Surfactant-mediated growth of giant magnetoresistance multilayers

A series of experiments have been conducted to evaluate the magnetotransport properties of rf-diode-sputterdeposited giant magnetoresistive multilayers with either copper or copper-silver-gold nonferromagnetic ~NFM! conducting layers. The study revealed that rf-diode-deposited multilayers utilizing Cu80Ag15Au5 as the NFM conducting layer possess significantly superior giant magnetoresistance to otherwise identical device architectures that used pure copper as the NFM conducting layer. To explore the origin of this effect, copper and Cu80Ag15Au5 films of varying thickness have been grown under identical deposition conditions and their surface morphology and roughness have been investigated. Atomic-force microscopy revealed significant roughness and the presence of many pinholes in thin pure-copper films. The surface roughness of the Cu80Ag15Au5 layers was found to be much less than that of pure copper, and the alloying eliminated the formation of pinholes. Using an embedded-atom-method alloy potential, molecular-dynamics simulations have been used to investigate the role of silver and gold upon the multilayer growth process. The smoother growth surface of Cu80Ag15Au5 was found to predominantly result from the addition of silver, which acts as a surfactant during growth. Molecular statistics estimates of atom migration energy barriers indicated that both silver and gold have significantly higher mobilities than copper atoms on a flat copper surface. However, gold was found to be incorporated in the lattice whereas silver tended to segregate ~and concentrate! upon the free surface, enhancing its potency as a surfactant. The atomic-scale mechanism responsible for silver’s surfaceflattening effect has been explored. We found that silver, when present at a ledge edge, reduces the EhrlichSchwoebel barrier for copper, promoting a step-flow growth mode. Gold was also found to reduce the EhrlichSchwoebel barrier, but its potency was less than that of silver due to its lower surface concentration. These observations suggest that small alloy additions can be used to manipulate the energy barriers that fundamentally control atomic assembly during vapor deposition, and provide a potentially powerful means of controlling the structure of thin films.