An increase of areal density can be achieved only if the medium SNR is increased. This requires a magnetically separated smaller grain and a smaller transition width. To reduce transition width, low $M_r t$ is also required in addition to a small grain size. As a result, as a grain size is reduced, energy barrier for magnetization reversal becomes smaller and thus thermal instability of written information is induced. Thus, the combination of SNR and thermal activation of small grains limit the potential areal density of the longitudinal recording media. Thus, this thesis is focused on a study on grain refinement and thermal stability improvement in longitudinal recording media.
In the first part, since magnetic properties and microstructure of magnetic layer depend strongly on microstructure of its underlayer, a novel VMn underlayer has been studied to reduce grain size and to improve magnetic properties of magnetic layer. It was found that VMn underlayer had (002) preferred orientation and finer and more uniform grain size compared with pure Cr underlayer. Also, the coercivity of magnetic layer on VMn underlayer was higher than that of magnetic layer on Cr underlayer. Contrary to the grain size of underlayer, however, the grain size of CoCrPt/VMn film was larger than that of CoCrPt/Cr film, due to the diffusion of Mn from VMn underlayer into magnetic layer. To use the fine and uniform grain of VMn underlayer, a CrMo intermediate layer as diffusion barrier layer was inserted at interface between magnetic layer and VMn underlayer. The CoCrPt/CrMo/VMn film showed fine and uniform grain size. The CoCrPt/CrMo/VMn film applied sub-bias voltage showed high coercivity. This implies that CoCrPt/CrMo/VMn type longitudinal media may be a good candidate for high density recording.
In second part, the effects of Co or CoCr-alloy interlayer on interlayer exchange coupling and thermal stability of AFC medium have been studied. In case of the conventional AFC medium, thermal stability of AFC medium has been restricted since the thickness and anisotropy of the stabilization layer has to be small enough to realize an anti-parallel magnetization configuration at remanence. In this thesis, only interfaces have been modified by inserting very thin Co or CoCr interlayers on both interfaces of Ru spacer layer to obtain higher interlayer exchange coupling, and thus higher thermal stability. In particular, the addition of 1 nm-thick Co interlayer yielded about 5 ~ 6 times enhancement of interlayer exchange coupling. This provides flexibility in tuning thermally stable media with the optimum read-write performance, because the stabilization layer with a high anisotropy and thicker thickness can be used.
This enhancement of interlayer exchange coupling is due to the increase of magnetic moments at interfaces. Also, it was found that interlayer exchange coupling depends on magnetic moments of magnetic layer as well as interlayer. Even though thermally stability of AFC medium with pure Co interlayer is larger than that of AFC medium with CoCr-alloy interlayer, the latter with low inter-granular exchange coupling will be more desirable than the former with high inter-granular exchange coupling from the view-point of media noise.
In the last part, magnetic properties and thermal stability of a new AFC medium with a CrMo spacer layer have been studied and compared with those of single-layer medium and conventional AFC medium with a Ru spacer layer. In addition, the magnetic interaction characteristic in AFC medium has been investigated by ΔM, MFM, and micromagnetic simulation. The reduction in $\frac{M_r}{M_s}$ (or $M_rt$) at remanent state was also found in the new AFC medium due to the antiferromagnetic coupling, similar to the conventional AFC medium. Thermal stability ratio ($\frac{K_U V}{k_B T}$) of the new AFC medium was slightly smaller than that of the conventional AFC medium, but was larger than that of single-layer medium.
In case of multilayer such as AFC medium, it was observed experimentally and predicted by micromagnetic simulation that ΔM curve depends on the interlayer exchange coupling ($J_ex$) as well as the inter-granular exchange coupling ($A^*$). The higher ($J_ex$) is, the smaller positive ΔM value. The converse is true for the inter-granular exchange coupling. The positive ΔM value of the new AFC medium was smaller than that of the conventional AFC medium, even though the $J_ex$ of the former was smaller than that of the latter. This indicates that the inter-granular exchange coupling of the new AFC medium is weaker than that of the conventional AFC medium. The possible mechanism of this may be the diffusion of Cr from CrMo spacer layer into grain boundaries of magnetic layer. This was also supported by smaller and more uniform magnetic cluster sizes of the new AFC medium as observed by MFM image. Thus, it is expected that noise of the newly developed AFC medium lowers compared with that of the conventional AFC medium without compromising a large thermal stability.
