Dominant Role of the Epitaxial Strain in the Magnetism of Core-Shell $\mathrm{Co}/\mathrm{Au}$ Self-Organized Nanodots

Self-organized Co nanodots on a Au(111) surface have been surrounded by controlled Au rings that progressively cap the entire dots. The magnetic susceptibility of these dots has been measured in situ as a function of the Au coverage. The blocking temperature increases when the Co bilayer dots are surrounded by the first Au atomic layer and decreases with the subsequent capping. This result cannot be explained by interfacial anisotropy which is generally assumed to be the dominant term in the magnetic anisotropy of nanostructures. Using molecular dynamics simulations, we evidence that the large strain inside the Co clusters is the main driving force for the anisotropy changes during the Au encapsulation.

Figure 1

Three dimensional view of a 50 nm STM image of 0.8 ML of Co and 0.4 ML of Au on Au(111). Au is surrounding and partially capping the initial Co dots. In inset, STM image of 0.35 ML of bilayer Co dots surrounded by 0.1 ML of Au on Au(111).

Figure 2

(a) Real part of the magnetic susceptibility measured as a function of the temperature for an initial 1.15 ML Co deposition. Different curves correspond to different Au coverage. The solid lines show the calculation of the susceptibility based on a macrospin model and taking into account a MAE distribution among the sample. (b) Variation of the activation energy extracted from the susceptibility versus the Au coverage for two different initial Co coverage (0.35 and 1.15 ML). The solid lines are guides to the eye. Sketches represent the configurations of the Co bilayer dots as function of the Au coverage.

Figure 3

(a) Some configurations calculated by the molecular dynamics simulations : pure bilayer Co dot (1), surrounded by an Au ring of one atomic layer (2), by two Au atomic layers (3) and covered by Au (4). (b) Variation of the mean Co-Co in-plane distance $a$ and inter-plane distance $c$ during Au encapsulation. The error bars indicate the standard deviation of these parameters over the island for 10 different simulations. The dotted lines indicate the completion of the first, second and third Au layers. (c) Variation of the mean magnetoelastic anisotropy of a Co cluster during its encapsulation calculated by taking into account the strain components ${ϵ}_{xx}$ and ${ϵ}_{zz}$ deduced from (b). Labels 1 to 4 refer to the corresponding configurations described in (a).