2021 Fiscal Year Annual Research Report
Theoretical Study of Neuromorphic Devices Based on Two-dimensional-based Magnetic Tunnel Junctions
| Project/Area Number |
21J22520
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| Allocation Type | Single-year Grants |
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| Research Institution | Osaka University |
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Principal Investigator |
Harfah Halimah 大阪大学, 基礎工学研究科, 特別研究員(DC1)
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| Project Period (FY) |
2021-04-28 – 2024-03-31
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| Keywords | hexagonal boron nitride / graphene (Gr) / magnetoresistance (MR) / proximity effect / magnetic tunnel junction / optical-based MTJ / new MTJ mechanism |
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| Outline of Annual Research Achievements |
First-principles calculations was done on Ni/nhBN/Ni (n=1, 2 ,3 ,4, ...) and Ni/hBN-graphene-hBN/Ni magnetic tunnel junctions (MTJ). The calculations found that Ni/nhBN/Ni MTJ exhibits high tunneling magnetoresistance (TMR) at energy slightly higher than Fermi energy (~0.34 eV). This shifting of high TMR ratio originates from the transmission of electrons through the Ni atoms' surface states at the interfaces, which acts as an evanescent wave through the insulator barrier. The highest TMR ratio is achieved when the number of three hBN layers is considered with a TMR ratio of ~900%. By changing the middle hBN of the three hBN insulator barrier with graphene creating Ni/hBN-graphene-hBN/Ni MTJ, the TMR ratio becomes as high as ~1200%. This increment comes from the proximity effect in the form of the damped wave characteristics of the evanescent wave. When 3hBN is considered, the damping nature is more prominent since the surface state of Ni acts on the pz-orbital of B atoms which is unoccupied. However, when the middle hBN of Ni/3hBN/Ni MTJ is replaced with graphene, the surface state works on the pi-orbital of graphene, leading to a less significant damping wave nature. Because of that, the conductance of Ni/hBN-graphene-hBN/Ni becomes higher than that of Ni/3hBN/Ni MTJ when the MTJ is in a parallel configuration. Besides the high TMR ratio, it is also envisaged that this MTJ has a new reading process mechanism.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
The findings of high tunneling magnetoresistance ratio (TMR) by ~1200% and the new reading mechanism using optical irradiation was within the original plans. However, understanding the proximity effect characteristics at the interface opens insight into the possible controllable mass gapped Dirac cone of graphene in Ni/hBN-graphene-hBN/Ni magnetic junctions. From the band structure calculation of the most stable structure of Ni/hBN-graphene-hBN/Ni MTJ, a controllable mass-gapped Dirac cone (MGDC) of graphene was found. Although the MGDC size is not significant, my coworker was able to perform a further investigation to create a bigger size of MGDC and propose unique memory state characteristics by using mechanical motion. This finding is beyond our original plan.
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| Strategy for Future Research Activity |
Two analyses on the conductance of Ni/hBN-Gr V111-hBN/Ni MTJ will be done; 1.) The transmission magnetoresistance in the current-perpendicular-to-plane (CPP) direction, and 2.) the Analysis of the magneto-transport properties of graphene V111 in the current-in-plane (CIP) direction in the quantum transport simulations. The first calculation is done to determine the TMR ratio of the proposed MTJ. On the other hand, the second calculation is done to find possible multi-level memory states and the IMR ratio of the proposed MTJ. Additionally, the investigation on the importance of interface, especially the surface state of Ni slabs, will be done. A Ni/X-graphene-X/Ni and Ni/X-hBN-X/Ni MTJ system with X is adsorbed gas atoms, e.g., H, O, S, P, or F, will be considered. The investigation will be done through spin-GGA DFT. Then, transmission probability calculation will be done using non-equilibrium Green's function within Landauer-Buttiker formalism.
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