Twist pz Orbital and Spin Moment of the Wavy-Graphene/L10-FePd Moiré Interface

Abstract

A crystallographically heterogeneous moiré interface of hexagonal graphene (Gr) and a tetragonal L10-FePd alloy is bonded via van der Waals (vdW) forces. Robust interfacial perpendicular magnetic anisotropy was discovered at the L10-FePd side of the Gr/L10-FePd heterogeneous interface (H. Naganuma et al. ACS Nano, 2022, 16, 4139). This study focuses on the Gr side of the Gr/L10-FePd interface. X-ray absorption spectroscopy measurements of Gr from two different angles demonstrated that in addition to the π* orbital peak being observed at a glance incident angle (θΑ = 30°), it was also observed at a nominal incident (NI) angle (θΑ = 90°). The appearance of the π* peak at NI is attributed to the wavy Gr and strong bonding of the chemisorption-type vdW force. The densities of states of px, py, and pz from first-principles calculations indicate another reason for the π* peak at NI. The pz orbital of C twists into the x–y plane owing to its chemical bond with Fe or perturbation of the Fe ion to the C orbital. This twisted pz orbital appears near the Fermi level. Thus, the π* peak at NI can be interpreted to appear owing to three reasons: (i) the wavy Gr, (ii) the twisted pz orbital near the Fermi level, and (iii) the chemisorption-type vdW force. The X-ray magnetic circular dichroism of the C K-edge and first-principles calculations revealed that wavy Gr has a spin magnetic moment of 0.018 μB/C atoms but no orbital magnetic moment.