Perpendicular magnetic recording(PMR) is the most possible candidate for the ultra high density(UHD) recording beyond 1 Tb/in2. The requirements for the UHD PMR are high anisotropy energy(Ku) for the thermal stability, small grain size with little distribution and low exchange coupling between grains for the low medium noise. The CoCrPt type medium is the most feasible one to meet the requirements mentioned above due to relatively high Ku, easy c axis orientation alignment, and well grain isolation. Grain isolation can be achieved by Cr segregation to the boundary at the elevated temperature for the conventional CoCrPt PMR media or oxide formation at the grain boundary in case of CoCrPt-oxide PMR media.
In this thesis, the methods and mechanism for grain size refinement and exchange coupling reduction in CoCrPt type media were studied by using conventional CoCrPt PMR media and CoCrPt-oxide PMR media. In the first half, we modified Cr and Pt composition to investigate the compositional change of magnetic properties and microstructure and found out the optimum composition in CoCrPt PMR media. Moreover, CoCrPtB and CoCrPtTa bottom recording layer(RL) were inserted beneath the RL to reduce grain size and exchange coupling furthermore. In the second half, the oxide type CoCrPtO and $CoCrPt-SiO_2$ media were studied for their improved grain isolation and Ku in comparison with conventional CoCrPt media. The dependence of sputtering condition of RL and UL on the oxide formation at the grain boundary and magnetic properties were also investigated in the CoCrPt-oxide media. There are four parts in this thesis.
In the first part, effects of Cr and Pt composition on the magnetic properties such as coercivity(Hc), negative nucleation field(-Hn), exchange slope, and microstructure in the conventional CoCtPt media were studied. As Pt composition increases, Hc and -Hn increase, but exchange slope also increase slightly. On the other hand, as Cr composition increases, Ms decreases drastically and exchange slope also decreases. These results are caused by the fact that Pt increases Ku and exchange coupling between grains, but Cr deteriorates Ku and enhances grain isolation by Cr segregation. Pt and Cr affect the magnetic properties of CoCrPt media contradictorily. The optimized condition was found at the composition of $CoCr_{18}Pt_{19}$ where Hc is 3850 Oe, Hn is -1600 Oe and exchange slope is 1.2. FeHfN soft underlayer and Ti layer were used to make double layered PMR media. From the read/write test, very clear and sharp transition bit pattern at the linear density of 310 kfci was measured by MFM. However, SNR begins to decrease drastically beyond the density. This indicates that the grain size and exchange coupling of the CoCrPt media are not small enough for the higher density.
In the second part, CoCrPtB and CoCrPtTa layers which are well known for small grain size and exchange coupling are inserted beneath the RL to reduce the grain size and exchange coupling furthermore for the CoCrPt media studied at the previous part. The effects of CoCrPtTa and CoCrPtB bottom recording layers (RL) on the magnetic properties and read/write performance of CoCrPt/soft magnetic layer perpendicular recording media were investigated. Even though the perpendicular coercivity of the media with these bottom RLs was reduced by 500 Oe, these media still showed low exchange slope of 1.4 and large negative nucleation field of about -1000 Oe. Additionally, the reduced grain size of the media with these bottom RLs was observed by TEM. From the read/write test, these media with the CoCrPtB bottom RL showed improved performance of 3~5 dB higher SNR and OW compared to the media without bottom RL. These enhancements could be attributed to the reduction of grain size of the magnetic layer and weakening of intergranular interaction between grains by insertion of the bottom RL.
In the third part, sputtering power dependence on magnetic properties and microstructure of CoCrPtO PMR media was examined. Also, change of the magnetic properties of CoCrPtO layers on the different soft underlayers was investigated. As oxygen gas ratio increases, Hc increases regardless of the variation of the sputtering power up to a certain critical oxygen gas ratio which depends on the sputtering power. Then Hc drops sharply in all cases. The higher the power is, the higher the critical oxygen gas ratio is. As the oxygen gas ratio increases, exchange slope also decreases more rapidly at the lower sputtering power than higher power. The small grains of the size of ~ 7 nm with thick amorphous-like grain boundaries are observed in the media sputtered at the power of 75 W. It is considered that the selective oxidation of grain boundary is performed more effectively at low sputtering power. CoCrPtO layer on NiFeNb soft magnetic underlayer showed higher Hc of 5000 Oe and well isolated grain in comparison with CoCrPtO layer on CoZrNb SUL. This difference of coercivity behavior may be associated with difference CoCrPtO layer on residual stress or surface roughness.
In the last part, effects of Ru underlayer deposition conditions on microstructure and magnetic properties in $CoCrPt-SiO_2$ perpendicular recording media were studied. $CoCrPt-SiO_2$ media have been widely studied for their excellent grain isolation and high magnetic anisotropy energy. Effects of composition and sputtering condition of $CoCrPt-SiO_2$ recording layers(RLs) on magnetic properties have been studied by many researchers. However, the magnetic properties and microstructure of RLs are much affected by the underlayer(UL) condition. In this part, we modified Ru UL condition by changing the sputtering pressure from 3 mtorr to 10 mtorr and studied the magnetic properties and microstructural changes of RLs. Both perpendicular coercivity(Hcp) and in-plane coercivity(Hci) of RLs increase as Ru UL sputtering pressure increase, while the slope of hysteresis loop at Hc keep decreasing with the sputtering pressure. The increase of Hci is caused by the poor textured Ru UL deposited at high pressure. On the other hand, the enhancement of Hcp is attributed to the improved grain isolation in spite of the poor textured Ru and RL which is supported by cross sectional and plane view TEM observation. This also can be explained by the decrease of exchange slope with the increase of sputtering pressure of Ru UL. The surface of Ru deposited at 10 mtorr is very rough and it has surface morphology like dome shape. The grain boundaries of this dome shaped grains have more open structure and easily make formation of oxide. In addition, we deposited Ru UL at 3 mtorr and continued to grow Ru UL at 10 mtorr to make well textured Ru UL with the dome shaped surface. High Hcp and low Hci were obtained from the double layered Ru UL. These results indicate that the enhanced grain isolation and Hcp are mainly caused by the grain size and surface roughness of Ru UL, whereas Hci is mainly affected by Δθ50 of Ru UL.
