Fabrication of Pt-Rich CoPt/Co-Rich CoPt Bilayer Films with Perpendicular Magnetic Anisotropy for Vertical Domain Wall Memory
Authors: Araki Daiki , Sonobe Yoshiaki , Homma Takayuki 
Published: December 2024

Vertical domain wall memory (V-DWM) is an advanced racetrack memory, whose recording density reaches more than 10 Tb･cm-2 due to the racetracks installed vertically [1]. Conventionally, horizontal racetrack memories have been fabricated by dry etching [2-5]. However, ferromagnetic materials are difficult to etch into high-aspect-ratio structure vertically. Therefore, electrodeposition in nanoholes is a suitable process to fabricate V-DWM. In addition, V-DWM has multilayer structures in the vertical racetracks, which improves controllability of domain walls and lowers power consumptions [1]. In the multilayers, magnetic layers with perpendicular magnetic anisotropy (PMA) and those with weak anisotropy are stacked. CoPt alloy is a candidate for the material of V-DWM because its magnetic anisotropy can be altered by changing its composition [6]. Until now, though Co/Pt multilayer films have been fabricated by electrodeposition [7, 8], they do not have both PMA and high coercivity enough for storage medium. In this study, to establish the method of stacking CoPt layers which can be applied to V-DWM, the bilayer film where PMA Co-rich CoPt layer and Pt-rich one were stacked was fabricated, and its structures and magnetic properties were characterized.
At first, monolayer Co-rich CoPt films with PMA were electrodeposited on Pt substrates potentiostatically. The plating bath consisted of 1 mM CoSO4, 0.1 mM H2PtCl6, and 0.1 M Na2SO4. The counter and reference electrodes were Pt mesh and Ag/AgCl. The deposition potential was set to -650 mV. Deposition time was varied from 100 to 1300 s and suitable time for PMA was found. From the results of vibration sample magnetometer (VSM) measurements, the electrodeposited CoPt films showed PMA when the deposition time was from 500 to 700 s, and their easy axis changed into in-plane direction from 1100 s. The coercivity of PMA sample deposited for 500 s was 1.5 kOe. Cross-sectional transmission electron microscope (TEM) images showed the thickness of the CoPt film with PMA deposited for 500 s was 6 nm while that with in-plane magnetic anisotropy deposited for 1300 s was 10 nm. Energy dispersive X-ray spectroscopy (EDS) results showed the composition of Co in deposited films were 70-90 at%. Considering that crystal magnetic anisotropy keeps PMA even if the thickness of CoPt film with almost the same composition is 20 nm [9], the deposited film could have weak crystal magnetic anisotropy, which suggests that interface magnetic anisotropy between the CoPt layer and Pt substrate is the cause of the PMA.
Next, Pt-rich layer was deposited on the PMA CoPt layer by changing deposition potential from -650 mV to -500 mV. The deposition time of the Pt-rich layer was 2000 s to design its thickness to be 10 nm. A bilayer structure was observed in a TEM bright field image and EDS mapping. The average compositions of top and bottom layers were Co42Pt58 and Co72Pt28, respectively. The total thickness of the bilayer was confirmed to be 12 nm. From VSM measurements, fabricated the bilayer film showed PMA and its coercivity was 1 kOe. However, PMA in the bilayer film was smaller than that of the bottom Co-rich layer. The reason why PMA was weakened would be that large total thickness of bilayer weakened interface anisotropy. However, smaller Co composition of the top layer than that of the bottom layer decreases demagnetization in the film, which could prevent the easy axis to become in-plane direction.
Thus, Pt-rich CoPt/Co-rich CoPt bilayer film with PMA and the coercivity of 1 kOe can be fabricated. With this stacking method, CoPt multilayered structures which can be applied for V-DWM will be fabricated in the future.
Acknowledgements
This work was supported in party JST Strategic Basic Research Programs CREST (No. JR-MJCR31C1). Also, part of this work was the results of using a research equipment (G1026) shared in MEXT Project for promoting public utilization of advanced research of advanced research infrastructure (JPMXS0440500024).
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