Modification of microstructure and magnetic properties of electrodeposited Co nanowire arrays: A study of the effect of external magnetic field, electrolyte acidity and annealing process

High aspect ratio Co nanowire arrays are fabricated through the ac electrodeposition method using anodic aluminum oxide templates. The effect of electrolytic bath acidity (pH) as well as applied external magnetic field during the electrodeposition process together with the effect of annealing process after sample preparation on the crystal structure and magnetic properties of nanowires are investigated. The experimental results show that the pH value of electrolyte affects the magnetic properties of the nanowire arrays due to modification of the microstructure and quality of electrodeposition. The Co nanowires form in fcc, a mixture of fcc and hcp, and hcp crystal structure at pH = 2.5, pH = 3.5, and pH = 5, 6, respectively. The experimental results also reveal that applying appropriate external magnetic field parallel to the nanowire axis improves the magnetic properties of deposited Co NWs probably due to increase in mass transport in the diffusion layer by applied external magnetic field, which results in lowering the porosity generated due to formation of hydroxide at high pH value. Our studies show that appropriate annealing process can also enhance the crystal texture in the hcp phase and therefore significantly improve the magnetic properties of NW arrays deposited at either high pH regime or under external magnetic field. Moreover, it is shown that although each factor individually affects the magnetic properties of the Co NW arrays, the combination of annealing process, external magnetic field and electrolyte acidity allows the control of the quality of samples and enhance the magnetic properties of Co nanowire arrays.