Using metallic interlayers to stabilize epitaxial metal-metal interfaces  

            Over the last few years the technology of thin film devices has evolved to the point where individual layers in a multilayer structure may be only a few nanometers thick, literally just tens of atoms thick.  In particular, for thin film magnetic structures, such as the read heads for magnetic hard drives based on magnetoresistive technology, metallic layers of Cu and Co are on the order of two nm thick.  Maintaining a well-defined architecture means that the films must be flat on an atomic scale and ideally have abrupt interfaces between layers.  At a minimum one expects that the interface itself should be thinner than the films making up the structure if the electrical and magnetic properties of the device are to be determined by the characteristics of the film rather than by those of the interface.

                One example of such interfaces is the Fe-Ni-Al system with possible applications in the fabrication of magnetic tunnel junctions for magnetic memory.  We have shown that the interface for thin films of Fe or Ni  deposited on Al single crystals at room temperature can be up to one nm thick, with Fe and Al atoms interdiffusing to form an intermetallic compound (Fe and Al interdiffusion (pdf 676 kb)).  The schematic crystal (Figure 1) shows the amount of interdiffusion and mixing which occurs at room temperature based on Monte Carlo simulations of the Ni-Al interface using embedded atom potentials.  The behavior for Fe-Al interfaces is expected  to be very similar.  Based on our work with metal-metal interfaces we have looked for a diffusion barrier that is only one or two atoms thick, an ultra-thin interlayer of atoms that stabilizes the interface with minimal perturbation of magnetic properties.  The resulting structure might resemble the ideal case shown schematically in Figure 2 where the interlayer (yellow) now prevents interdiffusion and the overlayer grows epitaxially on the stable template.  Recently we have shown that a single layer of Ti metal at the Fe-Al interface prevents the interdiffusion of Fe and Al observed for the bilayer without Ti (Preprint Fe-Ti-Al(100) (pdf 496 kb)).