Tadashi Yogi

Magnetic force microscopy : general principles and application to longitudinal recording media

This paper discusses the principles of magnetic force microscopy (MFM) and its application to magnetic recording studies. We use the ac detection method which senses the force gradient acting on a small magnetic tip due to fields emanating from the domain structure in the sample. Tip fabrication procedures are described for two types of magnetic tips: etched tungsten wires with a sputter‐deposited magnetic coating and etched nickel wires. The etched nickel wires are shown to have an apex radius on the order of 30 nm and a taper half‐angle of approximately 3°. Lorentz‐mode transmission electron microscopy of the nickel tips reveals that the final 20 μm is essentially single domain with magnetization approximately parallel with the tip axis. Images of written bit transitions are presented for several types of magnetic media, including CoPtCr, CoSm, and CoCr thin films, as well as γ‐Fe2O3 particulate media. In general, the written magnetization patterns are seen with high contrast and with resolution better ...

Longitudinal media for 1 Gb/in/sup 2/ areal density

Media with low noise at high transition density that demonstrate satisfactory recording performance at an areal density, of 1 Gb/in/sup 2/ when combined with dual-element (magnetoresistive read/inductive write) heads have been fabricated. A media structure of C/CoPtCr/Cr was utilized over a range of magnetic parameters: coercivity approximately=1600-1800 Oe, remanence-thickness product approximately=0.7*10/sup -3/ emu/cm/sup 2/, and coercive squareness approximately=0.7-0.8. Media noise reduction was accomplished by optimizing the film-growth characteristics to reduce intergranular exchange coupling in the magnetic layer. The low-noise characteristics of the media are manifested in their low transition jitter values, 5 nm for 3- mu m track width, and the absence of supralinear increase in media noise power with linear density up to 3000-3500 fc/mm. The -6-dB rolloff densities are in the range 4000-5000 fc/mm. Overwrite values are typically better than 40 dB. Microstructural analysis indicates that the reduced transition noise of the present media is due to physical separation of the grains in the magnetic films, which reduces the exchange coupling between the magnetic grains. The reduced coercive squareness of the low-noise media degrades the overwrite performance and is also expected to decrease the linear density resolution of the media. >

Gigabit density recording using dual-element MR/inductive heads on thin-film disks

Inductive-write and magnetoresistive (MR)-read dual-element heads of very narrow trackwidths and narrow gaps have been designed, fabricated, and recording-tested on thin-film media of high coercivity and squareness. Results show excellent writability at modest write currents of 50 mA, large readback amplitudes with good signal-to-noise performance, very high linear-density resolution up to 5000 fc/mm (-6-dB density), and trackwidths close to the design dimensions. These results show that such head-disk systems are capable of recording operations at a target areal density of 1 Gb/in/sup 2/. Another result is that novel techniques for measuring nonlinear transition shifts as well as offtrack and squeeze performance have been developed. >

A magnetic evaluation of interaction and noise characteristics of CoNiCr thin films

The nature of media recording noise in metallic, quasiparticulate thin films is principally related to the grain size, crystallographic orientation, and intergranular exchange and magnetostatic coupling in the films. In this study the results of a magnetic evaluation of magnetostatic interactions in CoNiCr thin films of varying Cr underlayer thickness are reported. The evaluation is undertaken through the measurement and comparison of remanence curves. The results presented here indicate enhanced cooperative switching as Cr underlayer thickness is reduced from 2000 to 100 A, with a strong correlation between signal‐to‐noise measurements. In addition, the transition from principally exchange‐coupled to quasiparticulate thin films, as Cr underlayer thickness increases, has been established.

Ultra high density media: gigabit and beyond

The authors review the recent advances made toward the attainment of high areal density in the Gb/in/sup 2/ regime. they review recent progress made in understanding the relationship between the media magnetic properties and recording characteristics such as linear resolution, transition noise, and overwrite. For high-linear-resolution media, a coercivity in the 2000-3000-Oe regime is now attainable, depending on the alloy, underlayer, and other process parameters such as substrate temperature bias. Various noise reduction schemes have been developed in recent years which are based on the decoupling of exchange interactions between magnetic grains or between multiple magnetic layers. As the bit cell size is further reduced in the Gb/in/sup 2/ regime, one of the main challenges will be to retain a high coercivity with decreasing grain size of the magnetic film. >

The effect of Cr underlayer thickness on magnetic and structural properties of CoPtCr thin films

Thin Co‐based alloy films have shown important recording, magnetic, and structural changes when grown on Cr underlayers of different thicknesses. We have investigated these properties using several different CoPtCr compositions on Cr underlayers ranging from 0 to 200 nm in thickness. We report epitaxial growth of the hcp Co (11.0) planes on the (100) planes of bcc Cr for the first time on a disk appropriate for magnetic recording. The 〈11.0〉 Co preferred orientation occurs only when the Cr underlayer has a 〈100〉 preferred orientation. The 〈100〉 preferred orientation in the Cr layer results from the use of sputtering rates above 150 nm/min and only persists to thicknesses of about 50 nm. The thin Cr underlayers (<50 nm) with a 〈100〉 preferred orientation have fine well‐packed grains. Thicker Cr underlayers have larger grains which are uncoupled and have a more random crystal orientation. The CoPtCr film morphologies follow these trends. The noise performance of these films improves with increasing Cr under...

Role of atomic mobility in the transition noise of longitudinal media

The relationship between grain growth morphology and the transition noise of high-density CoPtCr media on Cr underlayers is examined. The growth morphology is shown to depend on the sputtering pressure, substrate bias, and substrate temperature. Development of isolated magnetic grains is promoted when the mobility of the sputtered atoms is reduced, i.e. at high sputtering pressure, low substrate temperature, and no substrate bias. As the magnetic grains become more isolated, the media transition noise is significantly reduced, accompanied by a simultaneous reduction in the coercive squareness. This indicates that the transition noise is primarily governed by the intergranular exchange coupling among the magnetic grains, which can be optimized by controlling the grain growth morphology. The microstructural features of the decoupled media correspond to the zone I structure in J.A. Thorntons (1986) microstructure zone diagram. >

Dependence of magnetics, microstructures and recording properties on underlayer thickness in CoNiCr/Cr media

The effect of Cr underlayer thickness on the magnetic, microstructural, and recording properties of sputtered CoNiCr media is reported. Cr thickness was varied from 100 to 2000 AA while the magnetic layer was maintained in the range of 260-300 AA. Measurements using a vibrating sample magnetometer show a monotonic enhancement of coercivity from 600 to 1500 Oe with increasing Cr thickness. Media noise was measured as a function of transition density using a spectrum analyzer. The media noise power at 2400 flux changes per millimeter decreased by a factor of about 10 with the increase in Cr thickness. This reduction is accompanied by a reduction in the coercivity squareness and the development of granular features in CoNiCr grains growing on the columns of the Cr underlayer. These features are consistent with a reduction of exchange coupling among CoNiCr grains. >

Influence of coercivity squareness on media noise in thin-film recording media

Signal and media noise measurements have been made for a wide variety of Co‐alloy longitudinal magnetic recording media. A strong correlation between media noise and the coercivity squareness, S*, is observed at high transition density. The optimum ratio of isolated pulse amplitude to media noise is found for S*≂0.75. This behavior arises from reduced ferromagnetic exchange coupling between neighboring grains of the film which influence both the coercivity squareness and also the uniformity of the domain boundary at the recorded transitions. The low‐noise characteristics of sputtered γ‐Fe2O3 also appear to correlate with low values of coercivity squareness. This work confirms that for systems in which media noise is a limiting factor, optimum performance may require considerably lower values of coercivity squareness than commonly used, determined by the relative contribution of media noise to the overall system noise.

Composition effects in high density CoPtCr media

The optimization of the CoPtCr alloy for use in high density longitudinal magnetic recording is discussed. Low noise media can be obtained with a combination of high Cr concentration (22 at.%) and high deposition temperature. The results are consistent with a Cr segregation mechanism that provides reduction of exchange coupling without the need for a voided microstructure. The Curie temperature decreases significantly at 25 at.% Cr preventing further increase in the Cr concentration. Pt is added to the alloy to increase coercivity with little effect on other recording properties. >