It has been reported recently that a single layered perpendicular medium with a negative nucleation field is very favorable for high signal output at higher density recording in both theoretically and experimently. In the previous work, it was reported that a CoCrPt/Ti single layered medium showed a negative nucleation field when CoCrPt thickness was less than 40 nm. In this research, change of negative nucleation field and magnetic properties of CoCrPtX single layered perpendicular media as a functon of Pt content of the magnetic layer(ML) and magnetic layer thickness have been studied.
In chapter 4, change of structural and magnetic characteristics with the increase of Pt composition of ML was reported. As Pt composition increases from 8.3 at% to 14.9 at%, FWHM of CoCrPt(00.2) peak in rocking curve is 2.8 to 4.2 degree and grain size and distribution decrease. Saturation magnetization slightly decreases with the increase of Pt content. Negative nucleation field increases and at Pt 14.9 at%, a medium of 20 nm ML thickness shows high negative nucleation field of beyond 3000 Oe. This increase of negative nucleation field with increasing Pt content is associated with increase of the crystalline anisotropy energy by increased Pt content nad decreased Cr content.
In chapter 5, change of magnetic properties and negative nucleation field with increasing the ML thickness from 10 nm to 40 nm was studied.
The saturation magnetization(Ms) drastically increases when the ML thickness increases from 10 nm to 20 nm and gradually increases with further increase in the thickness from 25 nm. One of the origin of the Ms behaviour is associated with the existence of amorphous-like reacted layer by intermixing between Co and Ti at CoCrPt/Ti interface and this contribution is more pronounced at thinner thickness. The other origin of the Ms behaviour is change of the degree of Cr segregation with the increase of the ML thickness. That is, as the ML thickness increases, Cr segregation to column boundaries is enhanced. Shearing of the slope in the switching region of MH loop and change of magnetic domain images are the evidences for the above prediction. Especially, when the film thickness is 10 nm, domain size is rather large and domains are connected. However, as the film thickness increases to 20 nm, domain size becomes smaller and domains look like a dot type due to better isolation among grains.
In this study, we have tried to analyze the different degree of Cr segregation for the different ML layer thickness although the reason of Cr segregation in CoCr alloys is not established. The lattice parameters, a and c of Co alloy films were measured as a function of the CoCr alloy thickness and it was found that the lattice parameters are expanded at thinner magnetic layer. Therefore, the reasons for the different segregation with the thickness are considered to be associated with change of residual stress within films. The tensile residual stress at thinner thickness hinders Cr segregation. The negative nucleation field increases with increasing magnetic layer thickness and shows a maximum, then decreases with further increase of magnetic layer. The increase of negative nucleation field in thinner thickness is associated with the increase of the shape anisotropy energy with increasing film thickness and the increase of crystalline anisotropy energy by the decrease of Cr contents within the column. After a maximum point, the decrease of nagative nucleation field must be associated with change of rotation mode from coherent rotation to incoherent rotation as the aspect ratio of the column increases.
In order to clearify the understanding of the Ms behaviour with increasing magnetic layer thickness, a CoCr paramagnetic intermediate layer was inserted between the Ti underlayer and the CoCrPt magnetic layer. The Ms increase by the insertion of the CoCr intermediate layer is associated with exclusion of the reacted layer at the interface of the magnetic layer and Ti layer and Cr segregation enhancement. Relaxation of tensile residual stress with the increase of CoCr layer thickness enhances Cr segregation.
In the last chapter, the recording characteristics of CoCrPt(20 nm)/Ti(20 nm)/CoZr(20 nm)/ 2.5inch disk were discussed. The composition of the magnetic layer was $CoCrPt_14.5.$ Hn was 2800 Oe, Hc was 4500 Oe, and Exchang Slope was 3.93. The medium showed a high signal of 42 dB at 400 KFCI and a detectable signal at 800 KFCI. However, transition noise of this medium showed a significantly high level. To reduce the transition noise, the exchange slope must be reduced. In the last part, controlling of exchange slope was reported.
Based on these results, it is concluded that CoCrPt single layered perpendicular media with a high negative nucleation field and a low exchange slope will be favorable for higher density recording region.
