Role Of Orbital Bond and Local Magnetism In Fe₃GeTe₂ and Fe₄GeTe₂: Implication For Ultrathin Nano Devices

We investigated the role of the bonding environment and Coulomb screening in the van der Waals (vdW) metallic ferromagnets Fe3GeTe2 (FGT3) and Fe4GeTe2 (FGT4). The electronic origins of the contrasting magnetic anisotropy of FGT3 and FGT4 were examined using X-ray absorption spectroscopy (XAS) and ab initio calculations. We found that the orbital magnetic moment of FGT4 (0.050 ??B/Fe) is smaller than that of FGT3 (0.11 ??B/Fe). Here, the difference in Fe dumbbell structure gives rise to the more (less) localized d3z2???r2 states in FGT3(FGT4), and an effective orbital level splitting of t2g manifolds results in the large magnetocrystalline anisotropy in FGT3. The XAS measurements and the calculations additionally show that the Coulomb interaction is more effectively screened in FGT4 than in FGT3. The following theoretical analyses show that the screened Coulomb interaction becomes a key parameter to determine the magnetic anisotropy energy, especially in the thickness dependence. Finally, we discuss that the recent experimental demonstrations can be understood upon the effect of the bonding environment. Our investigation reveals essential electronic degrees of freedom to control the magnetism of the vdW ferromagnet for ultrathin nano devices.