For high density information recording, magnetic disks for HDD must have low noise characteristics and at the same time must have good thermal stability to overcome the superparamagnetic effect. In perpendicular recording, DC erased noise caused by reversed domain in the recorded bit as well as the transition noise becomes important noise sources. In order to reduce the DC erased noise, the number of reversed domains in a recorded bit must be reduced. The reduction of DC erased noise can be achieved only if a nucleation field of a medium exists in the second quadrant of major hysteresis loop. And to reduce transition noise, media should a low α value suggesting a decrease in the exchange coupling. In this research, the relationship between nucleation field and exchange coupling of CoCrPt single layered perpendicular media as a function of Pt, Cr content of the magnetic layer and magnetic layer thickness have been studied. The effects of introduction Pt underlayer and Pt-Nb12at.% to CoCrPt/Ti have been also examined.
Firstly, The change of magnetic characteristics with the increase of Pt content of magnetic layer was discussed. The Hc and $K_{eff}$. had a maximum values of 4700Oe and $2.1\times10^6 erg/cc$ at 13.2 at.% Pt, 30 nm thickness. This increasing with Pt content resulted from crystalline anisotropy increase. For Pt contents larger than 13.2 at.%, the Hc and $K_{eff}$. were decreased due to the generation of fcc phase or stacking fault with excess Pt content. The other reason was the change of magnetization reversal mechanism from spin rotation to domain wall motion because Cr content relative to increasing Pt content became less. The variation of Hc seemed to correlate with the dependence of $K_{eff}$. The media with 17.6 at.% Pt had negative nucleation field with 2500 Oe and exchange slope with 2.4. To reduce exchange coupling by Cr segregation enhancement, Cr content was increased from 16.1 at.% to 19.2 at.%. The Hc, Hn and exchange slope were decreased with Cr content. The increase of the Cr content in the Co-enriched grains due to Cr addition gave rise to a decrease of the magnetocrystalline anisotropy and negative nucleation field was decreased. But the exchange coupling was decreased by strengthening of Cr segregation.
Negative nucleation field was increased with the magnetic recording layer thickness showing a maximum value at 30 nm and decreases with further increase of the magnetic layer thickness. Exchange slope suggesting exchange coupling was decreased with the magnetic recording layer thickness. That is, as the magnetic layer thickness was increased, Cr segregation to column boundaries is enhanced. Shearing of the slope in the switching region of M-H loop was the evidence for the above prediction. The saturation magnetization(Ms) increases when the ML thickness was increased from 20 nm to 50 nm. This is another reason of Cr segregation enhancement with magnetic layer thickness. The increase of negative nucleation field with 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. The reason of decrease of negative nucleation field with increasing the magnetic layer thickness is associated with magnetization reversal mechanism by incoherent rotation. Therefore, The thickness of magnetic layer showing maximum nucleation field depended on the composition of magnetic layer and inter-granular exchange coupling. The $CoCr_{19.2}Pt_{18}$ media showed negative nucleation field with 1250 Oe and exchange slope with 1.18.
Secondly, Pt underlayer was introduced between Ti and CoZr to investigate the effects of Pt layer on magnetic layer. A fcc (111) can be used as a underlayer for heteroepitaxial growth of hcp (00.2). By deposited on Pt underlayer, Ti would have better crystallographic property. Thus Ti caused the magnetic and crystallographic properties of magnetic layer to be enhanced. Introduction of 10 nm Pt underlayer resulted in a strong c-axis orientation of magnetic layer. It can be seen that by introducing of 10 nm Pt underlayer, the perpendicular coercivity was increased by 350Oe and the in-plane coercivity was decreased by 170Oe due to enhancement of c-axis alignment. Also negative nucleation field and effective crystalline anisotropy energy were increased 750Oe and $0.8×10_6 erg/cc$, respectively. A 30 nm CoCr19.2Pt18 on Ti(20 nm)/Pt(10 nm) double underlayer showed negative nucleation field with 1500Oe and exchange slope with 1.26.
The addition of 12 at.% Nb to Pt caused the grain of Pt to be smaller. But when Pt was deposited on CoZr seed layer, the grain of Pt was small originally. Thus when Pt-Nb12at.% was deposited on CoZr seed layer, the smaller grain of Pt-Nb was obtained. But the amount of decrease was not so much more than without CoZr seed layer.
The recording characteristics of our experimental media with 2.5inch disk were discussed. All media showed high signal more than 20 dB at 600 KFCI and signal more than 10 dB at 800 KFCI.
In conclusion, It is found that when Pt content was from 10.2 at.% to 13.2 at.%, negative nucleation field was nearly zero and exchange slope was nearly a unit. The smaller Pt content relative to Cr content made Cr segregation and exchange decoupling enhance. And then the Pt content was added to around 18 at.%. As a result, It was obtained media with negative nucleation field greater than 2500Oe and exchange slope with 2.4. As increasing Cr content from 16.1 at.% to 19.2 at.% at nearly constant Pt content, negative nucleation field and exchange slope were decreased to 1200 Oe and 1.1, respectively. It is assumed that a negative nucleation field for high thermal stability and low D.C. erase noise is mutually contradictory to low exchange coupling for low transition noise. The negative nucleation field was proportionally varied with exchange coupling. If the distribuionf grain size large and t magnetic interaction of grains is small, the negative nucleation field is small, also. The this behavior is occurred by easy magnetization reversal of grain on accout of thermal agitation at room temperature.
It is necessary to researched further CoCrPt single layered perpendicular media with a high negative nucleation field and a low exchange slope favoring for higher density recording region.
