Preparation And Investigation Of One Dimensional Magnetic Nanostructures Base On The Controllable Growth Of Porous Anodic Alumina

With the development of nanotechnology, one dimensional magnetic nanostructures have attracted great attention and have been studied widely for their novel and unique physical properties which are induced by their unique structures and the potential applications in many fields, such as high density recording media, optical device, electronics, catalysis, nanosensors, and so on. Porous anodic alumina(PAA) films have been widely used as tempaltes to prepare one dimensional materials for its cylindrical, uniformly sized pores forming a hexagonal close-packed array and the pore diameter, length and inter-pore distance can be continuously adjusted over a wide range by choosing appropriate anodization conditions.In this paper, the PAA films with different parameters were first studied and on the basis, one dimensional magnetic material was prepared and the relationship between properties and the structures were analyzed. ① For the PAA films with small pore diameters, the effect of ethanol joining in solution which containing one acids, such as sulfuric acid, oxalic acid, on parameters of PAA films were studied, especially for the growth rate. ② For the PAA films with big pore diameters, ordered PAA films with interpore distances continuously tunable from 100 to 750 nm were fabricated using one simple method, i.e., the mixture of citric acid and oxalic acid using for electrolyte, and the mechanism of modulation was analyzed by means of oxidation current. ③Fe, Co and Ni nanostructures were deposited by electrodeposition in PAA films with small pore diameters, and the magnetic properties were also studied changing with the nanostructures. ④ Ni nanowirs and nanotubes with the same outer diameters were fabricated using the PAA films with the same big pore diameters, and the mechanism of formation of nanowires or nanotubes were discussed. The main contents are summarized as follows:(1) The influence of ethanol addition in sulfuric acid with different volume ratio was studied in details, especially for the growth rate of PAA films. Results show that the addition of ethanol can extend the scope of self-ordering voltages and the growth rates are significantly increased when the additional ratio is small. It is the fastest when ethanol addition is 10%, and the growth rate increases by 5 times compared with those in pure sulfuric acid. At the same time, the changes of the PAA parameters were also studied after addition of ethanol in oxalic acid, and the results are different with those in sulfuric acid. The growth rates decrease directly after ethanol addition, and what’s more, the thickness of PAA films is thinner when the ratio of ethanol increases. This is because that the ethanol will react with oxalic acid, i.e., esterification reaction, and so the concentration of oxalic acid in solution is reduced.(2) The PAA films with continuously tunable interpore distances were prepared in the mixed solution of citric and oxalic acids. By changing the ratios of the two acids in solution, the range of anodizing voltages within which the self-ordered films can be formed were extended to between 40 and 300 V, and correspondingly the interpore distances are changes between 100 to 750 nm. The dependence of the interpore distances on acid ratios in mixed solutions was discussed by means of anodizing current and it was found that the dissociation constant of solution is of great importance. The effective dissociation constant of the mixed solution changes when the proportion of the two kinds of acid change. And correspondingly the self-ordering voltage scopes and interpore distances are different. Oxalic acid can be arbitrary mixed with citric acid and so the interpore distance can be tuned within a certain specific range.(3) On the basis of the preparation of PAA films with many kinds of pore diameters and interpore distances, one dimensional magnetic metal nanoarray were fabricated by electrodepostion using PAA templates and the different emphasis on magnetism were studied for different materials. For Fe nanowires with different diameters, the magnetization easy axis is aligned along the nanowire axis which shows that the shape anisotropy is dominant. The changes of coercivity with diameters of Fe nanowires show that the magnetization reversal proceeds by curling mode while the magnetostatic interaction between nanowires are must considerable. The magnetic properties of Ni nanochains and Ni nanowires are directly decided by their nanostructures. For Ni nanochains, there is no magnetization easy axis and the coercivity does not change with the angle beween the magnetic field and the chain axis which because the Ni nanochains are made of many nano-particles which aligned along the nanopores, and so the reversal has nothing to do with the direction. While Ni nanowires are polycrystalline and the magnetizaton easy axis are parallel with the wire axis which owing to the dominant function of the shape anisotropy. The crystal structures of Co nanowires changed with the electrolyte temperature and when the temperature is 30 ℃, the Co nanowires grow along the(002) plane. In this instance, the direction of magnetocrystalline anisotropy is the same with the direction of shape anisotropy and so the coercivity and remanence ratio of the Co nanowires are the biggest in all samples prepared under selected temperatures.(4) Ni nanowires and nanotubes prepared in the same big pore diameter PAA films are polycrystalline and the magnetization easy axis of nanowire align along the nanostructure axis while those of nanotube perpendicule to the nanostructure axis. After analysis it is found that the deposition current density play a vital role in preparation of nanowires or nanotubes using PAA templates assisted chemical electrodeposition. When the current density is small the productors are more likely to be nanotubes and when the current density is big the productors are easy to be nanowires. And when the current density is medium, the productors will be the mixture of nanowires and nanotubes, or first the nanotubes then nanowires. The corresponding mechnism is that there are competition bewteen the electric field energy and absorption energy at the PAA pore walls.