Characterization of Electrodeposited Co-Pt Nano-Dot Array at Initial Deposition Stage

Electrodeposition is a promising method to fabricate micro-nano structures and devices featuring its high controllability and areal selectivity at the nano scale. We have proposed [1] a process of fabricating the Co-Pt nano-dot arrays by the combination of electrodeposition and electron beam lithography (EBL), for the application to bit patterned media (BPM), which has been proposed as an ultra-high density magnetic recording medium. Since the individual dot consisting of a single grain of magnetic metal is ideal for the application of BPM, control of the deposition from the initial stage (nucleation and growth) is significant.

In order to investigate this point, in the present work, the effect of deposition parameters, such as applied potential, on the initial deposition behavior of Co-Pt was analyzed. In addition, in order to analyze the effect of morphological changes of Co-Pt through applied potential on the magnetic property in conjunction with the analysis of initial deposition stages, magnetic properties of the Co-Pt thin films were analyzed.

Nano-patterned pores with different diameters (10 nm to 65 nm) were fabricated by EBL onto Ru underlayer, which was sputter-deposited on n-type Si (100) substrate. Co-Pt was potentiostatically electrodeposited in these nano-pores by different applied potential from －400 mV to －1100 mV vs. Ag / AgCl. The bath composition and electrodeposition condition are shown in table 1.

Figure 1 shows SEM images of Co-Pt at the initial deposition stage in the nano-pores with different diameters of 10, 15, 25, 35, 50 and 65 nm. In Fig. 1, multi nucleation was clearly observed in nano-pores with relatively large diameters, and the number of nuclei was decreased for the smaller diameter nano-pores. However, with the applied potential of －900 mV, Co-Pt was deposited as multi-grains even in a nano-pore with a 10 nm diameter due to multi nucleation of fine nuclei with the size of 2 nm. In order to obtain a single grain, the nucleation density should be reduced to form a larger grain by applying positive applied potential. Figure 2 shows representative SEM images of the Co-Pt nano-dots deposited at －500 mV and －600 mV in 10 nm diameter nano-pores. In Fig. 2, nucleation of Co-Pt was suppressed in the nano-pores of 10 nm. Furthermore, Co-Pt of about 9 nm that did not show any grain boundary was observed, which is indicating the deposition of a single grain. This was due to progressive nucleation that preferentially occurred to increase the grain size of Co-Pt in low overpotential.

In addition, increase in the coercivity at the initial stage was observed when Co-Pt thin films were deposited with more positive potential such as －600 mV, since Co-Pt nucleated and grew along with a columnar-like morphology of the Ru underlayer to form a magnetically isolated structure. Thus, in the case of Co-Pt nano-dot arrays, it was suggested that high values of coercivity and saturated magnetization, which are significant for the BPM, could be obtained with lower overpotential deposition, to form single or single-like grains grown along the grain of Ru underlayer with a high crystal orientation from the initial deposition stage.

These results indicated that formation of a BPM consisting of a single grain exhibiting high magnetic property is possible by depositing the Co-Pt in ultra-fine nano-pores by applying the potential in which progressive nucleation preferentially occurs.

This work was financially supported in part by Grant-in-Aid for Scientific Research from MEXT, Japan and by The Storage Research Consortium.

[1] T. Ouchi, Y. Arikawa, Y. Konishi, T. Homma, Electrochimica Acta, Vol. 55, No. 27, pp. 8081-8086, 2010