Magneto-impedance Properties of AlOx-based Magnetic Tunnel Junctions and Mn- or Co/Al- doped ZnO Diluted Magnetic Semiconductors

Spin-dependent materials, due to the charge and spin degrees of freedom accommodated into single matter, are promising candidates for a wide range of spintronic applications. Recently, the study of frequency-dependent transport properties of these materials has received considerable interest because most of their applications require high speed functionality. The transport in these kinds of materials depends sensitively on the details of the growth conditions and geometric structures. The essential underlying question is how these factors affect the frequency-dependent transport properties. Therefore, in this dissertation we study this fundamental problem using impedance spectroscopy methodology.
In the first part of this dissertation, we study the bias voltage dependence of tunnel magneto-impedance in two types of AlOx-based magnetic tunnel junctions (i.e., CoFeB/AlOx/CoFeB and CoFeB/AlOx). The equivalent circuit model is applied to characterize the transport properties and barrier/interface behavior of the tunnel junctions. The bias voltage dependence of the impedance spectra shows different behaviors for each type of junction, thus contributing different physical parameters to the equivalent circuit model. By analysis of the physical parameters we discuss the effects of the barrier high and spin-dependent screening on the frequency-dependent transport properties. The results indicate that the decrease in magneto-impedance is the results of decreased effective barrier high and increased inverse screening length. Impedance spectroscopy can be also used as a tool for studying the microstructure-related transport properties. The second part in this dissertation we have carried out a systematic study of the dependence of magneto-electrical properties of Mn-doped ZnO thin films deposited in various gas (Ar, Ar + N2, and Ar + O2) ambiences. The magneto-impedance spectra of the Mn-doped ZnO thin films have been analyzed using brick layer equivalent model. Different contributions are identified, and the results show that both electrical conductivity and dielectric relaxation of grains and grain boundaries contribute to the magneto-dynamics of the polycrystalline film. The grain boundary is found to make a larger contribution in sample grown in Ar + O2 during the sputtering process, resulting in larger resistance and lower relaxation frequency. Also, the results of the impedance spectroscopy are found to agree well with the SEM inspection.
Finally, the Al doping effects on high-frequency magneto-electric properties of Zn1-x-yAlxCoyO (x = 0 – 10.65 at.%) thin films are systematically studied in this dissertation. The Zn1-x-yAlxCoyO thin films have been deposited by magnetron co-sputtering onto quartz substrates. The magneto-impedance spectra of the thin films are measured by an impedance analyzer. Among all the doped films studied, the thin film with 6.03 at.% Al-doping shows the highest ac conductivity and relaxation frequency. To characterize the relaxation mechanism underlying the magneto-electric properties, a Cole-Cole impedance model is applied to analyze the impedance spectra. The analyzed result shows that the magneto-impedance of the Zn1-x-yAlxCoyO is contributed by multiple processes of magnetization dynamics and dielectric relaxation.