Effect Of External Magnetic Field On Magnetic Properties Of La-Co Nanotubes And Nanowires

Well-ordered La-Co nanotubes and nanowires have been fabricated into anodic aluminum oxide (AAO) templates by potentiostatic electrodeposition method. Among various fabrication methods, template-based electrodeposition method has become a better pathway, due to the easy controllable length, diameter and wall thickness and the low price. The crystalline structure of samples was investigated by X-ray diffraction (XRD), the morphology was investigated by transmission electron microscope (SEM), the composition was analyzed by energy dispersive spectrometry (EDS), and the magnetic properties of samples were measured by vibrating sample magnetometer (VSM).The rusults show nanowires and nanotubes are poor crystalline due to a random distribution of the grain. With increasing magnitude of voltage, La content gradually decreases, the magnetic anisotropy become more and more weak, and saturation magnetic field become big. The magnetic measurement shows the easy axis is perpendicular to the nanotubes but parallel to the nanowires. The reason is that the different stucture of nanotubes and nanowires lead to different effective anisotropy field which determine the direction of the easy axis. The magnetostatic interaction is favorable for the nanotubes, however, shape anisotropy and magnetocrystalline anisotropy occupy a dominant position for the nanowires. For the magnetization reversal mode determining the size of the coercivity, there is a transition from curling mode being parallel to the nanotube axis to transverse mode being perpendicular to the nanotube axis for the nanotubes. But for the nanowires, there is only one magnetization reversal mode which is coherent rotation.The external magnetic fields (EMF) of 1.5 T parallel and perpendicular to the nanotubes/nanowires axis have been applied. They have a little effect on the magnetic properties for nanotubes, because the nanotubes have small magnetocrystalline anisotropy due to poor crystalline structures. But for nanowires, there are many random grains. The magnetic moment orientation of them tends to align in the direction of (EMF). When EMF is parallel to the nanowires axis, an induced magnetic anisotropy is generated and effective anisotropy is more apparent. But the effective anisotropy is small when EMF is perpendicular to the nanowires axis because of the geometrical confinement of nanotubes, and the direction of induced magnetic anisotropy is perpendicular to the nanotubes axis. Thus, induced magnetic anisotropy and magnetostatic interaction compete with shape anisotropy, and further weaken the advantage of shape anisotropy field. They lead to effective anisotropy is more unconspicuous, and corcivity of nanowires become small.