T.C. Arnoldussen

Zigzag transition profiles, noise, and correlation statistics in highly oriented longitudinal film media

Lorentz microscopy images of recorded bit patterns on a metal film disk are used to demonstrate that even though the magnetization transitions are zigzag domain walls, the effective transition profile for a sufficiently wide track resembles an error function, rather than an arctangent. However, the Williams-Comstock arctangent model accurately predicts the slope of the transition. Digitized Lorentz-image zigzags, as well as the Lorentz images themselves, can be used to characterize the nature of the zigzags and the noise behavior associated with them. A proposed noise model based on the micromagnetics of film domains is included here.

10 Gbit/in.2 longitudinal media on a glass substrate (invited)

This article reports on the properties of the media prepared on glass substrates which were used in IBM’s 10 Gbit/in.2 demonstration. In order to support a linear density of 315 kbpi and a track density of 33 ktpi, the remanant coercivity Hcr and remanant moment thickness product Mrt of the magnetic layer were 3450 Oe and 0.37 memu/cm2, respectively. The media used a NiAl seed layer, a CrV underlayer, a Co alloy magnetic layer, and a carbon overcoat protection layer. The magnetic film had a grain size of 12 nm as observed by transmission electron microscopy. The preferred orientation (PO) of the magnetic layer was (1010). This PO enables one to sustain high coercivities at low values of Mrt. It is observed that the c-axis in-plane texture of the magnetic layer is critical to achieve a low noise medium. Using a focused-ion-beam (FIB) trimmed giant magnetoresistance head and conventional partial response maximum likelihood channel, the on-track-error rates were measured at the 10−10 level.

Microstructure and thermal stability of advanced longitudinal media

Thermal stability will ultimately limit the maximum areal density achievable with conventional longitudinal recording. The key aspects of the media microstructure contributing to thermal stability are the grain size and grain size distribution, alloy composition, alloy segregation, lattice defects and strain. Grain size distributions are created by the random nucleation processes occurring during media deposition. For media on glass substrates, c-axis in-plane preferred orientation can be achieved with either Co (112~0) or (101~0) planes parallel to the substrate surface. Improved squareness, S, is observed with the (112~0) orientation due to stronger crystallographic texture, however, larger changes in coercivity with decreasing magnetic layer thickness are observed compared to (101~0). Continued increases in areal density will require tighter grain size distributions and improved microstructural control of very thin magnetic layers.

A modular transmission line/reluctance head model

A transmission line model for calculating recording-head fields and efficiencies is described. Exact solutions are obtained for transmission line segments with fixed or varying gap and width. Using these solutions. Thevenin T-reluctance circuit equivalents are obtained for each segment of the total head transmission line structure, and the magnetic flux and fields are obtained by circuit analysis. External fringe fields at edges are also included. Using three basic building blocks: (1) constant-gap, variable or fixed-width regions, (2) graded-gap, variable or fixed-width regions, and (3) external fringe reluctances, in any number and sequence, virtually any head geometry can be quickly and reasonably modeled, without the need for reformulating the problem or respecifying boundary conditions. >

Side writing/reading in magnetic recording

Side writing is a significant effect for narrow track widths used in high‐ areal‐density magnetic recording. Lorentz electron micrographs can help to understand the nature of side writing and are used in this paper, in conjunction with analytical modeling, to characterize side writing in isotropic and strongly oriented recording media. Isotropic media exhibit a wide side‐written fringe, including a strip often termed an ‘‘erased band,’’ although it is not truly erased. Strongly oriented media, however, show no erased band.

Demonstration of 35 Gbits/in/sup 2/ in media on glass substrates

A recording density of 35 Gbits/in/sup 2/ was achieved in longitudinal recording media with high-sensitivity GMR heads. The media displayed excellent thermal stability as a result of a CoPtCrB alloy with high magnetocrystalline anisotropy and relatively narrow grain size distribution. The degree of Co easy-axis orientation in the plane of the /spl mu/m was greatly improved and the grain size was reduced in the media on glass substrates. Estimates of the switching volume from dynamic coercivity and signal-to-noise measurements are larger than the physical grain size, suggesting that intergranular interactions improve stability. A potential path to further increases in recording density above 35 Gbits/in/sup 2/ is to use antiferromagnetically coupled magnetic layers in the media.

Bit cell aspect ratio: An SNR and detection perspective

Future gains in areal density may require using bit cells which have smaller width-to-length aspect ratios than have been customary in magnetic recording (15 to 20:1). Common thinking is that such squarer bits have a media signal-to-noise advantage. This conclusion depends on the SNR formulation used, e.g, raw voltage SNR vs. a timing window/jitter SNR. Despite this conundrum, it is found that (PR4) equalization transforms the voltage signal and noise in such a way that the equalized voltage SNR, like the raw timing window/jitter, favors squarer bits. The presence of significant electronic noise weakens this conclusion.

Obliquely evaporated iron-cobalt and iron-cobalt-chromium thin film recording media

Thin films of Fe-Co and Fe-Co-Cr were deposited onto a variety of substrates by e-beam evaporation at an oblique angle of incidence of 60 degrees to the substrate normal. Deposition at this angle results in the formation of an easy axis in the plane of the magnetic films, oriented perpendicular to the plane of vapor incidence. Electron microscopy reveals the anisotropy to be due to the formation of parallel crystallite chains, which are, on the average, aligned transverse to the plane of the vapor beam. The lateral dimensions and the extent of alignment of these chains depend mainly on the substrate microtopography, so that with increasing surface roughness the alignment decreases, thereby leading to a decrease of the squareness value S and the in-plane anisotropy Mr(easy)/Mr(hard). The film coercivity, too, is to some degree determined by such aligned crystallite chains. However, Hc in general increases with increasing substrate microtopography and with decreasing film thickness.

Vacuum‐deposited thin‐metal‐film disk

The fabrication process used in an all‐vacuum‐deposition thin‐metal‐film disk will be described. The process consists of planar dc‐magnetron sputter deposition of a stainless alloy undercoat onto a 14‐in.‐diameter AlMg‐4 disk substrate, e‐beam evaporation of an Fe–Co–Cr alloy magnetic layer at an oblique angle of incidence, planar dc‐magnetron sputter deposition of a rhodium overcoat and spray deposition of a liquid lubricant layer. Some mechanical and magnetic performance characteristics of disks, deposited in that fashion, are described.

Correlation of thermal stability and signal-to-noise ratio of thin film recording media

The thermal stability of data stored in thin film recording media is closely tied to signal-to-noise ratio (SNR), yet is not correlated in a simple way. Indeed grain size strongly affects both SNR and thermal stability, but macroscopic magnetic properties as well as recording conditions are equally influential. This is illustrated by measurements of SNR and signal thermal decay along with a simple model analysis. Dependence of areal density on key magnetic properties, under SNR and stability constraints, is formulated.