Double-layered Perpendicular Recording Media Research Articles

1-6 二層垂直記録媒体のためのPRML方式の一検討

1-6 二層垂直記録媒体のためのPRML方式の一検討

Write-field gradient effect on transition width in perpendicular recording media

Perpendicular magnetic recording (PMR) system using a single-pole writer and a double-layered perpendicular recording medium with a soft underlayer (SUL) is a promising candidate for high recording densities of over 100 Gb/in/sup 2/. The relationships between medium characteristics and recording performance have been studied. In this paper, we have clarified the dependency of the write-field gradient on the transition width using by a micromagnetic modeling. Also, the dependence of medium parameters is also discussed in the paper.

Distributions and dynamics of spike noise in double-layered perpendicular recording media

Spike noise of double-layered perpendicular magnetic recording media was studied by being classified readback signals of a GMR head into noise which comes from a recording layer and noise which comes from a soft magnetic layer. On the medium with an FeCoAlO soft magnetic underlayer, no domain structure was observed with the magnetic field which is smaller than 0.1Oe. External magnetic field induces rounded shape domains in the FeCoAlO layer from the outer edge of the medium. The size of these domains is of the order of millimeters, and the width of the domain wall is a few micro-meters. While the medium with an FeSiN soft magnetic layer has many domains. The width of the domain wall is around 0.5μm. The domain wall is moved by the magnetic field which is smaller than 0.1Oe, and is moved by the field from magnetization of recording layer.

Transition position shift in double layered perpendicular recording media with soft underlayer

In perpendicular recording, double layered media with soft underlayer (SUL) are outstanding in view of increasing writing head field and the head field gradient. However, the SUL could serve as various noise sources. In order to investigate the effect of SUL on noise, writing and reading processes in a perpendicular system including a single pole head and a recording medium with the SUL were performed by micromagnetic simulation. It was found that when an abrupt head field is applied, the SNR of the double layered medium shows much lower SNR compared with the single layered medium. The higher noise in the double layered medium is not associated with transition jitter but with transition position shift. The transition position shift is associated with change of magnetization pattern of the SUL with time during writing process. In the present paper, we discuss the change of magnetization patterns of the SUL with time, for two different head sizes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of soft underlayer on read/write properties in double-layered perpendicular recording media

Micromagnetic simulations of writing and reading processes in a perpendicular system including a single pole head and a recording medium with soft underlayer (SUL) have been performed. The effect of SUL on noise in double-layered perpendicular recording media was investigated by comparing three different cases (i) ideal bit in single layer, (ii) realistic bit in single layer without SUL, and (iii) realistic bit in double layer with SUL.

Recording performance of double-layered perpendicular recording media with an antiferromagnetic layer

For a double-layered perpendicular media having an antiferromagnetic layer, it has been reported that increasing the exchange bias field H/sub ex/ is effective to reduce spike noise generated from the domain walls of the soft magnetic underlayer (SUL). In this study, a high exchange-coupling energy J/sub ex/ of 0.345 erg/cm/sup 2/ was obtained by using the combination of a NiFeB underlayer and a CoFeNiB enhancement layer. By investigating the effects of H/sub ex/ on the recording performance, it was revealed that increasing H/sub ex/ caused the writability of a head to deteriorate. However, reducing the nonmagnetic interlayer thickness, which reduces the magnetic spacing between the head and the SUL, could minimize the deterioration in writability. In addition, when the writability was unaffected by H/sub ex/, the signal-to-media-noise ratio was improved by the effect of SUL domain noise reduction.

Thermal stability of written bits in double-layered perpendicular recording media

The thermal stability of written bits on single- and double-layered perpendicular magnetic recording media were compared from micromagnetic simulation. It was found that, although double-layered media show a higher thermal stability than single-layered media, the difference is not large. Under the calculation conditions used here the gain in thermal stability by double-layering was found to correspond to 7.5% increase in the volume of a magnetic grain. It was also found that this difference tends to decrease at the low and high ends of recording density. Investigation of the magnetostatic field exerted in the recording layer shows that the change in the thermal stability is due to the change in the layer-thickness component of the static field exerted on the recording layer. Although the increase in the dispersion in the easy axes of magnetization was found to decrease thermal stability, the decrease was greater in single-layered films than double-layered films.

Hilbert filter for double-layer perpendicular recording media

Summary form only given. Readback waveforms from GMR heads in perpendicular magnetic recording are completely different from the regular longitudinal recording waveforms. Proper signal equalization is therefore critical. For this purpose a Hilbert filter is discussed. This filter is known to transform the dipulse response from a single layer perpendicular media into a longitudinal signal, however, for double layer perpendicular media with a soft magnetic underlayer its properties have not been reported yet. In this paper the authors discuss the performance of the Hilbert filter for double-layer perpendicular recording media.

FeCoC film as the soft magnetic underlayer for perpendicular media

FeCoC granular thin films are prepared and studied as the soft magnetic layer in order to obtain a low noise double-layered perpendicular recording medium. FeCoC granular soft magnetic underlayer prepared by cosputtering at room temperature showed good soft magnetic properties; with coercivity of a few oersteds and Bs of 1.4–1.7 T. Using a spin-stand and a ring-type head, sharper transitions can be obtained in the [Co/Pd]n media with an FeCoC soft underlayer at higher recording density compared to media without a soft magnetic layer. No spike noise was observed in the read back signals for [Co/Pd]n media with an FeCoC granular film as the soft magnetic underlayer. Preparing the [FeCo/C]n laminated films by alternately sputtering FeCo and C layers allows the properties of the films to be flexibly adjusted. A Bs of 1.9 T with relative permeability of 100–400 and coercivity lower than 40 Oe can be obtained with laminated [FeCo/C]n (210 nm) multilayer thin films.

Current progress of single-pole-type GMR heads for perpendicular recording

We studied several proposals to address the problems encountered when single-pole-type giant magnetoresistive (SPT-GMR) heads are applied to an actual recording system. The first area of problems was regarding side erasure when there is a head skew. We fabricated a trapezoid-shaped main pole using a focus-ion-beam trimming or using a new wafer-processing technique and examined their write performances and their effects on reducing side erasure. The second area of problems concerned the stray field robustness of SPT-GMR heads when combined with a double-layered perpendicular recording medium. We tried to optimize the robustness and write performance by changing the width of a yoke in the main pole or by positioning a shield above the main pole. The stray field robustness and write performance were adequate in both cases, but a tradeoff relationship exists between both performances.

Dynamic read/write performance characterization of perpendicular recording

Double-layer perpendicular recording media employing high squareness magnetic properties have been developed and tested with single-pole perpendicular heads. Dynamic performance tests revealed that double-layer perpendicular media exhibited high output signal, high resolution, and superior write performance at the same time. Signal decay tests also indicated superior thermal stability. Drive integration test with a 42 kTPI (1654 track/mm) system showed comparable tracking servo performance to conventional systems.

Detection of extremely high bit density signals with a narrow track width GMR head in double layered perpendicular recording media

Recording resolution, medium noise and thermal stability have been investigated for double layered perpendicular magnetic recording media. The recording performance was improved by introducing a stacked intermediate layer between a soft magnetic backlayer and a storage layer due to increased crystal orientation with a small magnetic domain size and suppressed initial growing layer of the storage layer. The reproduced output at an extremely high linear density of 950 kFRPI was detected by using a contact-type CF-SPT head for write and a GMR head with a narrow read track width of 0.4 μm.

Perpendicular recording with high squareness CoPtCrO media

Double layer perpendicular recording media employing highly square CoPtCrO recording layer with low noise granular structure and multi-layered [FeAlSi/C] n soft underlayer have been developed. Dynamic performance test revealed that the double layer CoPtCrO perpendicular media exhibited the high output signal and the high resolution recording performance at the same time. Signal decay test also indicated the superior thermal stability.

Distributions and characteristics of spike noise

Distributions and characteristics of spike noise in double-layered perpendicular recording media were investigated using a spin-stand and a digital storage oscilloscope. It was revealed that spike distributions are strongly dependent on the soft magnetic underlayer material and that the reproduced signal amplitude is modulated at the spike positions.

Low noise multi-layered FeAlSi soft magnetic films for perpendicular magnetic recording media

One of the solutions to suppress the noise from the soft magnetic underlayer of double-layered perpendicular recording media is investigated. Laminating FeAlSi films thinner than 10 nm drastically reduced the noise. By utilizing multi-layered FeAlSi as a soft magnetic underlayer, CoPtCrO perpendicular recording media showed features of high output voltage, low normalized DC noise, and high over-write performance.

Preparation of double layered perpendicular recording media with extremely high resolution

In a double layered perpendicular magnetic recording medium, recording resolution and time decay of output signal have been studied. Introduction of a stacked intermediate layer (NiFeNb/Ti) between a soft magnetic back layer and a storage layer improves magnetic property of the storage layer. The stacked intermediate layer can reduce the initial growth layer of the storage layer by hetero-epitaxial growth. Time decay of output signal for a medium with the stacked intermediate layer was suppressed within -0.4 dB at 1000 seconds after recording at 100 kFRPI. A reproduced signal at an extremely high linear density of 950 kFRPI has been confirmed by a CF-SPT head for writing and a GMR head with track width of 0.4 micrometer for reading.

Low noise perpendicular magnetic recording media for deep submicron track width recording

Noise reduction in double-layered perpendicular recording media with a thick soft magnetic backlayer was studied. Although the medium noise was increased with the backlayer, the noise for quaternary alloy films could be substantially reduced by introducing a certain intermediate layer between the alloy layer and the backlayer. The double-layered medium obtained exhibited a waveform with sharp transitions as well as a low noise property when recorded with a single pole head. The recording performances suggested the potential of the medium for a high areal density recording around 50 Gbit/in/sup 2/.

垂直磁気記録における縦続接続ターボ符号の性能評価

The performance of serially concatenated turbo coding systems using various RLL codes was evaluated and compared with that of PRML systems in a perpendicular magnetic recording channel. The error rates of these systems were experimentally estimated by an actual R/W channel using a spinstand with a double-layered perpendicular magnestic recording medium. The experimental results show that among the PRML systems, PR1ML, PR2ML, and E2PR3ML had good performance. We also found that our turbo coding system using a PR2 system had the best performance among all RLL-coded serially concatenated turbo coding systems we investigated. A 64/65 (0, 8) code was found suitable for our turbo coding system.

Effect of a Ti seed layer on the magnetic properties of CoCrTa double-layered perpendicular media

The effect of a Ti seed layer on the magnetic properties and microstructure of Co77Cr19Ta4 was studied in Co77Cr19Ta4/Ti/M (M=Co91Zr3Nb6, Fe, Co) double-layered perpendicular recording media. A thin Ti film of ∼5 nm enhanced the c-axis alignment perpendicular to the film plane, resulting in a large perpendicular anisotropy Ku and the best magnetic properties under the present experimental conditions. The coercivity at this Ti thickness (3–3.7 kOe) was approximately half of the magnetocrystalline anisotropy field Hk estimated from the Ku value, even with the present thick 50 nm CoCrTa. The recording performance using a single-pole-type writing head showed that media with a 5 nm Ti layer had higher recording sensitivity compared to media without Ti layers, and a higher SN value at densities greater than 200 kFRPI. No significant loss of recording resolution due to the nonmagnetic Ti space between the CoCrTa and the back layers was observed by the use of the 5 nm Ti film. Moreover, the results for media with Fe back layers indicated that a thinner back layer with a high flux density can be used without any reduction of recording sensitivity and resolution.

ＣｏＣｒＴａ／Ｔｉ／Ｍ（Ｍ：ＣｏＺｒＮｂ，Ｆｅ，Ｃｏ）垂直二層膜媒体の磁気特性と記録再生特性

The Magnetic properties and recording performances of CoCrTa/Ti/M (M: CoZrNb, Fe, Co) double-layered perpendicular recording media were studied. It was revealed that a thin Ti film of 5 nm gave the most enhanced c-axis alignment perpendicular to the film plane in media with CoZrNb or Fe back-layers, resulting in the highest coercivity and squareness of CoCrTa. The recording performance of media with Fe back-layers, using a single-pole-type writing head, indicated that a thinner back-layer with a high flux density can be used without any reduction of the recording sensitivity and resolution, even though the Fe back-layers do not have good soft magnetic properties. However, hcp-Co back-layers degrade the recording sensitivity and resolution. The media noise of media with Fe or Co back-layers was significantly higher than that of CoZrNb back-layer media. It is suggested that these noise spectra are related to the domain structure of the back-layers.