Perpendicular magnetic recording has been considered to be a candidate technology for realizing high areal recording densities. Co/Pd multilayers are strong candidates for perpendicular recording media. Co/Pd multilayers have high perpendicular anisotropy energy. Also these multilayers can be easily controlled with respect to parameters such as Hc, Ms, and SQ. However, they have disadvantage of large transition noise. Because Co/Pd multilayers have columnar structure, grains oriented normal to the film plane and these grains are coupled strongly. To reduce the transition noise, the magnetic coupling among the grains must be decreased. This can be done by adding 3rd elements to Co or Pd sublayers or depositing at high sputtering pressure or using the underlayer which have small grains.
In this study, to reduce magnetic exchange coupling in Co/Pd multilayers, we studied on effects of the underlayer on grain size and magnetic properties in Co/Pd multilayers. The Co/Pd multilayers were deposited by a D.C. magnetron sputtering. The basic structure was $Ti(50Å)/Pd(200Å)/[Co(4.2Å)/Pd(9.0Å)]_30$.
In the first part, instead of Pd underlayer, we deposited PtB and PtSi underlayer to reduce the exchange coupling. The solubility of B into Pt is very small, so if B atoms added into Pt, B-riched phases or B phase will be formed at the boundary region of Pt grains. This principles also can be applied to the case of adding Si atoms into Pt, because the solubility of Si into Pt is very small, too. If these segregation process is successful, the underlayer will have small prominence or depression morphology. Experimentally, the grains of PtB and PtSi became more smaller than that of Pd. And the grain of Co/Pd multilayers on the PtB or PtSi underlayer were also small, compared with the case of Pd underlayer. In the case of PtB or PtSi underlayer, the hysteresis loops were more sheared, that is, the intergranular exchange coupling becomes more weak. In the cross sectional TEM images, Co/Pd multilayer deposited on PtB or PtSi showed more clear physical separation at the column boundaries. Also from the MFM images, we can obtain more dotted and smaller domains in the Co/Pd multilayer deposited on PtB or PtSi, and this domain size have been reduced about 18% and 30%, respectively.
In the second part, effects of the grain size of underlayer on Co/Pd multilayer have been studied in detail. The grain size of CoCrPtO media can be tunable by changing the O2 gas ratio during deposition. So we used the CoCrPtO media as templates such as Si wafer. Pd or PtSi underlayer were deposited onto the CoCrPtO template and the Co/Pd multilayers were deposited on these underlayers sequentially. In the case of Pd underlayer, the grain becomes small due to the small grain of the CoCrPtO template. Also, more decreasing the grain size of these template, more reducing the exchange coupling. However, in the case of PtSi underlayer, the grain size of Co/Pd multilayers did not become small largely. Because the PtSi consists of very small grains, effects of CoCrPtO template is small. And the exchange coupling become strong rather than weak by using Pt underlayer on CoCrPtO template.
In the third part, we applied the Ar plasma etching (sputter-etching) to the surface of PtB or PtSi underlayer. Because the grains and their boundary regions have different sputtering rate in the PtB or PtSi underlayer, after etching, the surface morphology will be changed. The Co/Pd multilayers were deposited onto etched surface. But, in the hysteresis loop, the exchange coupling become more stronger. So we deposited intermediate layer between etched layer and Co/Pd multilayers to decrease the exchange coupling. When plasma etching was applied more times, the exchange coupling was increased. In the cross sectional TEM images, the spacing between columns was narrow in the 4 min etched case. This indicates the exchange coupling is strong. Also, this tendency coincides with result of MFM images. 4 min etched case showed more larger domains.
