Preparation And Magnetic Properties Of Gd-Co-Ni Metal Nanotubes/Wires

At present,the two-step anodic oxidation under a constant voltage is a better method to prepare the anodized aluminum oxide template(AAO).The AAO template prepared has such advantages as controllable pore diameter and aspect ratio,well-order pore arrangement,and low cost.Well-aligned Gd-Co-Ni nanotube/wire arrays were successfully electrodeposited into AAO templates by the electrochemical deposition method.In this thesis,the morphology of Gd-Co-Ni nanowires/tubes was investigated by scanning electron microscope(SEM)and transmission electron microscope(TEM).The content of every element in the sample under different deposition voltages was measured by energy dispersive spectrometry(EDS).The structure of nanoarrays was characterized by selected area electron diffraction(SAED).The magnetic properties of the samples were measured by vibrating sample magnetometer(VSM).The morphological characterization shows that such factors have a decisive influence on the pore size and thickness of the porous AAO template,as the two-step anodic oxidation voltage,the temperature of the oxidation tank,the oxidation time,the type of acid and the time of opening hole.As the increase of the opening hole time,the diameter of the pores increases,and the pores will be destroyed if the opening hole time is too long.During the deposition process,the pH of the deposition solution and the pore size of the AAO template have a great influence on the way that the sample is formed.When the pore diameter is less than 50 nm and the pH is less than 4.5,the nanostructures grow into nanowires from the bottom of the pores.Conversely,they would grow into nanotubes from the wall of the pores.It is found from the EDS spectrum that the variation of the elemental content of Gd/Co/Ni in the nanotube/line array becomes larger and larger as the deposition voltage decreases.SAED patterns of the annealed and unannealed nanowires/tubes demonstrate that the annealing has a great effect on the nanotubes/wires,and the amorphous sample becomes crystalline at 800 °C for 4 hours at last.We also measured the magnetic properties of the sample using VSM.Due to the large aspect ratio of samples,the nanowires/tubes have a large shape anisotropy.For unannealed sample,the magnetocrystalline anisotropy is basically negligible.As the deposition voltage decreases,the proportion of Gd ion doped in the nanotube increases in the alloy and the aspect ratio decreases.As a result,the shape anisotropy anisotropy is weakened and the magnetostatic interaction is enhanced.Finally,the easy magnetization axis is reversed from the direction parallel to arrays to the direction perpendicular to arrays.Moreover,the coercive force(Hc)and the squareness ratio(Sq)of the alloy nanotubes parallel to the nanotube axis reduce and those perpendicular to the nanotube axis enhance.Annealing treatment can enhance the magnetocrystalline anisotropy,so Hc and Sq of the nanotubes/wires significantly increase after annealing.This is because annealing process turns the amorphous nanotubes/wires into the crystalline under 800 °C.Finally,the results of VSM with different concentrations of Gd demonstrate that the greater the concentration of Gd ions are,the weaker the magnetic property of samples is.