Yoshihiro Ikeda

/spl Delta/H(M,/spl Delta/M) method for the determination of intrinsic switching field distributions in perpendicular media

We describe a method for the experimental characterization of intrinsic switching field distributions in perpendicular media. The method is based upon a comparison between the major loop and a set of recoil loops, which start at a certain distance /spl Delta/M away from saturation. In particular, we measure the applied field difference /spl Delta/H between the recoil loops and the major loop at identical M values, for which the average demagnetizing fields are the same. By analyzing complete /spl Delta/H(M,/spl Delta/M) data sets taken from multiple recoil loops, we gain a detailed measure of the intrinsic switching field distribution D(H/sub S/). Using polar-Kerr effect measurements of recording disk samples, we demonstrate the capabilities of our approach.

Head challenges for perpendicular recording at high areal density

To explore recording head challenges for perpendicular recording at 200 Gb/in/sup 2/ and beyond, the design, fabrication and performance of narrow track dual-element heads were studied using an ABS trailing shield writer design and a conventional CIP-GMR reader design. Parametric recording tests of these heads on low noise CoCrPt/SUL media show that, with the trailing shield design, good writability and low disk transition jitter around 2.5 nm were achieved at narrow write trackwidths down to 120 nm. In addition, peak-to-peak signal amplitudes around 1 mV and T/sub 50/ widths around 28 nm were also achieved at read trackwidths around 60 nm. The areal density potential of these heads was studied using a PRML channel at /spl sim/50 MB/s data rate. Results show linear densities around 1000 Kbpi at ontrack byte error rates of 10/sup -4/, and track densities around 200-240 ktpi using a criterion of 15% offtrack to trackpitch ratio. In all, areal densities of 210-230 Gb/in/sup 2/ were achieved with these head and disk components.

Control of Exchange Coupling Between Granular Oxide and Highly Exchange Coupled Cap Layers and the Effect on Perpendicular Magnetic Switching and Recording Characteristics

We report a systematic study of the switching and recording characteristics of perpendicular magnetic recording media in which the exchange coupling between granular oxide and continuous cap layers was varied. The interfacial exchange coupling strength was controlled by adjusting the magnetization (Ms) and the thickness (t) of the exchange control layer (ECL) between granular oxide and cap layers. The media switching mechanism highly depends on the oxide-to-cap exchange coupling strength as well as the relative moment ratio of cap and oxide layers. Reversal process is coherent for medium with only granular oxide layer and becomes incoherent with incorporation of ECL and continuous cap layers. Optimizing granular oxide-to-cap exchange coupling improves the media writeability as well as the media signal-to-noise ratio (SNRm). At optimum exchange coupling condition, the switching field is significantly reduced even with higher thermal stability factor (Ku V/kB T). However, when the interlayer coupling strength is too weak, independent switching of oxide and cap layers occurs, resulting in poor writeability and high media noise. An optimum design of oxide-to-cap exchange coupling is critical in attaining recording properties for high density recording through selection of appropriate ECL and cap materials.

Writeability Enhancement in Perpendicular Magnetic Multilayered Oxide Media for High Areal Density Recording

A systematic investigation of the switching behavior and writeability characteristics of perpendicular magnetic oxide media was performed by varying the number of oxide layers as well as the anisotropy Ku gradient in multilayered oxides. The media switching behavior highly depends on the magnetic volume of laterally exchange-coupled cap layer as well as the vertical exchange coupling between oxide and cap layers. The media switching field is significantly reduced with incoherent switching mode. The media writeability is further enhanced by employing the multilayered oxides whose anisotropy values vary. Higher Ku graded oxide media exhibits improved resolution, sharper transition, and higher signal-to-noise ratio. An analytical 9-spin model is used to study the impact of anisotropy grading, intergranular and interlayer exchange coupling on writeability and noise performance. The 9-spin model is found to qualitatively agree with the experimental results. Both model and experiment indicate that optimal tuning of the media anisotropy and exchange to the head design is critical in attaining good recording performance.

Design Consideration and Practical Solution of High-Performance Perpendicular Magnetic Recording Media

The core of the current granular perpendicular magnetic recording (PMR) media is the granular oxide magnetic layer (GOML). We observe that a fcc NiW seed layer, followed by a hcp Ru layer sputtered under low pressure (LP-Ru) and then by a hcp Ru layer sputtered under high pressure (HP-Ru) provides an excellent structural template for the granular oxide magnetic layer (GOML). Microstructure and magnetic properties of the GOML can be tailored through process conditions and alloy compositions to maximize the recording performance of the perpendicular magnetic recording media. Exchange coupled composite (ECC) media design provides significant improvement in overwrite (OW) and signal-to-noise ratio (S0 NR) for perpendicular magnetic recording media. Recording performance of the ECC media needs to be optimized with consideration of the magnetic properties and thicknesses of both the exchange control layer (ECL) and the cap magnetic layer (CML). At their respective optimum, the recording performance of the ECC media is mostly dependent upon the properties of the GOML and CML. This study indicates that a dual magnetic cap layer structure consisting of low-Ms and high-Ms sublayers can combine the benefits provided by the two types of cap magnetic materials.

Open-contact failure detection of via holes by using voltage contrast

We used two techniques to determine the sensitivity of a scanning-electron-microscope-based wafer-inspection system in detecting open-contact failures. (1) The correlation between the contact resistance and the brightness of the voltage-contrast image as captured by the system was obtained experimentally. (2) A voltage-contrast simulation was developed and applied to derive a correlation between resistance and brightness from these results. A close agreement between the experimental results and the calculated values was obtained. We succeeded in clarifying the determinants of the sensitivity of open-contact-failure detection. The brightness, over part of its range, appears to be proportional to log(R*Ip) where R is the resistance and Ip is the irradiating electron-beam current. This relationship indicates that the sensitivity of open-contact failure detection is determined by Ip. Control of Ip can be used to improve the voltage contrast, and this, in turn, can improve the sensitivity of detection.

Direct measurement of magnetodynamics in a perpendicular recording system

To characterize the magnetodynamic properties of a perpendicular recording system, consisting of a pole head, a recording layer, an exchangebreak layer, and a soft underlayer (SUL), we have built a test vehicle by depositing either the SUL, or the full medium stack on the air-bearing surface of the recording head. Using ultra-high-speed scanning Kerr microscopy, the intrinsic switching characteristics of the SUL and the impact of the recording layer on the former has been measured. One important feature found is the formation of vortices during reversal which are much larger than the pole area as well as evidence for spin wave excitation in the SUL. Both fast write current steps, and complex write current waveforms have been applied and significant differences of the magnetic responses are observed. Though the Kerr signal probes the temporal evolution of the SUL surface magnetization, it is possible to determine the onset of the recording layer switching by taking its magnetostatic coupling to the SUL into account. The magnetization state of the recording layer also impacts the switching speed of the SUL. The noise characteristics of the Kerr signal gives information about nonreproducible magnetization processes in the SUL.

Incoherent Switching Amount Analysis of Exchange Spring Media by Using Initial Minor Loop Slope Measurement

We describe a newly developed initial minor loop slope (IMLS) experimental method capable of measuring reversal switching in exchange spring perpendicular media. The slope peak height is centered near the mediums coercivity Hc. The magnitude of the peak signal is correlated with the soft top (cap) layers moment, Mst, and the exchange coupling strength between the cap and hard magnetic oxide layers. The peak position was correlated to the media Hc for media that is thermally stable and migrates to smaller fields in less stable media. The IMLS results are compared with both remanent angular dependence coercivity, Hcr, measurements and micro-magnetic simulations. This technique provides a simple method of characterizing incoherent switching behavior in different types of exchange spring media.

Switching Behavior and Writeability in Perpendicular Magnetic Dual Oxide Media With Capping Layer

We report a systematic study of the switching behavior and writeability characteristics of perpendicular magnetic recording media in which the anisotropy (K_u) of granular oxides, magnetic volume (M_s·t, product of magnetization and thickness) of cap layer, and exchange coupling between granular oxide and cap layer were varied. The media switching behavior highly depends on the M_st of cap layer as well as the oxide-to-cap exchange coupling strength but little on the K_u of granular oxides. The media switching field H_o at 1-ns scales is strongly affected by switching mode and is greatly reduced with incoherent switching mode. At constant switching mode with fixed M_st of cap layer, the media writeability is primarily determined by the oxide K_u value that affects the H_o. However, when the switching mode varies with different cap M_st or oxide-to-cap exchange coupling strength, no correlation between H_o and writeability is observed. Higher K_u oxide media with higher M_st of cap layer exhibits lower H_o due to more incoherent switching but substantially lower overwrite (OW) than the lower K_u oxide media with lower M_st of cap. Similarly, media with weaker exchange-coupling between oxide and cap layers shows more incoherent switching mode and worse writeability even with lower H_o. Our results have shown that oxide-to-cap exchange coupling as well as oxide K_u is critical in media design for good writeability at high recording density and can be optimized through selection of appropriate oxide and cap materials.

EBL thickness and transition parameter in perpendicular oxide media

This paper investigates the effect of varying the exchange-break-layer thicknesses (leading to a change in intergranular exchange) on spectral rolloff and the transition parameter (a-parameter). The rolloff curves are obtained by measuring the amplitude of the fundamental harmonic peak at each written density. A systematic increase of the low frequency amplitude is observed as the EBL thickness decrease.