Atomistic mechanisms triggered by Joule heating effects in metallic Cu-Bi nanowires for spintronics

Abstract

Joule heating may severely impact the response to electric current injection of nanomaterials for spintronics. Here, the effects of heating in Bi doped Cu nanowires, a system where 1% Bi doping promotes a giant spin Hall effect (SHE), are studied by in situ high resolution electron microscopy. High quality Bi-Cu nanowires are grown by room temperature electrodeposition. The large size of Bi cations precludes insertion into the dense Cu face-centered lattice. Still, homogeneous compositions up to a nominal 7% Bi are successfully obtained with thicknesses <100 nm and grain sizes in the micron longitudinal scale, coated by a native oxide. In situ injection of current promotes fast Bi segregation out of solution. Controlled in situ annealing shows that the onset for segregation starts above temperatures of 250 °C. Within minutes, Bi atoms diffuse to grain boundaries or to exposed surfaces, such as the nanowire tips. Monoatomic thick Bi ordered decorations appear, preferentially on surface planes of the (Formula presented.) type. Annealing at 400 °C promotes the growth of pure Bi nanocrystals, coherent with the underlying Cu matrix. Still, the intra-grain Bi concentration remains finite at values near 1%. Density-functional theory calculations show that small amounts of Bi atoms are stable as substitutional impurities, confirming the potential of this system as building block for future spintronic devices.