2022 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) / localized state / spin coupling / vacancy / magnetic proximity / neuromorphic device |
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| Outline of Annual Research Achievements |
To understand graphene (Gr) and hBN stability on Ni(111) substrate when a vacancy is created and their magnetic properties around the vacancy site, first-principles calculations were performed on several graphene and hBN vacancy systems as follows: 1.) Gr-vacancy/Ni; A spin-polarized localized state was observed when vacancy was created on the hollow site of Gr with a magnetic moment parallel to the neighboring C atoms, but opposite direction with Ni-slab. 2.) hBN-vacancy/Ni; A spin-polarized localized state was observed when vacancy was created on the B-site of hBN. The spin direction of a spin-polarized localized state was parallel to the N atoms surrounding the vacancy. Because of that, the spin direction of the localized state is parallel to Ni slabs creating a perfect spin filter. 3.) Gr-vacancy-hBN/Ni; Any vacancy on C atoms gives the vacancy a spin-polarized localized state with a spin direction opposite to Ni's substrate spin. Furthermore, the magnetic proximity effect from the Ni slab enables possible control of the spin direction of the localized state by controlling Ni magnetic alignment. However, when H atoms were considered at vacancy, creating V111, the H atoms preferred to buckle up, causing no spin-polarized localized state created at vacancy. From these results, a possible multi-level magnetic state can be expected by controlling the localized state spin direction of multi-stacking hBN and Gr vacancies.
The importance of hybridization at the interface on affecting the proximity effect was also found when hBN/oxidized Ni surface and hBN/Ni3Au was considered.
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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
Investigation of the importance of hybridization at the interface on the proximity effect by considering gas atoms, e.g., O atoms, on the surface of the Ni slab separating Ni and hBN was understood and within the original plan. Further investigation was done on possible control at the interface to have optimum proximity effect creating ideal tunneling magnetoresistance. It was found by considering heavy elements, i.e. Au atom, on the surface of the Ni slab, creating Ni3Au/hBN/Ni3Au magnetic tunnel junction, spin filtering becomes optimum by looking at its local density states. The proximity is optimized since strong hybridization was not created at the interface and the surface state works directly on the hBN insulator barrier. This result is beyond our original plan. On the other hand, the investigation of the proximity effect on the vacancy site was also done as per the original plan.
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| Strategy for Future Research Activity |
From these results, a multi-stacking of several hBN and graphene vacancies sandwiched with a Ni electrode will be proposed, such as Ni/hBN(vacancy)-hBN(vacancy)/Ni, Ni/hBN(vacancy)-Gr-hBN(vacancy)/Ni, and Ni/hBN-Gr(vacancy)-hBN/Ni. The proximity effect from both electrodes will affect the localized state spin direction, creating a possible multi-level magnetic state for the neuromorphic device purpose. Multiple magnetic states of the junctions would be investigated further by using the Heisenberg spin model. Further investigation on the spin-wave dispersion on the system will also be investigated. Furthermore, different interface conditions, i.e., the van der Walls interface, will be considered by considering the Ni3Au electrode and various vacancy systems. Finally, an additional system by considering hBN(Ge-doped) will be considered to create a magnetic tunnel junction with the van der Walls interface caused by 2D materials doping.
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