pH-Controlled electrodeposition of diameter-modulated Co nanowires: crystal texture and magnetic properties

Diameter-modulated (DM) and homogeneous-diameter cobalt nanowires (NWs) were electrodeposited into the ordered pores of anodic aluminum oxide templates from sulphate solutions at pH = 4.0 and pH = 6.5. The role played by these pH conditions on the electrochemical growth, crystalline structure and magnetic properties of such nanowire arrays has been investigated. Microstructural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows that DM Co NWs electrodeposited at pH = 6.5 exhibit a highly textured polycrystalline hcp lattice with preferred growth orientation along the [100] direction parallel to the nanowires' axis. However, these NWs electrodeposited at pH = 4.0 exhibit a polycrystalline hcp lattice structure without any crystalline texture. It is concluded that this crystalline growth is a consequence of the co-evolution of hydrogen with metallic cobalt, whose growth rate decreases at low pH and higher hydrogen ion concentrations due to the limitation of growth sites, which are blocked by gas bubbles. XRD and TEM analyses inform that the diameter modulation did not show a clear effect on the crystallographic texture of nanowires. Magnetization loops, measured by vibrating sample magnetometry (VSM) under parallel and perpendicular magnetic field configurations, show reduced irreversibility and reveal a pH-dependent magnetic response. For pH = 6.5, coercivity and squareness (in parallel and perpendicular configurations) decrease from 164 Oe and 5.1% to 152 Oe and 2.6%, respectively, while for pH = 4, they increase from 182 Oe and 3% to 203 Oe and 4%, respectively. At pH = 6.5, DM nanowires show a (100) texture, aligning the c-axis perpendicular to the nanowires and yielding a competition between shape and magnetocrystalline anisotropies, whereas at pH = 4.0, polycrystalline nanowires have no defined magnetocrystalline easy axis. For both pH values, a significant magnetostatic energy term is concluded, arising from the shape anisotropy of individual nanowires and significant interactions among neighboring DM nanowires.