(Invited) Structural Control of Electrodeposited L10-FePt Nanodot Arrays Towards Reduction of Ordering Temperature
Authors: Wodarz Siggi , Hashimoto Shougo , Saitou Manabu , Zangari Giovanni , Homma Takayuki 
Published: February 2020

Electrodeposition has been widely used as a fabrication method of micro/nano structures due to its precise control of growth and high deposition uniformity at nanoscale. Utilizing these features, we have attempted to fabricate ferromagnetic nanodot arrays [1] by combining electron beam lithography (EBL) and electrodeposition for use in bit patterned media (BPM), which have been suggested as one of the few designs capable to achieve ultrahigh density in magnetic recording medium [2]. L10-FePt ordered alloy has been considered as one of the most promising candidates for the use in BPM due to its high magnetocrystalline anisotropy, Ku
[2]. In previous work, we have succeeded to fabricate FePt nanodot arrays with 35 nm in pitch [1], however, high temperature annealing at ca. 600 oC was needed for the phase transition to L10 ordered structure, which caused a deterioration of nanodot arrays. In order to solve this problem, in the present work, we have attempted to fabricate uniform L10-FePt nanodot arrays by applying multilayered FePt structure [3] and FePt-Cu alloy structure [4] to lower the ordering temperature.

Nanopore patterned substrates were fabricated by EBL onto sputter deposited Ru (60 nm thick) / Ti (5 nm thick) / n-Si (100) substrate. Utilizing these substrates, FePt was potentiostatically electrodeposited under the conditions summarized in Table 1 to fabricate FePt nanodot arrays. Post annealing treatment of deposited FePt was carried out in argon atmosphere (Ar 90 % and H2 10 %) using rapid thermal annealing system. Multilayered FePt were fabricated by pulse deposition of Fe-rich layer and Pt-rich layer with -1.4 and -0.8 V (vs. Ag/AgCl), and the thickness of each layer was set to around 2 nm by adjusting the pulse duration. For the fabrication of FePt-Cu alloy, CuSO4 was added into the bath condition of Table1.

In order to analyze the effect of multilayered structure on phase transition and coercivity of FePt, magnetic properties and crystal structures of multilayered FePt film and continuous FePt film, which was deposited with constant potential of -1.0 V (vs. Ag/AgCl), with thickness of 20 nm after annealing at 450 oC for 1 h were analyzed. As a result, perpendicular coercivity increased from 1.7 kOe to 6.6 kOe in the multilayered structure compared to continuous film, and x-ray diffraction (XRD) patterns of multilayered film showed clear diffraction peaks of L10 ordered structure, whose intensity were very small in the continuous film. In addition, ordering parameter, S, of FePt films were calculated form the axial ratio (c/a), which was determined from the XRD patterns, and it was found that S was increased from 0.24 to 0.80 in multilayered structure, indicating that phase transition of FePt was facilitated in the multilayered structure resulting in the increase of coercivity at low ordering temperature. Based on these results, fabrication of multilayered FePt nanodot arrays with 35 nm in pitch was attempted, and uniform nanodot arrays without deterioration of dots were successfully formed after post annealing process at 450 oC for 1 h.

Subsequently, lowering the ordering temperature of FePt was attempted by addition of Cu into FePt films to form ternary FePt-Cu alloy. XRD patterns of FePt-Cu films after annealing at 450 oC for 1 h showed peaks of L10 ordered structure without elemental Cu peaks, and (001) peak of L10 structure was shifted to higher 2θ angle compared to FePt binary alloy, providing evidence that Cu was alloyed with FePt to shrink the lattice in the c-axis. In addition, perpendicular coercivity of FePt-Cu film was higher than that of FePt film at annealing temperature of 450 oC, indicating that reducing the ordering temperature was successfully attained by the addition of Cu to FePt.

 These results demonstrated a successful fabrication of uniform L10-FePt nanodot arrays with Tbit/in2 level density, by controlling the fine structure of FePt nanodot arrays by applying multilayered structure or addition of Cu to lower the ordering temperature.

This work was financially supported in part by JSPS KAKENHI Grant Number 25249104.

[1] T. Homma, S. Wodarz, D. Nishiie, T. Otani, S. Ge, and G. Zangari, Eletrochem. Soc. Trans., 64, 1-9 (2015)

[2] B. D. Terris, and T. Thomson, J. Phys. D: Appl. Phys., 38, R199-R222 (2005)

[3] Y. Endo, N. Kikuchi, O. Kitakami, and Y. Shimada, J. Appl. Phys., 89, 7065-7067 (2001)

[4] T. Maeda, T. Kai, A. Kikitsu, T. Nagase, and J. Akiyama, Appl. Phys. Lett., 80, 2147-2149 (2002)

Figure 1