In situ real-time annealing of ultrathin vertical Fe nanowires grown by focused electron beam induced deposition

Focused Electron Beam Induced Deposition is a consolidated technique for the growth of three-dimensional (3D) nanostructures. However, this single-step nanofabrication method requires further efforts to optimize simultaneously dimensional and compositional properties, in particular for deposits with a high aspect ratio. More specifically, ferromagnetic 3D nanowires (NWs) with diameters in the sub-50 nm regime and high metallic contents up to 95 at. % attract great interest to improve the final performance of magnetic nanodevices such as magnetic tips for scanning probe microscopy. In this work, we report on real-time monitoring during chemical purification and structural crystallization processes of ultra-narrow 3D Fe NWs (<50 nm in diameter achieved) by post-growth in situ annealing in a transmission electron microscope. NW heating up to 700 °C in very high vacuum reveals the local increase of the metallic content along the entire NW length concomitant with the growth of large Fe single crystals from initially amorphous compounds. A metallic purity of 95 at. % is observed in several regions, dramatically boosting the initial Fe content of 40 at. %. The real-time in situ tracking of 3D nanostructures during thermal annealing is a key element to design and optimize novel purification processes for the fabrication of customized components to be integrated in spintronic, logic and sensing devices.