The surface magnetism of Fe(001) was studied in an atomic layer-by-layer fashion by using the in situ iron-57 probe layer method with a synchrotron Mössbauer source. The observed internal hyperfine field $H_{\mathrm{int}}$ exhibits a marked decrease at the surface and an oscillatory behavior with increasing depth in the individual upper four layers below the surface. The calculated layer-depth dependencies of the effective hyperfine field $|H_{\mathrm{eff}}|$, isomer shift $\delta$, and quadrupole shift $2ϵ$ agree well with the observed experimental parameters. These results provide the first experimental evidence for the magnetic Friedel oscillations, which penetrate several layers from the Fe(001) surface.

The origin of the interfacial perpendicular magnetic anisotropy (PMA) induced in the ultrathin Fe layer on the Au(111) surface was examined using synchrotron-radiation-based Mössbauer spectroscopy (MS), x-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). To probe the detailed interfacial electronic structure of orbital hybridization between the Fe $3d$ and Au $6p$ bands, we detected the interfacial proximity effect, which modulates the valence-band electronic structure of Fe, resulting in PMA. MS and XMCD measurements were used to detect the interfacial magnetic structure and anisotropy in orbital magnetic moments, respectively. In situ ARPES also confirms the initial growth of Fe on large spin-orbit coupled surface Shockley states under Au(111) modulated electronic states in the vicinity of the Fermi level. This suggests that PMA in the Fe/Au(111) interface originates from the cooperation effects among the spin, orbital magnetic moments in Fe, and large spin-orbit coupling in Au. These findings pave the way to develop interfacial PMA using $p\text{−}d$ hybridization with a large spin-orbit interaction.

The energy spectrum of positronium atoms generated at a solid surface reflects the electron density of states (DOS) associated solely with the first surface layer. Using spin-polarized positrons, the spin-dependent surface DOS can be studied. For this purpose, we have developed a spin-polarized positronium time-of-flight spectroscopy apparatus based on a ${}^{22}\mathrm{Na}$ positron source and an electrostatic beam transportation system, which enables the sampling of topmost surface electrons around the $\mathrm{\Gamma }$ point and near the Fermi level. We applied this technique to nonmagnetic Pt(111) and W(001), ferromagnetic Ni(111), Co(0001) and graphene on them, ${\mathrm{Co}}_2{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) and ${\mathrm{Co}}_2\mathrm{MnSi}$ (CMS). The results showed that the electrons of Ni(111) and Co(0001) surfaces have characteristic negative spin polarizations, while these spin polarizations vanished upon graphene deposition, suggesting that the spin polarizations of graphene on Ni(111) and Co(0001) were mainly induced at the Dirac points that were out of range in the present measurement. The CFGG and CMS surfaces also exhibited only weak spin polarizations suggesting that the half-metallicity expected for these bulk states was not maintained at the surfaces.