Manufacturing organized networks of magnetic nano-aggregates on a surface is an important challenge, both for fundamental and applied research. Novel device applications could result from such systems with fascinating properties provided a macroscopic number of similar islands could be grown. However the ordering, shape and size of clusters is extremely sensitive to growth conditions and a better understanding of the elementary processes involved is necessary to succeed in this challenge.

A very promising way is self-organized growth (SOG) on strain relief patterns made of 2D arrays of dislocations. Following this approach we have studied, in situ, in UHV, by GISAXS and GIXS using the BM32 SUV surface diffraction apparatus, the SOG of Co dots on a well ordered misfit DN emerging at the surface of an ultra-thin of Ag film on MgO(001).

We have developed methods to prepare Ag(001) films of thickness ranging from ~50 Å up to several µm on MgO(001) surfaces of high quality. This produces a well-ordered square network of misfit dislocations emerging at the Ag(001) surface, with a ?=100 Å period due to 3% lattice parameter mismatch between MgO and Ag. Figure 1 displays a GISAXS image measured on an ultra-thin (8nm thickness) Ag/MgO(001) film with an incident X-Ray beam parallel to the <110> direction with respect to MgO(001) crystalline axes. Sharp scattering rods in the Q// direction (parallel to the surface) reveal a laterally periodic nanopatterning of 10nm size and the diffuse scattering perpendicularly to the surface (Q?) is a signature of the vertical extension of the expansion field. All these features are observed at ?90° and 180° in agreement with a fourfold symmetry as expected from the Ag/MgO(001) symmetry.

Schematic view of a GISAXS measurement and experimental image of the dislocation network

Figure 1: Schematic view of a GISAXS measurement and experimental image of the dislocation network with the X-Ray beam along the <110> direction

 

To induce the SOG of Co clusters on Ag/MgO films, ultra-thin films of ~10nm thickness have been prepared in order to significantly modulate the surface morphology. The growth of Co has been investigated in real time by GISAXS with the X-Ray beam impinging along the <110> direction of the MgO(001) crystal (Figure 2). From the very beginning of the growth (0.2Å) peculiar interference effects on the scattering rod appear with a sinusoidal shape and with an out-off plane period corresponding exactly to the Ag film thickness. The oscillations are the signature of the interference effects between the scattered waves by the dislocation network and that of Co clusters. With time of deposit the modulation is more and more intense putting clearly in evidence the ordering of Co nanostructures on the surface.

Real time GISAXS measurements of the SOG of Co clusters during deposit

Figure 2: Real time GISAXS measurements of the SOG of Co clusters during deposit (0.4 Å, 1.2 Å and 2.4 Å). The images are subtracted from the reference image of the substrate without cobalt deposit. Interference effects are clearly visible and demonstrate the SOG of Co clusters.

 

To succeed in self-organizing Co clusters, two conditions had to be fulfilled: the thermal energy of Co atoms had to be minimized to improve efficiency of the surface dislocation potential well and the mean free path of Co atoms had to be maximized to allow them to probe the surface and reach the nucleation sites. Both conditions imply low temperature and very low Co deposition rate.