Low energy ion assisted control of interfacial structure: Ion fluence effects

Abstract

Multilayered thin films consisting of high electrical conductivity copper layers sandwiched between pairs of low coercivity ferromagnetic alloys can exhibit giant magnetoresistance. The magnitude of the magnetoresistance increases with the structural and chemical perfection of the interfaces. Recent atomistic modeling and experimental observations have shown that nickel and cobalt atoms in the ferromagnetic layer readily exchange with underlying copper atoms during the deposition of the ferromagnetic layer upon the copper spacer. This results in mixing at the ferromagnetic metal on copper interface. Low energy (1–20 eV) inert gas ions can be used during deposition to flatten the surface of layers, in some cases without causing interlayer mixing. Here we use the molecular dynamics simulation method to investigate the effects of the assisting ion fluence upon the surface roughness and interlayer mixing of a model Ni/Cu/Ni multilayer system. The results reveal that the surface roughness initially drops rapidly with ion fluence and then approaches a limiting roughness that is dependent upon the surface type, the ion energy, and the ion mass. For a Cu on Ni surface irradiated by 2.0 eV Xe+ ions, the flattening transition occurs at a fluence of about 0.2 ions/Å2 (corresponding to an ion to metal deposition flux ratio of about 5). The same transition was seen at a similar fluence for a Ni on Cu surface, but at a higher Xe+ ion energy of 14.0 eV. Threshold energies for flattening and mixing were identified for various surfaces. The probabilities of both flattening and mixing were found to increase with ion fluence and ion energy. Because the threshold energy for mixing was lower than that for smoothing, significant interfacial mixing was only seen during ion assisted flattening of the Ni on Cu interface. Simple models have been developed to establish the functional dependence of interfacial structural parameters upon the assisting ion fluence.