David T. Wilton

Effect of island distribution on error rate performance in patterned media

The size, shape, and distribution of islands in a patterned medium depends on the patterning process adopted. The off-track performance in this case is mainly dominated by inter-track interference due to the neighboring islands. In this paper, the effect that the island distribution has on the off-track performance is investigated with respect to read channel bit-error-rate performance.

One-sided fluxes in planar, cylindrical, and spherical magnetized structures

In planar structures, magnetization patterns exist that have the surprising property that all the external fringing field of flux emerges from one side of the structure, with precisely none appearing on the other side. In the last 25 years, this fact has led to a number of useful applications. More recently, the same concept has been found to hold for both cylindrical and spherical hollow structures, where all the fringing flux emerges either inside or outside the structure, with precisely none appearing on the other side. Again, this has led to a number of important practical applications. While many of the concepts have been published previously, this paper contains, for the first time, a uniform mathematical analysis of these one-sided flux configurations. Additionally, the paper reviews the principal applications of one-sided flux structures.

Feasibility of ultra-dense spin-tunneling random access memory

Three-dimensional (3-D) finite element models have been utilized to simulate electromagnetic behaviors in spin-tunneling random access memory (STram). The most significant contributors have been identified. Compared with conventional current-in-plane (CIP) giant magneto-resistive (GMR) memory, whose signal level is inversely proportional to the square root of the storage density, these current-perpendicular-to-plane (CPP) STram elements provide an excellent readout property in that their signal level is independent of their cross-section area. This result is so attractive that the density of STram should not be limited by signal degradation. Moreover, a magnetic flux closure design was found to reduce the crossfeed by about a factor of five, compared with conventional keeperless design, which is the most favored approach for achieving 10/sup 9/ bits/cm/sup 2/ areal density. Although the storage mechanism described in this paper is made of STram, the flux-closure design could be generally applicable to other magnetic solid state memories.

Effect of island geometry on the replay signal in patterned media storage

In a move to extend the storage capabilities of magnetic storage systems beyond 1 Tb/in/sup 2/, the use of patterned media has often been cited. Here, recorded domains are constrained by the geometry of the magnetic island and not the geometry of the recording head. Conventional two-dimensional readout modeling techniques, using the reciprocity integral, rely on the assumption that the across-track medium magnetization is uniform under the giant magnetoresistive replay head. However, in the case of a geometrically constrained medium this is not the case. This work investigates the effect that the island geometry has on the characteristics of the replay signal in perpendicular patterned magnetic media storage through the extension of the reciprocity integral to three dimensions. The paper describes replay pulses that offer different characteristics from those obtained by conventional two-dimensional techniques. The origins of these differences are explained by the variation in medium magnetization across the track.

Approximate three-dimensional head fields for perpendicular magnetic recording

This paper proposes a method for obtaining approximate, but very accurate, three-dimensional (3-D) head fields for perpendicular magnetic recording heads. The method uses an assumed form for the scalar magnetic potential variation between a pole or shield and the underlayer to give the potential in the head/shields-face plane. It then uses this approximate air-bearing surface potential to find the potential or field components at any other position of interest. The approach is illustrated here by applying it to a symmetric double-shielded single-pole head with an underlayer and with side shields.

An analysis of a shielded magnetic pole for perpendicular recording

The new solution of an idealized mathematical model of the field due to a symmetrically shielded magnetic pole suitable for perpendicular recording is presented. Accurate Fourier coefficients and corresponding magnetic fields are obtained for a representative pole dimension and a range of pole/shield separation distances, and comparisons are made with the field of an unshielded isolated pole. It is found that the presence of a shield may significantly increase both the peak field strength and the field gradient. The highest field gradients presented are predicted for in-contact recording with certain thin-film pole/shield geometries. >

Readback responses for complex recording media configurations

This paper describes a relatively simple approach to calculating the frequency response for a two-dimensional (2-D) magnetic recording system with a medium that can include an arbitrary number of layers. Each layer may have an arbitrary magnetization direction, anisotropic permeability, and exchange coupling. The approach relies on an initial transformation into the spatial frequency domain and then the use of transmission matrices to relate the fields in the different layers. The approach is general in that it provides a method for finding the field configuration for any set of 2-D magnetic sources embedded in a layered magnetic medium with a linear B--H relationship. Here, we focus on calculating the readback response for a variety of longitudinal and perpendicular recording configurations. Since the permeability may have any wavelength dependence, we can easily include the effect of exchange coupling in the underlayer.

Readback Responses in Three Dimensions for Multilayered Recording Media Configurations

We have applied a technique for modeling the transmission of three-dimensional (3D) magnetic fields through an arbitrary number of layers of a layered structure to the specific case of a recording medium in order to study and explain the readback response for a variety of perpendicular recording configurations. We examine in particular the low-frequency response associated with flux-flow in the soft underlayer and the behavior of the main response for finite read and write widths. Our work leads to a useful analytic approximation for the 3D on-track readback response.

Comparison of ring and pole head magnetic fields

Idealized mathematical models of the magnetic fields generated by ring and single-pole heads are examined. In each case, more accurate Fourier coefficients than have been published previously are provided. However, the main purpose is to consider, in a more rigorous manner than has been presented before, the correspondence between the fields of ring and pole heads. Ranges of parameters and areas of the domain are identified for which the fields are almost identical. In the region of each geometry occupied by the recording medium, the similarities were less clear. >

Accurate approximation of fields and spectral response functions for perpendicular recording heads

Solutions for the magnetic potential in rectangular regions with simple boundary conditions are used to approximate the two-dimensional potential in the head face plane of a recording head. This method of approximation can be applied to any recording head with rectangular corners, in the presence of an underlayer. The slot approximation is demonstrated for ring heads and shielded (magnetoresistive) pole heads. The quarter plane approximation is given for application to finite-length heads. Estimates of the field components and the spectral response function are derived from the approximate potential. The simple slot approximation is shown to be at least as accurate as other, often more complicated, methods of approximation. Most significantly, both the slot and the quarter-plane approximations are easy to apply and need no prior knowledge of the head field of the particular geometry under consideration.