Magneto-electrical Behavior Of Individual Magnetic Nanostructures In Situ SEM

Magneto-electrical transport behavior which is also called magnetoresistance effect(MR) is one of most important physical effects in the magnetic architectures. The efforts of MR in theoretical and practical exploriation have not only led to the births of magnetoelectronics and spintronics, but also formed the current high-tech industries about information storage and signal detection. The geometrical sizes of the basic unit in magnetic-electronic and spintronics devices has already reached the nanoscale, therefore the size effect starts to strongly determine their overall physical behaviors. The further development of the magnetic-electronic and spintronics devices demands a precise detection and comprehensive understandings of the physical properties of individual device units. In this thesis, we have designed and manufactured a magneto-transport instrument in situ SEM according to the requirement of current magnetism research and future industrial demamds. The magneto-electrical transport behaviors of individual Co Cu/Cu multilayer nanowire(MNW) have been systematically measured. Basic physical paprements and clear physical image of magneto-electrical transport behavior in one-dimentional multilayered structures are achieved. The main research results are summarized as follows:(1) Magneto-transport instrument in situ scanning electron microscope(SEM) was designed and manufactured for the first time. Its performance was tested as follows: the maximum movement range of its nano-manipulator along the x, y and z directions was 2.5 cm, 2.0 cm and 1.0 cm, respectively; smallest step precision was 1.80 nm; its magnetizing sample stage can rotate 360 degree in plane and have a 0.003° rotation precision; the magnetizing sample stage can provide a ?1800 Oe external field to magnitize the specimen, which only had a weak effect on the SEM image and can be easily corrected by SEM astigmators. The overall testing showed that our invented magneto-transport instrument in situ SEM can provide a simultaneous, dynamic and graphical measurement for individual magnetic nanomaterials or nanodevice units.(2) Co Cu/Cu multilayer nanowires have been successfully fabricated into anodic aluminium oxide templates by using an electrodeposition method, and their morphology, crystal structure, chemistry have been systemically characterized at the nanoscale. The results indicated that the average diameter of the bamboo-like Co Cu/Cu multilayer nanowires was about 55 nm. The average thickness of the Co Cu layers was 124 nm, and that of Cu layers for 14 nm. Meanwhile, the electron diffraction and high resolution images analysis revealed that Cu layers and Co-rich layers were polycrystalline fcc structure, a twin relationship of Cu layer {111} planes stacking on Co81Cu19 layer {111} planes was visualized at the atomic scale. The EELS line scan analysis indicated that the Co Cu layer was consisted of average 12 nm diameter Co grains and 5 nm Cu grains. Therefore, the concentration of Co and Cu grains per unit volume were calculaterd to be 57.27% and 26.12% respectively in Co Cu layers. These results provide basic geometrical and crystal parameters for the latter investigation on their electrical and magneto-electrical transport behaviors.(3) Electrical properties of individual Co Cu/Cu multilayer nanowire have been accurately measured by the magneto-transport instrument in situ SEM. The resistivity of individual Co Cu/Cu multilayer nanowire, Co Cu nanowire and Cu nanowire were measured to be 3.41×10-7Ω?m, 3.51×10-8 Ω?m and 1.19×10-7 Ω?m, respectively. The MS(Mayadas-Shatzkes) model was applied to correct the resistivity of the Cu layer for obtaining intrinsic resistivity of Co Cu/Cu multilayer nanowire. The physical image of internal resistance distribution of the individual multilayer naowire was revealed by combing the MS model and series resistance model. The resistivity of the interfacial transition layers for 2.17×10-6 Ω?m was simulated. Besides, the maximum current density was measured in situ SEM. The results indicated that the maximum current density of single Co Cu/Cu multilayer nanowire is only 1.01×1011 A?m-2,which is smaller than that of both Cu nanowire and Co Cu nanowire. The intrinsic reason originates from that the conduction electrons should be strongly scatted in the interfacial transition layers, the local Joule heating at the interfaces would lead to the failure of multilayer nanowires. This work provides not only the basic electrical parameters for the application of Co Cu/Cu multilayer nanowire, but also a clear physical picture of the magneto-electrical transport behavior of 1D multilayer structures.(4) The MR effect of individual Co Cu/Cu multilayer nanowire has also been accurately measured by the magneto-transport instrument in situ SEM. The MR value of individual Co Cu/Cu multilayer nanowire was 0.84%. Meanwhile, its theoretical value was simulated to be 0.89% according to Granular Films model, which has a good agreemnent with the experimental value. This result demonstrated that our invented magneto-transport instrument can be used to precisely and dynamically measure the magneto-electrical properties of individual magnetic nanomaterials and nanodevice units.(5) In order to extend the exploriation of magneto-electrical properties of low dimensional magnetic materials, two-dimensional composited materials of graphene and tri-iron tetroxide has been successfully prepared by using high-temperature liquid-phase method. Their morphology and structure were analyzed at the nanoscale by electron microscopes, showing that the Fe3O4 particles were uniformly distributed on the grapheme and the dispersion of Fe3O4 particles on the graphene can be controlled by adjusting reactant concentrations.