Correlating Synthesis, Structure, and Thermal Stability of CuBi Nanowires for Spintronic Applications by Electron Microscopy and <i>in Situ</i> Scattering Methods

Bi-doped copper (Cu_1-<i>x</i>Bi_<i>x</i>) nanowires (NWs), promising candidates for spintronic applications due to their potential for a giant spin Hall effect (SHE), were synthesized, and their structural properties and thermal stability were investigated. Using template-assisted electrodeposition, Cu_1-<i>x</i>Bi_<i>x</i> nanowires with varying bismuth (Bi) content (<i>x</i> = 0, 2, 4, and 7%) and different crystalline domain sizes were fabricated. Structural analysis by advanced electron microscopy and X-ray scattering techniques revealed the influence of synthesis conditions on the resulting NW crystal structure and microstructure, including Bi localization (within the lattice or in the grain boundaries), crystallite domain dimensions, and lattice distortions. While NWs with larger crystalline domains allow homogeneous Bi incorporation into the Cu lattice, NWs with smaller crystalline domains exhibit noticeable Bi accumulation at grain boundaries. The thermal stability of the NWs was examined using variable temperature X-ray diffraction and total scattering. Upon heating, lattice distortions consistent with Bi diffusion out of the Cu lattice were observed, with subsequent crystallization of rhombohedral metallic Bi upon cooling. Microstructural analysis of NWs post heating shows that the recrystallized rhombohedral Bi accumulates in localized regions within the NWs, most likely corresponding to grain boundaries. In some cases, the exsolution of Bi from these regions leads to wedge-shaped fractures in the NWs and the formation of independent Bi particles. These findings establish a foundation for optimizing the SHE performance of Cu_1-<i>x</i>Bi_<i>x</i> nanowires for spintronic devices by correlating synthesis parameters with microstructural features and thermal behavior.