Preparation And Study On Tunable Magnetic Anisotropy Of Magnetic Nanowire Arrays

Recently, one-dimensional magnetic nanowire arrays have been studied widely for their unique physical properties used in nanodevice design, pattern memory and their importance in many foundation physics. Magnetic anisotropy is one of the basic properties of magnetic materials, which determines their potential value. So adjusting the magnetic anisotropy of nanowire arrays becomes more and more important. As well known, total magnetic anisotropy of nanowire arrays originates from their shape anisotropy and magnetocrystalline anisotropy. For Fe and Ni nanowire arrays, their shape anisotropy is more than their magnetocrystalline anisotropy, so their total magnetic anisotropy originates from their shape anisotropy. For a HCP cobalt nanowire array, its magnetocrystalline anisotropic constant is comparable to the shape anisotropic constant. It is a chance to adjust the total magnetic anisotropy of magnetic nanowire arrays by controlling the easy magnetization direction of magnetocrystalline anisotropy. In this study, we investigated the properties of HCP Co nanowire arrays. Its total magnetic anisotropy and coercivity were controlled by changing the angle between the easy magnetization direction of magnetocrystalline anisotropy and shape anisotropy. The total magnetic anisotropy was changed by crystallinity of Co crystal grain use magnetic field heat treatment method. Co nanotubes were fabricated by Co nanowires using H2O2 for the first time.Various methods are used to prepare nanowires, especially anodic aluminium oxide (AAO) template method, because of its its convenience and inexpensiveness. In this study, we first fabricated cobalt nanowire arrays and adjusted their magnetic anisotropy and other magnetic properties; investigated the magnetization reversal mechanism of magnetic nanowire arrays with different diameters; studied the influence of magnetic field heat treatment on the structure and properties of FCC and HCP cobalt nanowire arrays. Second, we prepared magnetic multilayer nanowire arrays and magnetic nanobrush in order to adjust the amplitude and direction of the total magnetic anisotropy of magnetic samples. The main contents are as follows:1,HCP cobalt nanowire arrays with different microstructures were fabricated by electrodeposition, and their easy axis direction of magnetocrystalline anisotropy and effective anisotropy field were controlled by several methods.A,We found that the coercivity of Co nanowire with diameter of 20 nm strongly depends on the preferred orientation of nanowire arrays, the crystal textures of samples areâ‘ (100),â‘¡(100),(101),(002),â‘¢(002), respectively. The coercivity can be tailored from 925 Oe to 3310 Oe at 300 K, up to 4050 Oe at 5 K, by experimental parameters. This wide adjusting of coercivity supplies a more promising application field for Co nanowire arrays with a fixed diameter and length. We also found that the change of coercivity is related to the different Keff aroused by the samples with different textures. Micromagnetic simulation indicates that the angle between the direction of shape anisotropy and the easy magnetization direction of magnetocrystalline anisotropy is the key role in adjusting the magnetic properties of Co nanowire arrays.B,Co nanowire arrays with the diameters of 20 and 50 nm are fabricated into anodic alumina oxide templates by ac electrodeposition technology. The coercivity of nanowire arrays with diameter of 50 nm is 1990 Oe, whereas that of nanowire arrays with diameter of 20 nm is only 925 Oe. This is quite different from other former experiments results of magnetic metal nanowire arrays, which exhibit larger coercivity at the diameter of 20 nm. X-ray diffraction patterns suggest that the anomalous increase of coercivity is related to the appearance of (002) and (101) reflections. Micromagnetism simulation reveals the mechanism of how the (002) texture of nanowire influences the magnetic properties of Co nanowire arrays.C,HCP cobalt nanowire arrays, FCC cobalt nanowire arrays and FCC iron nanowire arrays which undergo the heat-treatment process with a proper applied magnetic field are studied by XRD and VSM. It is found that all of their structures can be changed by magnetic field heat-treatment and the diffraction peak of hcp Co nanowire arrays disappears, its magnetic anisotropy also changes. So it is an effective method to adjust the anisotropy of magnetic nanowire arrays.D,Co nanowire arrays with AAO template is immersed in H2O2 solution firstly. It is found that Co nanowire can be removed thoroughly, and the process is not from long to short, but becomes nanotube. Magnetic measurement shows that the magnetic anisotropy of cobalt nanowire arrays increase after being immersed in H2O2 solution. We believe this method can be applied in preparing other metal nanotube and complex nanostructures.2,Ordered Co/Cu multilayer nanowire arrays have been fabricated into the anodic aluminium oxide templates with Ag and Cu substrate by direct current electrodeposition. The morphology, structure and magnetic properties have been studied by transmission electron microscopy, selective area electron diffraction, X-ray diffraction, and vibrating sample magnetometer. X-ray diffraction patterns reveal that both of the as-deposited nanowire arrays films exhibit face-centred cubic structure. Magnetic measurements indicate that the easy magnetization direction of Co/Cu multilayer nanowire arrays films on Ag substrate is perpendicular to the long axis of nanowire, whereas the easy magnetization direction of the sample with Cu substrate is parallel to the long axis of nanowire. The reason for the changes of easy axis attribute to the lattice match between the substrate and sample.3,With a bottom-up assemble technology, heterogeneous magnetic nanobrushes, consisting of Co nanowire arrays and ferromagnetic Fe7oCo3o nanofilm, have been fabricated using an anodic aluminum oxide template method combining with sputtering technology. Magnetic measurement suggests that the magnetic anisotropy of nanobrush depends on the thickness of Fe7oCo3o layer, and its total magnetic anisotropy originates from the competition between the shape anisotropy of nanowire arrays and nanofilm. Micromagnetic simulation result indicates the switching field of nanobrush is 1900 Oe, while that of nanowire array is 2700 Oe. These suggest that the nanobrush film can promote the magnetization reversal processes of nanowire arrays in nanobrush.