| There has been great interest in oxide-based diluted magnetic semiconductors due to their unique magnetic, magneto-optical, magneto-electrical effects and high transparency in the visible region. Theoretical predictions pointed out that ferromagnetic coupling between magnetic ions could be formed more easily in p-type semiconductors. However, p-type doping is hard to achieve in most oxide semiconductors, which hinders the realization of high-Curie-temperature ferromagnetism. In this thesis, Cu Cr O2 delafossite with an ABO2 structure was selected as a p-type matrix semiconductor, and the magnetic properties have been improved through B-site Mg/3d-transition-metal codoping, with the help of high hole concentration produced by the Mg doping. X-ray diffraction, X-ray photoelectron spectroscopy, M?ssbauer spectra, scanning electron microscope, physical property measurement system and photoluminescence spectra were used to characterize crystal structure, valence states of ions, surface morphology, magnetic and optical properties.1. Cu(Cr0.96-x Mg0.04Fex)O2-δ(x=0~9 at%) nanopowders were prepared by combining solid-state reaction and ball milling. With the fixed Mg content, the influence of Fe content on microstructure and physical properties was investigated. The main results obtained are as follows:(1) All the samples have a single 3R-Cu Cr O2 delafossite structure. The Fe ions substitute for Cr3+ at the B site in the form of Fe3+. Due to the larger size ofFe3+ compared with Cr3+, the unit cell volume is increased almost linearly with the Fe content. The lattice expansion is favorable for the intercalation of interstitial oxygen atoms and therefore the interstitial oxygen content is increased gradually with the Fe addition. Accordingly, the concentration of holes which mainly originate from the interstitial oxygen atoms is enhanced.(2) The Mg/Fe codoping induces the hole-mediated Fe3+–Cr3+ ferromagnetic super-exchange interaction and the ferromagnetism is achieved. Compared with the 3d-transition-metal-doped Cu Cr O2 samples in the literatures, the saturation magnetization and Curie temperature are improved obviously by theMg/Fe codoping. This can be attributed to the enhanced p-type conductivityproduced by the Mg doping. The saturation magnetization is influenced by three main factors: the number of B-site ion pairs, the distance of B-site ions and the hole concentration.(3) The Mg/Fe codoping brings abut many interstitial oxygen defects. Hence the fluorescence peak which comes from the defect-energy-level-related transition is observerd located in the near-infrared region.2. Cu(Cr0.96-x Mg0.04Mnx)O2-δ(x=0~15 at%) ceramics were prepared by solid-state reaction. With the fixed Mg content, the influence of Mn content on microstructure and physical properties was investigated. The main results obtained are as follows:(1) All the samples have a single 3R-Cu Cr O2 delafossite structure. The Mn ions replace Cr3+ at the B site in a mixed valence state: Mn3+ and Mn4+. With the increasing of Mn content from 5 at% to 15 at%, the Mn3+/Mn4+ ratio is decreased gradually. As a result, the unit cell volume is decreased because of the ionic size effect. The lattice contraction is unfavorable for the oxygen insertion, and therefore the interstitial oxygen content is decreased gradually but always higher than that of undoped Cu Cr O2. Accordingly, the hole concentration is reduced with the Mn addition.(2) The Mg/Mn codoping induces the hole-mediated Mn3+–Mn4+ and Mn3+–Cr3+ ferromagnetic double-exchange interactions, responsible for the progressive conversion of paramagnetism into ferromagnetism. Compared with the Mn-doped Cu Cr O2 ceramics in the literatures, the Mg/Mn codoping improves the saturation magnetization and Curie temperature by 6.5 times and 50%, respectively.(3) A fluorescence peak located in the near-infrared region is observerd, which comes from the defect-energy-level-related transition induced by the Mg/Mn codoping. With a different excitation wavelength, a red fluorescence peak is observed. This band edge emission can be ascribed to the 3d94s1 and 3d10intraband transition involving Cu+.3. Cu(Cr0.96-x Mg0.04Mnx)O2-δ(x=0~15 at%) thin films were prepared on sapphire substrates by pulsed laser deposition. The x=9 at% sample is taken as an example here.The Cu(Cr0.87Mg0.04Mn0.09)O2-δ thin film has a single 3R-Cu Cr O2 delafossite structure with a c-axis quasi-epitaxial orientation. The Mn ions are present in a mixed valence state: Mn3+ and Mn4+. The hole-mediated Mn3+–Mn4+ and Mn3+–Cr3+ double-exchange mechanism produces ferromagnetism. The thin film exhibits comparatively high transparency in the visible region, 15% higher than that of Mn-doped Cu Cr O2 thin films synthesized by chemical method. With an excitation wavelength of 543 nm, an orange fluorescence peak and a near-infrared fluorescence peak occur at 608 nm and 820 nm, respectively. |
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