| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 91 === Following the technological improvement, the information industry is changing day by day and various several media are keeping researching to satisfy the request of data storage and recording. Since 1995 materials of the TMR (tunneling magnetoresistance) effect were improved evidently, in a few years it’s magnetic induction was doubled. TMR materials show much higher MR% (about 25~50%) than traditional GMR materials (about 10%) at room temperature, so it is considered be applied as the MR sensor in the magnetic recording industry. Accordingly, the first part of this research employs the chemical replacement that the Sr2+ with a bigger ionic radius is replaced by the smaller Ca2+ to synthesize a series of the TMR materials composed of (Sr2-xCax)FeMoO6 (0 < x < 2.0), to observe the influence of the chemical pressure effect on TMR materials in their crystal structure, valence, magnetic and electrical properties, which we discussed in this thesis in order to find the best TMR materials for the application in recording media.
In the second part of this study, the Sr2FeMoO6 (Sr2BB''O6) double perovskites compounds were fabricated by keeping the stoichiometry of the alkaline earth strontium ion and displace the position of B or B''with different 3d, 4d and 5d transition metals. Based on this procedure, 3d5(Fe) systems of Sr2FeMoO6 (Fe3+：Mo5+ <=> 3d5：4d1) and Sr2FeWO6 (Fe3+：W3+ <=> 3d5：5d1), 3d3(Cr) systems of Sr2CrMoO6 (Cr3+：Mo3+ <=> 3d3：4d1) and Sr2CrWO6 (Cr3+：W3+ <=> 3d3：5d1) have been synthesized. Moreover, their crystal structures, magnetic and electrical properties have been discussed, were may lead to develop the optimal composition and excellent properties of the TMR materials.
We apply XRD (X-ray diffraction) and Rietveld refinement to determine the materials’ purity and crystal structures. The HRTEM (high resolution electron microscope) and SEM (scanning electron microscope) have been used to determine the microstructure and morphology of materials. The four-point probe and SQUID (superconducting quantum interference device) techniques have been applied to measure the magnetic and electrical properties. XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) experiments were performed using synchrotron radiation to probe the valence states, coordination numbers, bond distances, and energy levels of structural disordering, respectively. Furthermore, the theory of FLAPW (full-potential theory linear augmented plane ware) has been used to calculate and analyze the density of states. The results calculated by FLAPW have been compared to those of the experimental data.
In the (Sr2-xCax)FeMoO6 (0 < x < 2.0) system, the chemical replacement of the bigger Sr2+ ion by the smaller Ca2+ has been carried out. An increase in the Ca-content leads to the phase transition from tetragonal to monoclinic unit cell, and raises the magnetic moments close to the theoretical value of 4 μB. An increase in conductivity with increasing the Ca-content is also found and is correlated with an increase in (Fe2+ + Mo6+)/(Fe3+ + Mo5+) band overlap rather than the change of bandwidth. The XANES and band structure calculations support the changes observed by physical measurements.
In the d5(Fe) systems of Sr2FeMoO6 (Fe3+：Mo5+ <=> 3d5：4d1) and Sr2FeWO6 (Fe3+：W3+ <=> 3d5：5d1) and 3d3(Cr) systems of Sr2CrMoO6 (Cr3+：Mo3+ <=> 3d3：4d1) and Sr2CrWO6 (Cr3+：W3+ <=> 3d3：5d1), we find that the structure transition based on the 3d5(Fe3+) system is from tetragonal Sr2FeMoO6 (Mo5+: 4d1) to orthorhombic Sr2FeWO6(W5+: 5d1). However, the structures based on the 3d5(Cr3+) system keeps the cubic symmetry through Sr2CrMoO6 (Mo5+: 4d1) to Sr2CrWO6 (W5+: 5d1). When replacement occurs in the B¢ site (4d1 (Mo) and 5d1 (W)), it is found that Sr2FeMoO6 is ferromagnetic, and Sr2FeWO6 is antiferromagnetic. Based on both electrical properties and band structure calculations, the results show that Sr2FeMoO6 is metallic due to small band gap between Fe3+ (3d5) and Mo5+ (4d1) ions. On the contrary, Sr2FeWO6 is an insulator owing to the wide band gap between Fe3+ (3d5) and W5+ (5d1) ions. When using the smaller electron density ion of Cr3+ (3d3) to replace the higher electron density ion of Fe3+ (3d5), the interaction between the transition metal decreases. Therefore, the Sr2CrMoO6 and Sr2CrWO6 are both ferromagnetic and semiconducting.
The influences of the chemical replacement on double perovskites compounds with theTMR effect have been studied in this research. Based on this study it is hoped to give useful information on the exploration of new TMR materials.
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