Performance of GPRML-AR system in perpendicular magnetic recording channel with thermal decay

The bit-error rate (BER) performance of the generalized partial response maximum likelihood with autoregressive (GPRML-AR) channel model system in perpendicular magnetic recording (PMR) channel with thermal decay is obtained. The 128/130(0,16/8) run-length-limited (RLL) code is used as a recording code. The GPR channel consists of the PR1 channel followed by the reduction circuit of predicted noise. The BER performance is evaluated by computer simulation using a thermal decay model. The model has been obtained by using an approximate equation that represents amplitude degradation of the reproducing waveform with elapsed time based on the experimental data for CoPtCr-SiO₂ PMR media. The Viterbi detector with an AR channel model is employed. Furthermore, long-term degradation of the required SNR to achieve a BER of 10⁻⁴ with elapsed time is obtained and the performance is compared with that of PR1ML system. The results show that the poorer the thermal stability of the medium becomes, the larger the SNR gain of the GPR1ML-AR system over the PR1ML system becomes. The SNR gain also increases with elapsed time.     

Role of magnetic circular dichroism in all-optical magnetic recording

Using magneto-optical microscopy in combination with ellipsometry measurements, we show that all-optical switching with polarized femtosecond laser pulses in ferrimagnetic GdFeCo is subjected to a threshold fluence absorbed in the magnetic layer, independent of either the excitation wavelength or the polarization of the laser pulse. Furthermore, we present a quantitative explanation of the intensity window in which all-optical helicity-dependent switching (AO-HDS) occurs, based on magnetic circular dichroism. This explanation is consistent with all the experimental findings on AO-HDS so far, varying from single- to multiple-shot experiments. The presented results give a solid understanding of the origin of AO-HDS, and give novel insights into the physics of ultrafast, laser controlled magnetism.     

Double-layer particulate magnetic recording media

Double-layer media employ two layers with different magnetic characteristics, with the objective of optimizing performance and/or reducing the overall cost. The idea has recently been applied to longitudinally oriented thin top layers of Ba-ferrite (BF) or metal particle (MP) coated over thicker Co-iron oxide (CoFe) or non-magnetic (NM) underlayers resulting in significantly improved recording performance for high recording density video, digital audio and data applications.     

Micro magnetic structure of magnetic clusters in CoCrTaPtB recording media

Micro magnetic structure in CoCrTaPtB recording media with H_c>3 kOe was studied by magnetic force microscope and LLG simulation. Two types of the characteristic magnetic structure were observed. One is the convergent-type magnetic structure, which is observed as localized magnetic cluster on bit. The other is the vortex structure, which is responsible for the recording bit peroration. These micro magnetic structures cause the medium noise in high-density recording and must be suppressed by adjusting coercive force H_c and inter-granular interaction to explore high density recording media.     

Constructive ITI-coded PRML system based on a two-track model for perpendicular magnetic recording

In this paper, we study not only the new constructive inter-track interference (CITI) code based on the equalized level but also Viterbi detection algorithm taking into account the ITI from adjacent tracks for the perpendicular magnetic recording channel without a differentiator of two-track model. Although the investigation of this paper was not practical but theoretical, the result shows that the permissible percentage of ITI for conventional Viterbi detection to attain better performance compared with the case of single track is 26%, while the percentage for the proposed one is improved up to 50%. Further investigation has to be required under a more realistic system and moreover may be expanded into patterned media perpendicular magnetic recording.     

Micromagnetic modeling for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is one of the candidate systems beyond the perpendicular recording technology. Here, a micromagnetic model and a heat transfer model are introduced to study the heating and cooling processes in the HAMR media; then, by integration of the SPT head and the laser heating source, the recording performance is simulated and investigated on a single track at an area density of 1 Tb/in². In the HAMR system, the temperature in the medium under the laser wave guide is increased by heating, and decreased by air bearing and heat conduction when the write process really occurred. The target of this study is to find the proper design of the head-laser assembly for optimum recording. It is found that the proper distance between the laser wave guide and the head's main pole rear/front edge is only 41.4/1.4 nm for optimum recording performance.     

Writability issues in high-anisotropy perpendicular magnetic recording media

Challenges and recent developments associated with writability issues in high-anisotropy perpendicular recording media are reviewed. The writing field is limited by the high coercivity caused by the high anisotropy. Some new alterna- tives are proposed to solve the writability issues, including texture-tilting-assisted, domain-wall-assisted, energy-assisted magnetic recording technologies, and so on, In addition, we propose new alternatives for the next-generation of magnetic recording media.     

First-order reversal curves of magnetic recording tapes

The interaction and its variation between magnetic grains in two kinds of magnetic recording tapes are investigated by first-order reversal curves (FORC) and the $\delta M$ method. The composition and microstructure of the samples are characterised by x-ray diffraction and scanning electron microscope. The FORC diagram can provide more accurate information of the interaction and its variation, but the $\delta M$ curves cannot. The positive interaction field and the large variation of the interaction field have opposite effects on the $\delta M$ curve.     

Past and present of perpendicular magnetic recording

Past and present of perpendicular magnetic recording (PMR) is described. PMR was born at a university institute 30 year ago and hard disk drive (HDD) industry is now converting the products from longitudinal magnetic recording (LMR) to PMR. The way to the present, however, was not flat. The research of PMR experienced the death valley in the 1990s, but was overcome by its unique research system. The commercialization of PMR was much later than it was expected in the beginning, which might have affected the position of HDDs in storage systems. The market of HDDs is expected to steadily grow towards the future being supported by PMR.     

Investigating recording patterns using magnetic force microscopy

The magnetic force microscope is a valuable tool for the qualitative analysis of local phenomena in magnetic recording media. To study the possibilities for a more quantitative analysis, the MFM results have been compared with recording results. It is shown that such analysis is hampered by the nonlinear transfer characteristic and by the masking of local high-definition details due to the application of an auxiliary electrostatic field in the current instrument.     

Erasure of high-energy magnetic tape recording media

The erasure characteristics of CrO₂, Co−γ-Fe₂O₃, Ba ferrite, metal-evaporated and metal particle tapes were examined. It was found that the Ba ferrite sample was the hardest to erase, and the CrO₂ the easiest. For all the media studied, except Ba ferrite, longitudinal ac fields yield higher degrees of erasure than dc fields. Time domain measurements indicated that particle orientation dispersion plays an important role in the erasure process.     

All-optical magnetic recording with circularly polarized light

We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.     

Particle coercivity distribution of magnetic recording medium

It is proved theoretically that the intrinsic distribution of particle coercivities in a magnetic recording medium can be obtained by differentiating the dc remanence curve. The coercivity distribution is hardly influenced by the orientation distribution of the particle easy-axis.     

Shingle magnetic recording assessment with spinstand measurement

Shingle Magnetic Recording (SMR) is a candidate to realize recording density of over 1 Tbit/in². At the cost of complex HDD firmware, we can expect many gains such as higher write-ability to break through the famous tri-lemma. In this paper, promising empirical data and concerns found on spinstand were introduced and discussed. More than 18 dB of better reverse overwrite (ROW) than conventional write recording (CWR) was confirmed at 40 nm of track pitch. Areal density capability (ADC) was improved by 25.5% by SMR. Another gain from SMR was observed at inner diameter (ID) and outer diameter (OD) that ADC loss due to skew effect was improved by using preferable side write pole edge. The dependence of ADC on magnetic read width measured with micro-track method (MRWu) indicates that side reading is limiting ADC. Reduction in side reading through narrower MRWu and inter-track interference cancellation technology are necessary to further improve ADC. BPI capability improvement by steeper down track field gradient at the track edge needs to be considered.     

Extracting intergranular exchange coupling in perpendicular magnetic recording media

A micromagnetic numerical technique has been used to demonstrate how intergranular exchange coupling and intrinsic anisotropy field dispersion can be extracted from measuring two types of M-H curves. A realistic grain configuration formed by planar Voronoi cells is used to simulate perpendicular magnetic media. This technique effectively separates the effects of intergranular exchange coupling and anisotropy dispersion by finding their correlation to differentiated M-H curves with different initial magnetization states, even in the presence of thermal fluctuation. The validity of this method is investigated with a series of intergranular exchange couplings and anisotropy dispersions for different media thickness. A relationship between the auto-correlation function of an ac-erased sample and dispersion of the exchange interaction is demonstrated. Utilizing magnetization auto-correlation functions, the magnetic intergranular exchange coupling statistics show a correlation with the auto-correlation function shape in terms of zero-cross and undershoot values.     

Angular-dependent remanent magnetization curve for perpendicular magnetic recording media determined by precise polar-Kerr detection system

We developed the polar-Kerr detection system and evaluated the angular dependence of magnetization curves with applied field of various directions during the newly developed system. The polar-Kerr detection system enabled precise evaluation of angular dependence of remanent coercivity (H_cr), as compared with the conventional VSM system. In addition, a cusp appeared in polar-Kerr hysteresis loop was observed to originate magnetization reversal by the thermal agitation.     

Nanodiamonds for biological investigations

Nanoparticles with a modified surface are prepared from nanodiamonds produced in Russia. The properties of modified nanodiamonds and their hydrosols and organosols are investigated by biophysicists with the aim of preparing nanoparticles with controlled properties for solving biological problems.     

Preparation and recording characteristics of granular-type perpendicular magnetic recording media with thin intermediate layer

Granular-type media with thin Ru intermediate layer were prepared on a highly oriented high-B_s FeCo soft underlayer (SUL). A CoPt–TiO₂ recording layer on a Ru intermediate layer of only 2 nm had high-crystal orientation, high H_c of 6.5 kOe, and a high squareness ratio (SQ) of 0.99. The magnetic property of the SUL was also good. The recording performance was measured for the media with different Ru intermediate thicknesses by using a single-pole-type (SPT) head. The media had large reproduced output even for the Ru intermediate layer thickness of 2 nm.