The Study Of Magnetoresistance In Chromium Dioxide Granules Treated By Ultrasound

Magnetoelectronics is a new multidisciplinary field whose central theme is the research and manipulation of transportation properties of spin-polarized electrons in mecroscopy scale. Magnetoresistance (MR) is the important content of this field, and it is the base of new generation sensor and high-density memory devices. The application of devices based on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) make memory devices have higher density. Tunneling magnetoresistance (TMR) discovered in magnetic tunneling junction (MTJ) has high sensitivity and adjustable junction resistance, so it has potential to be applied in new MRAM devices. Julliere's model justifies the quest for high spin-polarized materials, which would provide large TMR. The founding of half-metal magnets, which have full spin-polarized conduct electrons, brings vim to the research and application of TMR, meanwhile brings challenges. The biggest challenge is how to preserve such high spin polarization of these materials at room temperature and in the junction with other materials. Therefore, it is necessary to study magnetotransport of half metals. Powder magnetoresistance (PMR) is a simple and versatile method for investigating magnetotransport of half metals. Chromium dioxide (CrO₂) is the half metal with simplest structure whose spin polarization detected by Adreeve reflection reaches to 98.4% at the temperature of 77 K. These characters make it be a potential candidate for fabrication of MTJ devices. Coey's group in Ireland, Chien's group, Xiao's group, Tang's group, IBM company in America and Ziese's group in Germany have been studying the magnetotransport of CrO₂. It is worthy to be focused on that improved MRs have been observed in particle complexes, such as CrO₂/Cr₂O₃, CrO₂/TiO₂, CrO₂/Polymer, etc. The improved MR is achieved by adjusting interface states. It is of special interest to study the influence of surface properties on the magnetotransport of CrO₂ cold-pressed powders. In addition, experiments have found that ultrasound not only disperse particles but also can change surface properties of particles. In this work, CrO₂ powders are treated by ultrasound with different time. The thesis is intended to study the influence of surface properties on magnetotransport of CrO₂ granular compacts. We have discussed the origin of changed MR through detecting surface component, crystalline structure, particle size and electromagnetic properties of CrO2 with different treatment. The results show that the change of surface component of CrO2 particles induces to the difference in tunneling barrier, which alters physical properties and magnetotransport properties of CrO2.