Atomistic Mechanisms Triggered by Joule Heating Effects in Metallic Cu-Bi Nanowires for Spintronics

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 $\left\{320\right\}$ 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.