| Magneto-electronics lies in the area of current scientific interest in condensed matter physics and material sciences, due to abundant physical conceptions and applied potentials. It is a base of magnetic devices with high density and sensitivity. Recently, non-volatile magnetic computer memory (MRAM) is expected to have a large economic impact, and has been significantly focused on its experimenatal research. The MRAM needs to be of a large magnetoresistance (MR) effect at room temperature and lower magnetic field, in order to demands of practical application. However, the large tunneling MR effect is strongly dependent upon electrode material, which is of high spin polarization. On the other hand, half-metallic material is theoretically predicted to be perfect spin polarization, nearly 100%. Indeed, in recent years, some half-metallic oxides have been fabricated and measured, such as CrO2 (P=98.4%), Fe3O4 (P=-80%), LaCaMnO3 (P=78%), etc. These measured results are very well agreement with theoretical prediction. Hence, half-metal is considered as an ideal, potential material in spintronic devices. Previous work has indicated that MR is very small in crystal C1O2, but OO2 polycrystalline film and powder compact exhibit large negative MR (-23%?0%) at low temperature. Furthermore, an enhancement of MR (-50%) was also observed in CrO2/Cr2O3 composite. Therefore, it is an efficient way to dilute CrO2 granules for enhancing extrinsic MR, due to the formation of new Metal-Insulator microstructure,which adjusts natures of barrier. In much of work published, much interest involves the fabricating technique and magnetotransport of CrO2 ferromagnet, experimentally. These original outcomes are primarily from Coey's group in Ireland, Chien's group, Xiao's group, Tang's group, IBM company in America, and Ziese group in Germany, etc.; nevertheless the systematical work in CrO2 granular composite is still insufficient until now.The thesis was originally intended to reveal magnetoresistance effect in two kinds of novel composites: (1) acicular CrO2 / spherical insulating granular composite; (2) CrO2 granules coated with insulating polymer, respectively. The emphasis is placed on the magnetotransport of the composites that is concerned with the relation between microstructure and MR effect, and discussion of magnetotransport mechanism in these CrO2 composites. Particular interest is to find a way that can enhance room temperature MR, which always receives much attention in all research activity.The main work involves: 1. C1O2 / TiO2 granular compositeThe composite consists of acicular, conducting, ferromagnetic CrO2 particles and spherical, insulating, diamagnetic TiO2 particles. Work is focused on understanding the macroscopic properties in terms of microscopic mechanisms, in the novel binary composite with different geometric (shape and size), electrical and magnetic particles. (1) MR and percolation effect:A new microstructure is observed due to size of the TiO2 particle much more than that of the CrO2 particle, and obviously different shapes. This may decrease percolation threshold ( pc ) and enhance MR. The experimental results indicate pc=19.2, lower indeed than that with identical size and shape particles, which further supports to previous conclusion of maximum MR near pc . In this composite, it is observed that a maximum MR value reaches 9.2% at T=77K when p approaches increasingly pc. It is proposed that MR effect may depend on microstructure near M-I phase transition. We find that the MR vs p curve is different between two different composition regions (10 |
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