Three‐dimensional core‐shell ferromagnetic nanowires fabricated by focused electron beam induced deposition

Functional nanostructured materials often rely on the combination of more than one material to confer the desired functionality or an enhanced performance of the devices. One of the challenges for Focused Electron Beam Induced Deposition (FEBID) technology is the fabrication of three‐dimensional (3D) heterogeneous nanostructures which can be applied in magnetic memories, logic and sensing [1]. A novel procedure to create nanoscale heterostructured materials in the form of 3D core‐shell nanowires by FEBID technologies has been developed. This new approach has been applied to synthesize standing nanowires with cylindrical metallic ferromagnetic cores of Co‐ and Fe‐FEBID (less than 100 nm thick) coated with a protective Pt‐FEBID shell ranging 10‐20 nm of uniform thickness, using Co 2 (CO) 8 , Fe 2 (CO) 9 and CH 3 CpPt(CH 3 ) 3 precursor gases. This architecture aims at minimizing the degradation of magnetic properties caused by the natural surface oxidation of the core to a non‐ferromagnetic material. This is a key issue in such thin ferromagnetic objects with a high surface‐to‐volume ratio [2]. The structure, chemistry and magnetism of FEBID nanowires with cores of Co and Fe have been characterized in Pt‐coated and uncoated nanostructures. Scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) experiments have revealed that the surface oxidation is suppressed from the magnetic cores and confined to the Pt layer, while keeping the cylindrical shape of the nanowire (Figure 1). Local magnetic states of uncoated and coated Co‐ and Fe‐FEBID nanowires in remanence have been obtained by off‐axis Electron Holography (EH). After substraction of the phase contribution of the mean inner potential and thanks to the cylindrical shape of the cores, quantitative magnetic induction maps have been obtained which demonstrates that the average magnetization of the ultrathin coated cores is strengthened up to 30% in the thinnest nanowires (50‐nm‐thick cores) with respect to unprotected ones (Figure 2).