Presentation of the crystallographically heterogeneous chemisorbed graphene/FePd interface

The Center for Innovative Integrated Electronics System (CIES) at Tohoku University and a collaborative team announced an analysis of the crystallographically heterogeneous chemisorbed graphene/FePd interface, revealing that robust interfacial perpendicular magnetic anisotropy (IPMA) emerged in due to high electron density and van der Waals (vdW) chemisorbed type force.

Graphene/FePd is expected to act as a recording layer in the next generation X nm magnetic random access memory.

The results were published Feb. 28 in ACS Nano.

The project saw Tohoku University work in collaboration with Paris-Saclay University and Kobe University under the core-to-core project of the Japan Society for the Promotion of Science (JSPS) as well as with Tokyo Institute of Technology, Waseda University, CNRS/Thales, and High Energy Accelerator Research Organization. CIES Director Tetsuo Endoh served as the project manager.

The state-of-the-art MRAM recording layer, with an IPMA interface in CoFeB/MgO, has already achieved generation 1X nm magnetic tunnel junctions (MTJs) for magnetic random access memory (MRAM) applications. However, further innovation was needed in the Gen X nm MTJs to improve the retention characteristics.

Professors Pierre Seneor from Université Paris-Saclay and Hiroshi Naganuma from CIES unveiled the performance of the graphene/FePd bilayer recording layer as one of the candidates for the next generation of MTJ X nm for MRAM.

The sample was fabricated by a chemical vapor deposition (CVD) method for hexagonal graphene (Gr) on a tetragonal FePd epitaxial film that was developed by rf magnetron sputtering. As shown by the scanning electron microscopy (STEM) images in fig. 1, the FePd is arranged alternately between Fe and Pd in the vertical direction. This means that FePd has a high degree of L10 order.

The atomic relationship of Gr/L10-FePd, which has an energetically stable interface, theoretically claims [Fig. 1(b) and 1(c)] and experimentally [Fig. 1(a)] that the axis of the chair Gr is parallel to the FePd [100]L10. The Gr also underwent slight deformation by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the research group theoretically and experimentally determined it to be around 0.2 nm. The shorter distance between the graphene and L10-FePd layers can be explained by the chemisorption type vdW force having strong orbital hybridization.

(a) Cross-sectional observations by scanning tunneling electron microscopy (STEM) using three different detectors providing brightfield (BF), annular brightfield (ABF), and high-angle annular darkfield (HAADF ). Acceptance angles for BF, ABF, and HAADF were 0-34.8, 10.1-105.0, 79.5-200 mrad, respectively. STEM observation for bilayer graphene. (b) Simulated BF, ABF and HAADF-STEM images from an electron beam incident [110]L10. The atomic model of the cross-sectional image [110]L10 view of Gr/FePd and picture-in-plane [001]The L10 view for carbon atoms only has been described. (c) The atomic positions calculated via first principles calculation based on the van der Waals force (vdW) and the stable structure of the crystallographically heterogeneous Gr/FePd interface of the 3D image were depicted. The chair axis of Gr is parallel to the FePd [100]L10. © Tohoku University

Deep-resolved X-ray magnetic circular dichroism analyzes performed by the High-Energy Accelerator Research Organization (KEK) revealed that the orbital magnetic moment of Fe in FePd emerges at the Gr/L10-FePd interface. The enhanced interfacial orbital moment showed obvious anisotropy to the perpendicular direction, explaining the main cause of IPMA. Moreover, the interfacial enhanced orbital moment and interfacial enhanced electron density showed robustness. Interatomic distance shortening is considered to produce a robust high electron density at the interface, resulting in a chemisorption-like vdW force and orbital hybridization, ultimately causing the emergence of the robust IPMA at a Gr/L10- interface. Crystallographically heterogeneous FePd.

(a) Experimental setup for fluorescence-yielding depth-resolved X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD). Fluorescence X-rays emitted after XAS were acquired separately at different detection angles θd on a CCD device. The lowest θd was determined at around 0.1°. The magnetic field was tilted by rotations θi and θis of 90° and 30°, respectively, implying that a magnetic field of 0.87 T was applied out of plane and rotated 30° from the direction in the plan. (b) Fe L3 and L2 edge spectra with different λ ranging from 0.25 to 2.5 nm for θ i = 90°. A magnetic field was applied to the depth profile of the out-of-plane direction (θi = 90°) of the XMCD spectrum obtained based on the differences of circularly polarized XAS. XAS and XMCD spectra at (c) interface and (d) inner layer. © Tohoku University

Finally, the X nm generation suitability potential of MRAM was calculated by micromagnetic simulation. The IPMA in Gr/L10-FePd is useful because it can be incorporated into the large perpendicular magnetic anisotropy (PMA) of L10-FePd. Then a micromagnetic simulation can assume both PMA and IPMA. It is predicted that p-MTJs using Gr/L10-FePd can achieve 10-year data retention in an extremely small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively.

Release details:

Title: Unveiling of a crystallographically heterogeneous chemisorbed 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

Review: ACS Nano

DOI: doi.org/10.1021/acsnano.1c09843

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