The structure and properties of interface regions in nanostructured Co/Si thin films

Sputtered co-deposited Co-Si alloys were found to have a hcp structure up to #approx# 25 atm. % Si. For Si compositions beyond this, and less than 60 atm. %, a single amorphous phase exists with features in the electron diffraction patterns that suggest a dense-random-packed-structure. These structures are ferromagnetic. For higher Si concentrations #approx# 60 atm. % Si, additional haloes are observed that suggest the formation of at least two amorphous phases and production of thermally unstable magnetic particles. Beyond 80 atm. %. Si, Co atoms are incorporated in the random network structure of Si and paramagnetism is found. A correlation of the PEELS Co L_2/L_3 edge intensity ratio with the effective Co moment suggests that the magnetic changes could arise from the local environment changes, with less importance given to the change in the particle size distribution with composition. Amorphization (visible in TEM) also occurs within layered structures but is dominant only with nominally thin layers < 6 nm, and a similar magnetic behaviour, as above, is found. This can be induced either by increasing the Si or reducing the Co layer thickness implying a symmetry; alloy formation over a limited distance with a variable average composition. However, with multilayers, the features of a dense-random-packed-structure are completely lost, prompting a greater degree of local atomic disorder. This is thought to be linked to the fact that the interface region is able to exist over a broad spectrum of compositions, A basic model for Co/Si layered structures follows. Changes in the multilayer periodicity have been quantified by x-ray reflectivity and atomic mixing is held responsible for a contraction of the bilayer. The estimation of the magnetic anisotropy, determined from blocking temperature and TEM magnetic particle measurements, for structures with different nominal Si layer thicknesses, supports a crystalline to amorphous transformation, as do a series of thermal electrical resistivity curves. The coexistence of a Kondo type resistivity minimum in these curves with ferromagnetism demonstrates some degree of compositional disorder within the amorphous films. Ion beam assisted deposition of Co/Si multilayers seems to favour the removal of material rather than the promotion of further interfacial mixing and induces complex magnetic properties, leads to small particle formation and eventual amorphization at higher energies. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN029129 / BLDSC - British Library Document Supply CentreGBUnited Kingdo