These results support formation of a highly exchange-coupled thin magnetic layer on top of Ru interlayer, which can explain the large difference between angular dependence of Hcr predicted by the Stoner-Wohlfarth theory and the experimental values.

Effect of temperature on nanowear of platinum-coated probes sliding against coated silicon wafers for probe-based recording technology

In one of the new alternative information storage technologies, a phase-change medium is used and phase change is accomplished by applying either a high or low magnitude of current which heats the interface to different temperatures. Tip wear is a serious concern. For wear protection of the phase-change chalcogenide medium with a silicon substrate, diamond-like carbon (DLC) film with various lubricant overcoats was deposited on the recording layer surface. Nanowear properties of platinum (Pt)-coated probes with high electrical conductivity have been investigated in sliding against the coated medium using an atomic force microscope (AFM). The nanowear experiments were performed at sliding velocity of 0.1 mm s −1 and at sample temperatures of 20 and 80 °C. In high-temperature experiments at a sample temperature of 80 °C, wear rate is higher compared to that at a sample temperature of 20 °C. The results suggest that wear mechanism appears to be primarily adhesive, abrasive and tribochemical.

L1 0 ordered FePt based double-layered perpendicular recording media with (002) oriented FeCo films as a soft magnetic underlayer

The introduction of the soft magnetic underlayer (SUL) in perpendicular recording technology is to further increase the recording areal density. However, problems such as growth of the uncontrollable recording layer and additional media noise contributed from the SUL could be resulted. In this work, a synthetic antiferromagnetically (SAF) coupled (002) oriented Fe 65Co 35 film as an SUL was developed for L1 0 ordered FePt based double-layered recording media. The crystallography of hetero-epitaxially grown double-layered media CrRu/(Ru/FeCo) 2/Pt/FePt/Ru was demonstrated. The L1 0 ordered FePt based double-layered perpendicular recording media with SAF coupled FeCo films as the SUL were developed.

Rewrite Characteristics of a Scattered-Type Superresolution Near-Field Structure Optical Disk with a Ag2O Mask Layer

On the basis of the working mechanism of a silver-oxide-type superresolution near-field structure (super-RENS) disk, we realized excellent rewriting by performing the following steps: (1) initialize the disk at a continuous power of Pi=8.0 mW for 3 s, (2) write at a pulsed power of Pw=8.0–9.0 mW, (3) read (reinitialize only the mask layer) at a superresolution power of Psr=5.5 mW, which lies between the powers under reversible conditions of Pil=4.5 mW and Piu=6.0 mW for the mask layer, (4) erase at a continuous power of Pe=7.5 mW for 3 s, and (4) rewrite at a pulsed power of Pw=8.0–9.0 mW. The carrier-to-noise ratio (CNR) difference between writing and erasing slightly varies for successive rewritings, resulting in CNR=23 dB after 20 rewrite operations for a mark length of 400 nm, which is less than the resolution limit λ/4NA=413 nm. The Ag2O mask layer is decomposed into Ag nanoparticles at 160 °C that aggregate to form nanoclusters dispersed in the mask layer after the initialization operation at Pi=8.0 mW (3 s), and the recording layer made of Ge2Sb2Te5 (GST) is transformed by a second phase transition from a rocksalt crystal structure of fcc to a hexagonal crystal structure (the phase transition temperature range is estimated to be 250–450 °C) at a pulsed power of Pw=8.0–9.0 mW, and vice versa at a continuous power of Pe=7.5 mW for a disk velocity of 2 m/s.

The Nanocomposite Thin Film Applied to High-Density Write-Once Optical Data Storage

This work demonstrates the feasibility of producing composite thin films containing nano-scale recording particles for high-density optical data storage. The nanocomposite recording layers were found to exhibit an ultrafast transition rate and a unique phase transition temperature compared with the bilayer recording systems. For type I disk samples containing a nanocomposite recording layer and a Ag reflection layer, modulations higher than 0.8 and 0.5 could be achieved when 11T and 4T signals were respectively written into the disk samples using a blue-laser dynamic tester. As for type II disk samples containing a sole nanocomposite recording layer, modulations better than 0.8 and 0.6 were obtained. Transmission electron microscopy (TEM) characterization attributed the main signal recording mechanism to the coalescence and coarsening of nano-recording particles, which, in turn, implies a sufficient reflectivity difference for signal readout. The reactions and/or redistribution of elements involved in the layer structure of optical disks also contributed to the signal recording. The nanocomposite recording layer provides various advantages including a fast phase transition rate, initialization-free, disk structure simplification, and a sputtering-compatible fabrication process and thus, promising applications to next-generation write-once optical data storage are expected.