Analysis of Shingle-Write Readback Using Magnetic-Force Microscopy

Shingle recording experiments were conducted on a precision spin-stand using conventional recording components. Although both head and medium correspond to a 250 Gb/in² (Gigabits/sq. inch) product, the raw written densities were pushed beyond 1 Tb/in² (Terabits/sq. inch). High-resolution Magnetic-force microscopy (MFM) was used to examine the resulting written patterns. The MFM images have an apparent resolution down to 16 nm. From these images we are able to characterize the signal and noise in two dimensions up to the highest densities. Two-dimensional signal-processing was applied to the "waveforms" recovered from these MFM images. At the highest written densities, very poor raw error-rates were encountered and customer data could only be recovered successfully by employing relatively low rate codes. Although the experiments pushed the raw recording density to 1039 Gb/in², the highest customer information density at which data was successfully recovered was 623 Gb/in².

I. Introduction

Shingled magnetic recording (SMR) and two-dimensional magnetic recording (TDMR) have been proposed for pushing storage densities beyond 1 (Terabits/sq. inch) [1]. Conventional recording is expected to reach a limiting density at about this point and all of the alternative technologies under consideration appear very challenging [2]. Shingle-writing takes advantage of the high fields and gradients associated with the corner of a wide write head. Tracks are written such that they overlap one another yielding a track pitch much less than the head-width [1]–[5]. The track-pitch and bit aspect ratio can readily be pushed far below that of a conventional system, to the point where inter-track interference (ITI) necessitates the use of 2-dimensional (2D) read-back and signal-processing [6]–[9].