High-density Perpendicular Magnetic Recording Research Articles

Growth of high quality Si-based (Gd2Ce)(Fe4Ga)O12 thin film and prospects as a magnetic recording media

Ce3+-doped rare-earth iron garnet is a hot magneto-optical material in the field of optical communication and information technology. However, the integration of magneto-optical devices has encountered difficulties due to the poor-quality film deposited on Si. This paper focuses on high-quality Si-based (Gd2Ce)Fe5O12 and (Gd2Ce)(Fe4Ga)O12 thin films by using radio frequency magnetron sputtering method. It was found that the introduction of Ga3+ ions can protect Ce3+ from oxidation, stabilize the garnet phase, improve the optical transmittance, and reduce the lattice mismatch between the thin film and the Si substrate. And heating the substrate at 500 ℃ is conducive to grow high-quality Si-based films. The (Gd2Ce)(Fe4Ga)O12/Si thin film has the strong Kerr magneto-optical effect and the pretty high coercivity. Moreover, this film has the obvious out-of-plane magnetic anisotropy and its vertical matrix ratio close to 1, which indicates that it has the considerable prospects for high-density perpendicular magnetic recording.

Investigation of FePt-SiO2-C granular film in columnar structure

We worked on a FePt granular film with a columnar structure, for the purpose of ultrahigh density perpendicular magnetic recording media. In experiments, a Fe53Pt47-SiO230%-C25% (10 nm) film was deposited on a silicon substrate with a MgO (12 nm) interlayer at 510 °C. The perpendicular coercivity of the film is 29 kOe, with a squareness of 1. Bright-field TEM images show that the FePt granular film has small and uniform grains of 8.3 ± 1.9 nm with a columnar structure. More work of high-resolution TEM imaging shows excellent L10 ordering inside the FePt grains. The measurement of remnant coercivity proves that it has an energy barrier of 290kBT at room temperature, meaning excellent thermal stability. We demonstrate FePt granular film to be a qualified candidate for high-density perpendicular magnetic recording media, and both SiO2 and C addition helps form columnar structure.

Multi-Level Read/Write Signaling Using Pulse Width Modulation for High Density Perpendicular Magnetic Recording

In this letter, we propose a novel read/write (R/W) signal processing method by 3-ary amplitude shift keying for high density perpendicular magnetic recording. We also provide a 3B2T modulation to generate 3-ary signal, which will be converted to a pulse width modulation (PWM) signal, and then recorded on magnetic media. The PWM signal can be simply demodulated to derive the 3-ary signal through a low-pass filter at readback side. The demodulated signal in the presence of intersymbol interference is detected by the Viterbi algorithm as partial response signal. Nonbinary low-density parity-check codes over GF(8) are applied as error correcting codes in our evaluation. Our simulation result shows the proposed R/W method has +4% more user bit density gain than conventional binary recording.

Discovery of hard-magnetic domains in two-dimensional arrays of soft-magnetic Fe3O4 nanocubes

In this study, abnormal hard-magnetic domains were discovered in Fe3O4@C composite material, in which well-ordered 16-nm-sized Fe3O4 cubes were tightly embedded into carbon sheets of tens of nanometers thick. It was found that ca. 40 columns of Fe3O4 nanocubes magnetically self-assembled into a single strip-type domain with perpendicular magnetic anisotropy. More strikingly, remarkable domain misalignments, which were very similar to common edge dislocations among atomic planes in crystal lattices, were clearly observed and termed as “domain dislocation” in this work. The hard-magnetic properties of Fe3O4@C material, including large coercivity of 2150 Oe, high MR/MS value of 0.9, and strong anisotropy energy of 3.772 × 105 erg/cm3, were further ascertained by carefully designed electromagnetic absorption contrast experiments. It is anticipated that the discovery of hard-magnetic domains and domain dislocations within 2-D arrays of soft-magnetic nanomaterials will shed new light on the development of high-density perpendicular magnetic recording industry.

The Magnetic Properties of L10-FePt Thin Film with Perpendicular Anisotropy

FePt thin films in the thickness range of 5∼40 nm, have been prepared by magnetron sputtering on MgO (001) substrates. The magnetic measurements have shown that the decrease in the thickness is helpful to improve perpendicular anisotropy of FePt thin films. The perpendicular anisotropy gradually changes to in-plane anisotropy with increase in the thickness leading to weaker (001) texture of FePt thin film. After vacuum annealing at 600°C, the 5 nm thick FePt thin film has been obtained with about 240 kA/m of perpendicular coercivity, good squareness and only about 7nm of average crystalline size, which is suitable for applications of future ultra high density perpendicular magnetic recording.

Reduced-Nonlinear Effect on Magnetic Recording Channels Using MMSE Equalizers

This paper considers the nonlinear effect on magnetic recording channels in order to reduce the system distortion. In this work, we propose the designed MMSE equalizer and target for a high density perpendicular magnetic recording to combat the nonlinear transition shift (NLTS) in a channel characterized by jitter noise and additive white Gaussian noise (AWGN). Due to a complex nature of nonlinear effects, the nonlinearity of a read-back signal is designed by the second order Volterra model consisting of linear and nonlinear part. The bit-error rate (BER) of the nonlinear-affected system using the designed MMSE equalizer is significantly decreased. The results show that the proposed minimum mean square error (MMSE) equalizer with constraint can decrease the inter-symbol interference (ISI) that causes nonlinearity and also improve performance.

Potential perpendicular magnetic recording material: Supported and unsupported vertically-grown ferromagnetic iron nanowire arrays

We fabricated vertically-grown ferromagnetic iron nanowire arrays on glass substrates (unsupported) and inside aluminum-oxide nanopores (supported) by thermally decomposing iron pentacarbonyl precursor in the gas phase in the presence of a magnetic field. High coercivities and easy magnetization parallel to the nanowire’s direction were observed at room temperature due to shape anisotropy. These results show that iron nanowire arrays have potential for high-density perpendicular magnetic recording.

Perpendicularly magnetized τ-MnAl (001) thin films epitaxied on GaAs

Perpendicularly magnetized τ-MnAl films have been epitaxied on GaAs (001) by molecular-beam epitaxy. Crystalline quality and magnetic properties of the samples were strongly dependent on growth temperature. The highest coercivity of 10.7 kOe, saturation magnetization of 361.4 emu/cm3, perpendicular magnetic anisotropy constant of 13.65 Merg/cm3, and magnetic energy product of 4.44 MGOe were achieved. These tunable magnetic properties make MnAl films valuable as excellent and cost-effective alternative for not only high density perpendicular magnetic recording storage and spintronics devices but also permanent magnets.

High resolution switching magnetization magnetic force microscopy

We introduce switching magnetization magnetic force microscopy based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within this approach the sum of the scanned data with reversed tip magnetization depicts local van der Waals forces, while their differences map the local magnetic forces. Here we implement this method by fabricating low-momentum magnetic probes that exhibit magnetic single domain state, which can be easily reversed in low external field during the scanning. Measurements on high-density parallel and perpendicular magnetic recording media show enhanced spatial resolution of magnetization.

Analysis of Nonlinear Transition Shift and Write Precompensation in Perpendicular Recording Systems

In high density perpendicular magnetic recording channels, nonlinear transition shift (NLTS) is one of the distortions that can degrade the system performance. Write precompensation is a standard method used to combat the negative effect of NLTS. In this paper, we present an analysis of the bit-error-rate (BER) for perpendicular recording systems with NLTS and write precompensation. Media jitter noise and additive white Gaussian noise are also considered in the model. A BER lower bound is derived, as well as a more easily computed estimate of the bound. The write precompensation values that numerically minimize the estimate of the BER lower bound prove to be very close to those found using Monte-Carlo channel simulation. We then apply these methods to the design of multilevel precompensation schemes, for which the optimization of precompensation values by Monte-Carlo channel simulation is computationally infeasible. The results show that for higher recording densities subject to increased ISI and noise, the use of more complex precompensation schemes does not significantly improve the system performance.

Effect of Anodized Aluminum Oxide Underlayer on the Magnetic Properties and Structure of Smtbco Magnetic Films

A self-ordered hexagonal array of nanopore was fabricated by anodizing a thin film of Al on glass and subsequently SmTbCo perpendicular thin film was prepared on the AAO underlayer by magnetron sputtering.The vibrating sample magnetometer(VSM) measurement reveals that introducing anodized aluminum oxide(AAO) underlayer can increase the coercivity of SmTbCo thin film from 370kA/m to 530kA/m.With the decrease of hole diameter of AAO underlayer,the coercivity and remanence squareness ratio of SmTbCo magnetic thin film are increased.SEM observations indicate that the SmTbCo thin film deposited on the AAO underlayer consists of a column structure which is attributed to the AAO underlayer self-ordered hexagonal array of nanopore structure,The enhancement of coercivity caused by heterogeneous structure extends SmTbCo thin film's application in super high density perpendicular magnetic recording.

Control of Curie Temperature of FePt(Cu) Films Prepared From Pt(Cu)/Fe Bilayers

Addition of Cu into <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> 1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -ordered FePt alloy thin films with (001) preferential orientation results in the change of Curie temperature. Due to these aspects, the films have potential for next-generation high density perpendicular magnetic recording media, especially, heat-assisted magnetic recording (HAMR) media. FePt(Cu) films were fabricated by annealing Pt(Cu)/Fe bilayers prepared by facing targets sputtering. Their magnetic properties were investigated from room temperature to 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C. As a result, it is possible to control Curie temperature from 150 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C to 250 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C by changing the amount of Cu in FePt(Cu) films. In addition, we discuss required characteristics of medium for HAMR and mention that the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55.3</sub> Pt <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">37.5</sub> (Cu) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7.2</sub> film exhibited favorable characteristics for the HAMR medium.

Mean-Adjusted Pattern-Dependent Noise Prediction for Perpendicular Recording Channels With Nonlinear Transition Shift

In high-density perpendicular magnetic recording channels, nonlinear transition shift (NLTS) is one of the distortions that can degrade the system performance. Write precompensation is a standard method used to combat the negative effect of NLTS. In this paper, we propose a modified pattern-dependent noise predictive (PDNP) detection algorithm for use on channels with electronics noise, transition jitter noise, and NLTS. We show that this detector can offer significant improvement in bit-error-rate (BER) compared to conventional Viterbi and PDNP detectors. Moreover, the system performance can be further improved by combining the new detector with a simple write precompensation scheme.

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.

Structure and magnetic property of c-axis oriented L10-FePt nanoparticles on TiN/a-Si underlayers

L 1 0 -FePt is a promising material for high-density perpendicular magnetic recording media. The authors previously reported that c-axis oriented L10-FePt nanoparticle monolayers can be formed on (200)-oriented polycrystalline template TiN underlayers on SiO2 by using a conventional sputtering method. In this study, TiN nanostructures, such as the degree of (200) orientation, were improved by first depositing a buffer layer, such as amorphous Si onto SiO2, and the grain size could be controlled by adjusting either the deposition temperature or TiN thickness. When FePt nanoparticles were formed on a template TiN underlayer with a buffer layer of amorphous Si, both their degree of c-axis orientation and their magnetic properties were improved; FePt nanoparticles with nominal thickness of 1.4 nm had coercivity of 12.9 kOe in the out-of-plane direction at 300 K.

Recent Progress of Patterned Media

Recent progress in the development of patterned magnetic recording media is presented, focusing on the results of patterned media fabricated by a self-assembled mask process. Several issues such as fabrication process, flyability, and signal processing have been discussed so far. However, recent progress seems to resolve these problems. A fabrication process using nano-imprint lithography is a possible solution in terms of productivity. Good results for flyability have been reported for discrete track recording media. Certain advantages with respect to signal processing have been reported. Although problems remain to be resolved, patterned media is expected to be commercialized following the introduction of the discrete track media, as a high density perpendicular magnetic recording media.

Nanocomposite magnetic films for high-density perpendicular magnetic recording media

Multi-layer nanocomposite structures of Ta/Ru/CoCr 1/FeCoTaCr(soft magnetic layer)/CoCr 2/CoCrPt–SiO 2(hard magnetic layer or recording layer)/C and Ta/Ru/CoCr 1/CoCrPt–SiO 2/CoCr 2/FeCoTaCr/C were proposed. This exchange coupled composite (ECC) media consisting of hard/soft stacked magnetic layers were promising in improving the writability of perpendicular magnetic recording media. A small CoCrPt c-axis orientation dispersion of about 3° was achieved with the optimized sputter conditions. The CoCrPt–SiO 2 grains were well segregated by SiO 2 at grain boundaries. The macro-magnetic properties showed that the stacked magnetic grains switched in a coherent mode and that switching field decreased with increasing the thickness of the soft magnetic layer.

Bit-Percolation Studies on CoPtCr–SiO$_{2}$ Perpendicular Magnetic-Recording Media

Bit percolation degrades the performance of high-density perpendicular magnetic recording. The magnetic cluster size determines the limit of the recordable bit length. In order to examine the parameter, which influences the high-density recording characteristics, we have investigated the relationship between magnetic cluster size and the recording bit length at which bit percolation begins to occur in a CoPtCr-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> perpendicular magnetic-recording medium to the range of 1500 kfci. Our experimental data indicate that the onset of bit percolation takes place at a recording bit length close to the magnetic cluster size of the medium

(0 0 1) textured CoPt-Ag nanocomposite films for high-density perpendicular magnetic recording

CoPt/Ag nanocomposite films were prepared by magnetron sputtering. The dependence of texture and magnetic properties on film thickness, Ag atomic fraction and annealing conditions is investigated. Films with a thickness about 20 nm are easy to form with (0 0 1) orientation. The existence of the Ag in the film plays a dominant role in inducing the (0 0 1) texture of the film and suppressing the growth of the CoPt grains during annealing. The Co 35Pt 38Ag 27 film after annealing at 600 °C exhibits a large perpendicular coercivity of 5.6 kOe and a squareness of 0.90 with a small average grains size of 12.5 nm.

Optimal detection for perpendicular recording channels with transition noise

This paper proposes a jitter-sensitive-sequence-detection scheme based on a realistic transition noise model for high-density perpendicular magnetic recording channels. In this scheme, the transition jitter is assumed to have one of the Q discrete values, and the discretized jitter sequence is estimated jointly with the recorded bit sequence using maximum a posteriori criterion. The detection performance is compared with that of the pattern dependent noise predictive scheme based on an autoregressive Gaussian noise model. Simulation results show that the scheme provides a significant gain when the transition jitter is large and the resulting transition noise dominates over the electronic noise.