Size-induced crossover from itinerant to localized magnetism observed for isolated Fe impurities embedded in different structural polymorphs of silver

Measurements of the local susceptibility and spin relaxation rate for isolated Fe atoms implanted in the nanocrystalline hexagonal phase of Ag (4H-Ag) show a large orbital moment, in stark contrast with the spin-dominated itinerant magnetism in conventional face centered cubic Ag. The orbital moment observed for Fe in hexagonal Ag $\left(\approx 1{\mu }_B\right)$ is the largest among $d$-block hosts, and resembles $f$-electron magnetism, suggesting Hund's rule type local moment with strong spin-orbit coupling. Calculations based on density functional theory suggest that the crossover from itinerant to local moment behavior results from localization of Fe-$3d$ states due to size-induced changes in unit cell volume and symmetry.

Figure 1

Spin rotation spectra $R\left(t\right)$ (left panel) and their Fourier transforms (right panel) for ${}^{54}\mathrm{Fe}$ in 3C and 4H-Ag.

Figure 2

Local susceptibility $\beta \left(T\right)$ of ${}^{54}\mathrm{Fe}$ in bulk 3C-Ag (triangles) and nano-4H-Ag (circles). Data from nano $(\approx 20$ nm) 3C-Ag (from Ref. [24]) are also shown. The solid lines are fits to the Curie-Weiss law. Inset shows the relaxation time ${\tau }_N\left(T\right)$ in bulk 3C-Ag and nano-4H-Ag. The solid lines are fits to the Korringa relation (see text).

Figure 3

LDOS of Fe in bulk 3C-Ag host (a) and (b) and nano-4H-Ag host (c) and (d). Calculations for both from unpolarized (a) and (c) and spin polarized (b) and (d) LDOS are shown. Line colors: black is Fe-$d$; red and green in (a) show $\text{Fe-}{d}_{e_g}$ and $\text{Fe-}{d}_{t_{2}g}$, respectively; cyan, dark red, and pink in (c) denote $d-z^2,d-(x^2{y}^2+xy)$, and $d-(xz+yz)$ of Fe. Dashed blue in (b) and (d) denote Ag-$4d$. The vertical dashed lines mark the position of $E_F$.