Inhomogeneous magnetization processes in advanced recording media

Abstract

Magnetization reversal processes of magnetic recording media are determined by the intrinsic properties such as crystalline anisotropy K1 and saturation magnetization JS, extrinsic properties such as shape, size and distribution of phases, and the external field gradient applied on the media. Taking advantage of finite element micromagnetic simulations, the detailed reversal processes and the influence of the intrinsic and extrinsic properties on the reversal behavior have been studied. The main objective of the thesis was to study advanced recording media, with a bit size smaller than 25 nm, corresponding to an areal density of at least 1 Tb/in2. For this dimension the media has a relatively small crystalline anisotropy K1 < 1 MJ/m3 which is sufficient for the "Stoner-Wohlfarth" coherent rotation process. The magnetization reversal process becomes inhomogeneous as the K1 increases larger than 1 MJ/m3 and the domain wall width gets comparable with the bit dimension.<br />The reversal by nucleation of reversal domains and the domain wall motion are more dominant, if the media exhibit a narrow domain wall followed by large K1 values. The concept of exchange spring media which consist of hard and soft magnetic layers, is one of the most promising structure for the switching of hard magnetic nano sized grains and avoiding the superparamagnetic effect of thermal switching. In the exchange spring media, a reversal domain is nucleated in the soft magnetic layer followed by the expansion into the the hard layer.<br />Various stacks of exchange spring media, FePtCu L10(hard) /[Co/Pt]N(soft) and FePt L10(hard)/ FePt A1(soft) were investigated. The microstructure of exchange spring media was directly studied using nanoanalytical and high resolution transmission electron microscopy (TEM). In the FePtCu/ [Co/Pt]N multilayer exchange spring media, the interface recrystallization and element interdiffusion between the hard/soft interface by heat treatment were found. For the case of the sputtered FePt L10/ FePt A1 structure a rough interphase boundary between the L10 and the A1 phase was found. The TEM results were implemented to the finite element models for micromagnetic simulations.<br />The switching field of the FePtCu hard layer was effectively reduced up to 40 % as the interlayer exchange coupling to the [Co/Pt]N soft layer increases. The prediction of the switching field reduction has been proved by the experiment. Strong interlayer coupling supported domain wall propagation to the hard layer. In the FePt L10/ FePt A1 structure, the spike of the hard layer at the rough interphase boundary helped domain wall propagation into the hard layer, therefore the switching field was reduced by 84 % compared with the single hard phase media only. The inhomogeneity of magnetization switching is also triggered by inhomogeneous external fields such as write head fields applied on recording media during the bit write process. The inhomogeneous write head field was obtained by analytic equations and the finite element method. Under the inhomogeneous magnetic head field, jitter transition between individual grains was reduced by 40 % in the exchange spring media with a top soft layer exposed to the stronger field. . On the other hand in the particulate media composed of hexagonal barium ferrite (h-BaFe), the individual particles were switched by coherent rotation only if the external field was larger than the switching field of the individual particle. The h-BaFe particulate media recording simulations were performed for linear densities larger than 400 kfci media varying head type, head field strength and head to media distance. Comparing various particulate media with sputtered CoCrPt-SiO2 granular media it have been shown that the Lindholm ring head and single pole tip (SPT) head exhibit a similar signal to noise ratio in the written state. The best performance was found for the case of the SPT head with a soft underlayer, if the particles are perpendicularly oriented to the recording plane. Another outcome of the thesis is to find out the contributions of the intrinsic and extrinsic properties on the switching field distribution (SFD) for 1 Tb/in2 hard disk media for based on the bit patterned media concept. The SFD increases proportional to the deviation of the crystalline anisotropy and the bit diameter, whereas a drastic increase is induced by a small amount of the easy axis misorientation. This study demonstrates the relationship between the inhomogeneity of the properties and the inhomogenous reversal processes. The inhomogeneities of the intrinsic and extrinsic parameters with a phase distribution effectively resulted in the inhomogeneous reversals, as well as inhomogeneous external field gradient does.