| Summary: | 碩士 === 國立臺灣大學 === 物理學研究所 === 92 === Abstract
Magnetic trilayer junctions(MTJs) based on half metallic oxides have attracted much attention because of their potential in memory and logic device applications. Among half metallic materials, the magnetite(Fe3O4) is the most promising candidate due to its high ferrimagnetic Curie temperature(~856K). And an enhanced TMR effect has been expected for this sort of MTJ. However, up to now, the MR effect of MTJ with Fe3O4 as an electrode has been found to be negligibly small. Several causes have been speculated to explain the reduction. For examples, the spin flip processes on the interface would have a deleterious effect on the MR. A less-than ideal insulating barrier containing impurities and defects would also lead to spin scattering. In order to explore the major origins of reduction in MR, we have systematically investigated the TMR of MTJs prepared under various conditions.
Our MTJs consist of Fe3O4 and Co as the electrodes sandwitched with cobalt oxide and AlOx thin layers as the tunneling barriers for comparison. The choice of Fe3O4 and Co as the electrodes is due to a large difference of coercivities from these two magnetic layers. Besides, a thin oxide layer can be directly obtained from oxidation of the metal layer right after deposition of the metal, where metals are Co and Al, respectively. We have adopted different oxidation time to acquire tunneling barrier of various thickness. Thus, the impact of tunneling barrier on TMR can be evaluated. Furthermore, since the polarization of Fe3O4 has been predicted to be negative, the inverse MR will be also studied. In Our MTJs, the resistance of electrode Fe3O4 is too large compared to the junction''s, then it would cause nonuniform current distribution over the junction area. Thus, negative resistance will be obtained by using four-probe measurement. We try to reduce the junction area from mm2-size to um2-size to pursuit a uniform coverage of the barrier on the electrode in MTJs. For this purpose, we learn to use E-beam lithography technique to make um2-size magnetic films. We also fabricate hundreds of nm size Co dot arrays to investigate its properties.
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