Tohoku University's Center for Innovative Integrated Electronics System (CIES) and a collaborative team has announced an analysis of chemisorbed crystallographically heterogeneous graphene/FePd interface, unveiling that the robust interfacial perpendicular magnetic anisotropy (IPMA) emerged because of high-electron density and van der Waals (vdW) chemisorbed-type force.
The graphene/FePd is expected to act as a recording layer in the future X nm generation of magnetic random-access memory.
The results were published on February 28 in ACS Nano.
The project saw Tohoku University work in collaboration with Université Paris-Saclay and Kobe University under the Japan Society for the Promotion of Science (JSPS) core-to-core project as well as The Tokyo Institute of Technology, Waseda University, CNRS/Thales, and High Energy Accelerator Research Organization. CIES Director Tetsuo Endoh served as the project leader.
The state-of-the-art MRAM recording layer, with an IPMA in CoFeB/MgO interface, has already achieved up to the 1X nm generation magnetic tunnel junctions (MTJs) for magnetic random-access memory (MRAM) applications. However, further innovation was needed in the X nm generation MTJs to improve retention characteristics.
Professors Pierre Seneor from the Université Paris-Saclay and Hiroshi Naganuma from CIES unveiled the performance of graphene/FePd bilayer recording layer as one of the candidates for the next X nm generation MTJs for MRAM.
The sample was fabricated by a chemical vapor deposition (CVD) method for hexagonal graphene (Gr) on a tetragonal FePd epitaxial film which was grown by r.f. magnetron sputtering. As shown in the scanning tunneling electron microscopy (STEM) images in fig. 1, the FePd is alternately arranged between Fe and Pd in the vertical direction. This means the FePd has a high degree of L10-ordering.
The atomic relationship of Gr/L10-FePd, which possess an energetically stable interface, purports theoretically [Fig. 1(b) and 1(c)] and experimentally [Fig. 1(a)] that the Gr armchair axis parallels the FePd L10. The Gr also underwent a slight strain by chemical bonding. Focusing on interatomic distance between the Gr and the FePd layers, the research group theoretically and experimentally determined it to be approximately 0.2 nm. The shorter distance between graphene and L10-FePd layers can be explained by the chemisorption-type vdW force having a strong orbital hybridization.
Depth-resolved X-ray magnetic circular dichroism analyses performed by High Energy Accelerator Research Organization (KEK) revealed that the orbital magnetic moment of Fe in FePd emerged at the Gr/ L10-FePd interface. The interfacial enhanced orbital moment showed obvious anisotropy to the perpendicular direction, explaining the main cause of IPMA. Moreover, the interfacial enhanced orbital moment and the interfacial enhanced electron density showed robustness. It is considered that the shortening interatomic distance produces a robust high electron density at the interface, resulting in chemisorption-type vdW force and orbital hybridization, eventually causing the robust IPMA to emerge at a crystallographically heterogeneous Gr/L10-FePd interface.
Finally, the potential of the MRAM's X nm generation suitability was calculated by micromagnetic simulation. The IPMA in Gr/L10-FePd is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. Then a micromagnetic simulation can assume both the PMA and IPMA. It is predicted that p-MTJs using Gr/L10-FePd can realize 10-year data retention in an extremely small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively.
- Publication Details:
Title: Unveiling a Chemisorbed Crystallographically Heterogeneous Graphene/L10-FePd Interface with a Robust and Perpendicular Orbital Moment
Authors: Hiroshi Naganuma, Masahiko Nishijima , Hayato Adachi, Mitsuharu Uemoto, Hikari Shinya, Shintaro Yasui, Hitoshi Morioka , Akihiko Hirata, Florian Godel, Marie-Blandine Martin, Bruno Dlubak, Pierre Seneor, Kenta Amemiya
Journal: ACS Nano