Roger William Wood

Parametric optimization for terabit perpendicular recording

A viable medium for perpendicular recording must be writeable by a realistic head and yet must also be thermally stable. These requirements place conflicting demands upon anisotropy and saturation magnetization and this paper seeks to identify viable parameter values for terabit/in/sup 2/ perpendicular recording. In this paper we determine viable operating regimes for Tbit/in/sup 2/ perpendicular recording.

A Soft Decodable Concatenated LDPC Code

In this paper, a concatenated low-density-parity-check (LDPC) coding scheme that is suitable for iterative soft decoding is introduced for magnetic recording systems. The concatenated code is constructed as the product of two linear codes, in which a conventional 4 kB LDPC code is used as the inner code and a single-parity check code serves as the outer code. The outer code in this concatenated construction spans many sectors, possibly an entire track, and the code rate and the block size can be flexibly configured. While individual sectors can be read and decoded independently, the full decoding power of the concatenated code can only be achieved by reading all the data sectors that are involved in the outer code. The concatenated code produces significant signal-to-noise gain over the baseline 4 kB LDPC. The soft decoding for the single-parity check code is very simple compared with the 4 kB LDPC code, and the concatenated code has a similar decoding complexity as the baseline 4 kB LDPC code. When the individual sectors within the outer code are updated in place, a read-modify-write procedure must be employed.

The effect of exchange coupling on switching fields in magnetic recording

Increased levels of exchange in a thin-film granular perpendicular recording media generally lead to reduced coercivity. However, this behavior cannot be deduced simply from the mean-field theory. To gain some insights into this behavior, a simple 2-D chain-of-grains model is employed. It can be recognized that the reduction in coercivity is due to the presence of in-plane magnetization and the corresponding in-plane fields that arise at the transitions between magnetized regions. In addition, a comparison is made between the two classical forms for exchange energy, 1-cosθ and 1/2θ2, that link adjacent grains. The latter form is found to have a much larger impact on coercivity and results in a more Kondorsky-like angular dependence.

Manifestation of higher-order inter-granular exchange in magnetic recording media

Exchange coupling between magnetic grains is essential for maintaining the stability of stored information in magnetic recording media. Using an atomistic spin model, we have investigated the coupling between neighbouring magnetic grains where magnetic impurity atoms have migrated into the non-magnetic grain boundary. We find that when the impurity density is low, a biquadratic term in addition to the bilinear term is found to better describe the inter-granular exchange coupling. The temperature dependence of both terms is found to follow a power law behaviour with the biquadratic exchange constant decaying faster than the bilinear. For the increasing grain boundary thickness, the inter-granular exchange reduces and also decays more quickly with temperature. Further simulations of a grain at a bit boundary show an unexpected energy minimum for in-plane magnetisation. This feature is reproduced if the biquadratic exchange term is included.

Ferromagnetic Resonance in Multilayer Perpendicular Films

Ferromagnetic resonance measurements can provide valuable information about thin-film structures, including recording media. This paper develops analytic expressions for the behavior of continuous-coupled multilayer perpendicular films and compares these with micromagnetic simulations. A frequency-domain equation is derived for small excitation, where the in-plane variables are expressed as complex quantities. The corresponding micromagnetic simulations implement the standard Landau-Lifshitz-Gilbert equation and are driven with an in-plane linear sinusoidal excitation. A four-layer graded medium is used as an illustrative example. The medium includes a relatively soft, lossy, weakly coupled cap. The analytic result reveals four resonant modes with the lowest mode at 16.40 GHz with zero external dc field. The simulation matches the analytic frequency and damping factor well but only for very low levels of excitation.