Fabrication of FePt and CoPt Magnetic Nanodot Arrays by Electrodeposition Process

Abstract

Electrodeposition processes have been extensively used to fabricate various magnetic thin films and micro/nano structures. We have attempted to utilize these processes to form magnetic nanodot arrays for use in bit patterned media (BPM) applications [1,2] for ultra-high density magnetic recording. In this paper, we will discuss our recent results on the synthesis of magnetic nanodot arrays using electrochemical deposition, focusing on FePt and CoPt. Patterned substrates with nanopore arrays were prepared by electron beam lithography (EBL), which has the capability to form smaller-scale patterns, as well as by UV-nanoimprint lithography (UV-NIL), with larger size but high throughput. N-Si(100) wafer covered with sputter deposited Cu or Ru was used as a substrate. Electrodeposition of CoPt [3] was successfully carried out into the patterned nanopores to form arrays with 20 nm diameter dots, and fabrication of even smaller dots with <10 nm diameter has been performed. In order to achieve precise control of growth, the initial stage of the deposition process was investigated in detail, and correlation between deposition conditions such as applied potential and microstructure of the deposited dots were studied. Based upon these results, we attempted to fabricate nanodot arrays using electrodeposition of FePt, which is expected to exhibit higher perpendicular coercivity, Hc, due to the formation of the fct L10 structure. Application of FePt to BPM has been widely studied, mainly using arrays of discrete nanoparticles [2]. Based upon the bath described in previous studies [4,5], deposition conditions were optimized. It was found that layered deposition of Fe-rich and Pt-rich regions, followed by RTA (rapid thermal annealing) process was most effective in forming the L10 structure, resulting in higher Hc at thinner thickness region; in particular, a Hc of 12 kOe was obtained at < 20 nm thick films. By using this method, the patterned deposition of FePt was attempted and the array with 15 nm diameter dots was formed, the structure of which has been maintained after the annealing process. This work was financially supported in part by Grant-in-Aid for Scientific Research, MEXT, Japan and by The Storage Research Consortium. [1] D. Weller et. al, Appl. Phys. Lett. 88, 222512 (2006). [2] B. D. Terris, T. Thomson, J. Phys. D: Appl. Phys., 38, R199 (2005). [3] T. Ouchi, Y. Arikawa, Y. Konishi, T. Homma, Electrochim. Acta, 55, 8081 (2010). [4] D. Liang, J. J. Mallett, G. Zangari, J. Electrochem. Soc., 158,149 (2011). [5] S. Wodarz, Y. Maniwa, H. Hagiwara, T. Otani, D. Nishiie, G. Zangari, T. Homma, IEICE Tech. Rep., MR2013-20, 7 (2013).