Electrodeposition of CoPt Magnetic Nanowires with Controlled Microstructure Using a Pt(111) Seed Layer

The CoPt nanowires (NWs) with a 100 nm diameter are grown by electrodeposition using a single bath containing Pt and Co cations. The NWs are electrodeposited into a porous membrane coated with a Pt(111) seed layer as the working electrode. The atomic structure, morphology, composition, and magnetic properties of CoPt NWs are investigated in detail using field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscope (HR-TEM), X-ray diffractometer (XRD), and vibrating-sample magnetometer (VSM). Results indicate that the use of a Pt(111) seed layer is crucial to controlling the microstructure of NWs. Specifically, a preferred (001) texture is obtained at constant potential E₁ = -1000 mV (vs Ag/AgCl). Moreover, using periodic pulse deposition between E₁ = -1000 mV and E₂ = -700 mV, one may modulate the Pt% content along the NWs. The work also explores for the first time the case where E₂ > E₀, the Co²⁺/Co standard potential. It is demonstrated that the resulting NWs are composite. They are made of an alternation of metallic CoPt segments and Co oxide segments, where the CoPt segments present a strong (001) hcp structure, although the oxide presents a defective structure. A mechanism is proposed to interpret this result. From a magnetic viewpoint, the CoPt NWs present a strong uniaxial magnetization anisotropy parallel to the NW axis, which is well-correlated with an hcp (001) texture of the metallic Co-rich segments. Quantitative analysis of the average magnetic moment confirms that CoOx segments have a negligible contribution to the total magnetic moment. Furthermore, MFM characterizations show that Co-rich CoPt NWs, deposited at a constant potential, have uniform magnetization along the NW axis. A periodic magnetic contrast is observed for multilayered NWs prepared by pulse deposition.