Ferromagnetic Stability in Fe Nanodot Assemblies on Cu(111) Induced by Indirect Coupling through the Substrate

We report collective ferromagnetic behavior with high Curie temperatures ($T_c$) in Fe dot assemblies supported by the Cu(111) surface. Our ability to tune the average size and spacing of the individual dots allows us to conclude that enhanced magnetic anisotropy cannot account for this high-$T_c$ ferromagnetic order. Because our Monte Carlo simulations have ruled out the dipolar interaction as the dominant factor in this system, we attribute the origin of the ferromagnetic order to indirect exchange coupling via the Cu(111) substrate.

Figure 1

(a) STM morphology ($100\mathrm{nm}\times 100\mathrm{nm}$) of an Fe dot assembly on Cu(111) prepared with 0.8 ML Fe and 200 L Xe. (b) MOKE hysteresis loops of the Fe dot assembly at various temperatures. (c) Time dependence of the magnetization of the Fe dot assembly. An in-plane external field was applied at time “zero” and was switched off at the points in time indicated by the arrows. (d) Remanent magnetization of the Fe dot assembly as a function of temperature. The critical temperature is around 120 K.

Figure 2

Average size and density of $\mathrm{Fe}/\mathrm{Cu}(111)$ dot assemblies as a function of Fe dosage (a), and Xe exposure (b). The solid and dashed lines are guides for the eyes. Two sets of Fe dosage/Xe exposure parameter combinations can produce Fe dot assemblies with different density but the same size. They are summarized in Table .

Figure 3

Morphology and magnetization of $\mathrm{Fe}/\mathrm{Cu}(111)$ dots prepared by conditions highlighted in Table . Top: STM images ($100\mathrm{nm}\times 100\mathrm{nm}$) of $\mathrm{Fe}/\mathrm{Cu}(111)$ dots with equal average sizes ($\sim 25{\mathrm{nm}}^3$) but different densities. Bottom: Their corresponding remanent magnetization measured as a function of temperature, with the critical exponent of power-law fitting (dashed lines) indicated.

Figure 4

Critical temperature ($T_c$) of Fe dot assemblies on various substrates of Cu(111), Cu(100), and Ge(111) as a function of Xe exposure. The Fe nominal thickness is fixed at 1 ML in all cases. The arrow for the $\mathrm{Fe}/\mathrm{Ge}(111)$ dots (200 L Xe) indicates that the $T_c$ is below 40 K.