Seed Layer Effect on Electrodeposited 3D CoPt Alloy Nanowires for Magnetic Storage Application
Authors: Hasan Md Mahmudul , Huang Tongshuang , Saito Mikiko , Takamura Yota , Allongue Philippe , Homma Takayuki 
Published: December 2024

Nanowires with modulated magnetic properties like perpendicular magnetic anisotropy (PMA) could be applicable for next-generation magnetic storage devices like Racetrack memory. The 3D nanowire structures in Racetrack memory can provides high-density storage, where digital bits are stored in a series of magnetic layers separated by domain wall (DW).1,2 Electric current-driven DW motion can be used to move digital bits through the nanowires. Compared to traditional information storage devices, 3D magnetic nanowires with many DW encoded into them offer high-density storage.
Template-assisted electrodeposition technique in nanoporous membranes is one of the few processes allowing the formation of void-free nanowires with modulated composition.3,4 Among magnetic materials, CoPt alloys show large magnetic anisotropy and have been studied widely for magnetic storage devices.4,5,6 The crystal structure and composition of CoPt alloy nanowires play a vital role in getting strong PMA. The composition of CoPt alloy can be controlled by adjusting the applied potential during the electrodeposition process.3

In this study, we report the crystal structures and magnetic properties of CoPt alloy nanowires electrodeposited into a polycarbonate membrane template (PT) with 1% porosity and an average pore diameter of 100 nm. A Cu (400 nm) or a Pt (100 nm) seed layer sputtered at the back side of the PT. CoPt nanowires were produced using the standard three-electrode electrodeposition procedure, in which Ag/AgCl (sat. KCl) and Pt mesh were utilized as the reference and counter electrodes, respectively, while the seed layer coated PT served as the working electrode. The bath solution used for electrodeposition contained CoSO4.7H2O (0.1 M), Pt(NH3)2(NO2)2 (0.001 M), and H3BO3 (0.5 M). The solution has a pH of 5.2 and is stirred at 200 rpm at 40 °C. The applied potentials were -1000 mV vs Ag/AgCl (Sat. KCl) to form Co-rich CoPt alloy nanowires. The deposition time was adjusted to form a similar aspect ratio of CoPt alloy nanowires.
The X-ray diffraction (XRD) patterns in Figure 1a showed that the CoPt alloy nanowires present a hcp structure on both seed layers. However, the Pt (111) seed layer favours the epitaxial growth along the hcp (002) direction while other orientations (hcp 100) are found with the Cu seed layer. As a result, the CoPt nanowires deposited with the Pt seed layer present better PMA than those deposited with the Cu seed layer. The magnetization curves M – H of CoPt nanowires on the Pt seed layer (vibrating-sample magnetometry, VSM) present a good squareness (M/M
S = 0.64), a strong coercive field H
C = 660 Oe and a saturation magnetization of 740 emu cm-3 (Figure 1b). These properties are suitable for 3D magnetic domain motion memory. In summary, our work may contribute to regulating the epitaxial growth of CoPt crystalline structure, enhancing the magnetic properties of 3D CoPt alloy nanowires for magnetic storage devices.
Acknowledgement
This work was supported by JST, CREST Grant Number JPMJCR21C1, Japan.
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Figure 1